unit 4- 8051 microcontrollermahalakshmiengineeringcollege.com/pdf/eee/visem/ee2354...to note that...

EE2324/ Microprocessors & Microcontrollers Department of EEE 1

MAHALAKSHMI

ENGINEERING COLLEGE

TIRUCHIRAPALLI-621213.

QUESTION BANK

DEPARTMENT: EEE YR/ SEM:III/ VI

SUB CODE: EE2324 SUB NAME: MICROPROCESSORS & MICROCONTROLLERS

UNIT 4- 8051 MICROCONTROLLER

PART A (2 Marks)

1. What are the main features of 8051 microcontroller?(AUC MAY 2012)

The features are

*single supply +5 volt operation using HMOS technology.

*4096 bytes program memory on chip (not on 8031)

*128 data memory on chip.

*Four register banks.

*Two multiple mode,16-bit timer/counter.

*Extensive Boolean processing capabilities.

*64 KB external RAM size

*32 bi-directional individually addressable I/O lines.

*8 bit CPU optimized for control applications

2. List the on chip peripherals of 8051 microcontroller. (AUC NOV 2011)

4KB on chip RAM

128 B RAM

2 Timers

32 I/O pins

1 serial port

6 Interrupt sources


3. What are the addressing modes of 8051 microcontroller? (AUC NOV 2011)

Immediate

Direct

Register

Register Indirect

4. List the alternative function assigned to port 3 pins of 8051 microcontroller. (AUC

MAY 2011)

P3 bit Function pin

P3.0 RxD ( Receive data,Serial communication) 10

P3.1 TxD(( Transmit data,Serial communication) 11

P3.2 INT0 12

P3.3 INT1 13

P3.4 T0 14

P3.5 T1 15

P3.6 WR 16

P3.7 RD 17

5. Mention the size of DPTR & stack pointer in 8051 microcontroller. (AUC MAY 2011)

Size of stack Pointer & DPTR registers are 16 bit.

PART B (8, 16Marks)

1. Explain with a neat block diagram the architecture of 8051 microcontroller. (AUC

MAY 2012)


Pin Configuration:

8051: Types of Memory

The 8051 has three very general types of memory. To effectively program the 8051 it

is necessary to have a basic understanding of these memory types .


On-Chip Memory refers to any memory(Code, RAM, or other) that physically exists

on the microcontroller itself. On-chip memory can be of several types, but we'll get

into that shortly.

External Code Memory is code (or program) memory that resides off-chip. This is

often in the form of an external EPROM.

External RAM is RAM memory that resides off-chip. This is often in the form of

standard static RAM or flash RAM.

Code Memory

Code memory is the memory that holds the actual 8051 program that is to be run.

This memory is limited to 64K and comes in many shapes and sizes: Code memory

may be found on-chip, either burned into the microcontroller as ROM or EPROM.

Code may also be stored completely off-chip in an external ROM or, more commonly,

an external EPROM. Flash RAM is also another popular method of storing a

program. Various combinations of these memory types may also be used--that is to

say, it is possible to have4K of code memory on-chip and 64k of code memory off-

chip in an EPROM. When the program is stored on-chip the 64K maximum is often

reduced to 4k, 8k, or 16k. This varies depending on the version of the chip that is

being used. Each version offers specific capabilities and one of the distinguishing

factors from chip to chip is how much ROM/EPROM space the chip has. However,

code memory is most commonly implemented as off-chip EPROM

Programming Tip: Since code memory is restricted to 64K, 8051 programs are

limited to 64K. Some assemblers and compilers offer ways to get around this limit

when used with specially wired hardware. However, without such special compilers

and hardware, programs are limited to 64K.

External RAM

As an obvious opposite of Internal RAM, the 8051 also supports what is called

External RAM. As the name suggests, External RAM is any random access memory

which is found off-chip. Since the memory is off-chip it is not as flexible in terms of

accessing, and is also slower. For example, to increment an Internal RAM location by

1 requires only 1 instruction and 1 instruction cycle. To increment a 1-byte value

stored in External RAM requires 4 instructions and 7 instruction cycles. In this case,

external memory is 7 times slower! What External RAM loses in speed and flexibility


it gains in quantity? While Internal RAM is limited to 128 bytes the 8051 supports

External RAM up to 64K.

Programming Tip: The 8051 may only address 64k of RAM. To expand RAM

beyond this limit requires programming and hardware tricks. You may have to do this

"by hand" since many compilers and assemblers, while providing support for

programs in excess of 64k, do not support more than 64k of RAM. This is rather

strange since it has been my experience that programs can usually fit in 64k but

often RAM is what is lacking. Thus if you need more than 64k of RAM, check to see if

your compiler supports it-- but if it doesn't, be prepared to do it by hand .

On-Chip Memory.

The 8051 includes a certain amount of onchip memory. On-chip memory is really one

of two (SFR) memories. the 8051 has bank of 128 bytes of Internal RAM. This

Internal RAM is found on-chip on the 8051 so it is the fastest RAM available, and it is

also the most flexible in terms of reading, writing, and modifying its contents. Internal

RAM is volatile, so when the 8051 is reset this memory is cleared. The 128 bytes of

internal ram is subdivided as shown on the memory map. The first 8 bytes (00h- 07h)

are "register bank 0". By manipulating certain SFRs, a program may choose to use

register banks1, 2, or 3. These alternative register banks are located in internal RAM

in addresses 08h through1Fh.The 80 bytes remaining of Internal RAM, from

addresses 30h through 7Fh, may be used by user variables that need to be

accessed frequently or at high-speed. This area is also utilized by the microcontroller

as a storage area for the operating stack. This fact severely limits the 8051’s stack

since, as illustrated in the memory map, the area reserved for the stack is only 80

bytes--and usually it is less since this 80 bytes has to be shared between the stack

and user variables.

Register Banks

The 8051 uses 8 "R" registers which are used in many of its instructions. These

"R"registers are numbered from 0 through 7 (R0, R1,R2, R3, R4, R5, R6, and R7).

These registers are generally used to assist in manipulating values and moving data

from one memory location to another. For example, to add the value of R4 to the

Accumulator, we would execute the following instruction: ADD A,R4When the 8051 is

first booted up, register bank 0(addresses 00h through 07h) is used by default.

However, your program may instruct the 8051 to use one of the alternate register


banks; i.e. register banks 1, 2, or 3. In this case, R4 will no longer be the same as

Internal RAM address 04h. For example, if your program instructs the 8051 to use

register bank 3, "R" register R4 will now be synonymous with Internal RAM address

1Ch. The concept of register banks adds a great level of flexibility to the 8051,

especially when dealing with interrupts However, always remember that the register

banks really reside in the first 32 bytes

Bit Memory

The 8051, being a communications oriented microcontroller, gives the user the ability

to access a number of bit variables. These variables may be either 1 or 0.There are

128 bit variables available to the user, numbered 00h through 7Fh. The user may

make use of these variables with commands such as SETB and CLR. It is important

to note that Bit Memory is really a part of Internal RAM. In fact, the 128 bit variables

occupy the 16 bytes of Internal RAM from 20h through 2Fh. Thus, if you write the

value FFh to Internal RAM address 20h you’ve effectively set bits 00h through

07h.But since the 8051 provides special instructions to access these 16 bytes of

memoryon a bit by bit basis it is useful to think of it as a separate type of memory.

However, always keep in mind that it is just a subset of Internal RAM—an that

operations performed on Internal RAM can change the values of the bit variables.

use bit variables, you may use Internal RAM locations Bit variables 00h through 7Fh

are for user defined functions in their programs. However, bit variables 80h and

above are actually used to access certain SFRs on a bit-by-bit basis. For example, if

output lines P0.0 through P0.7 are all clear (0) and you want to turn on the P0.0

output line you may either execute: MOV P0,#01h || SETB 80hBoth these

instructions accomplish the same thing. However, using the SETB command will turn

on the P0.0 line without affecting the status of any of the other P0 output lines. The

MOV command effectively turns off all the other output lines which, in some cases,

may not be acceptable.

Special Function Register (SFR) Memory

Special Function Registers (SFRs) area of memory that control specific functionality

of the 8051 processor. For example, four SFRs permit access to the 8051’s 32

input/output lines. Another SFR allows a program to read or write to the 8051’s serial

port. Other SFRs allow the use to set the serial baud rate, control and access timers,

and configure the 8051’s interrupt system. When programming, SFRs have the

illusion of being Internal Memory. For example, if you want to write the value "1" to


Internal RAM location 50 hex you would execute the instruction: MOV

50h,#01Similarly, if you want to write the value "1" to the 8051’s serial port you would

write this value to the SBUF SFR, which has an SFR address of 99 Hex. Thus, to

write the value "1" to the serial port you would execute the instruction: MOV

99h,#01hAs you can see, it appears that the SFR is part of Internal Memory. This is

not the case. When using this method of memory access (it’s called direct address),

any instruction that has an address of 00h through 7Fh refers to an Internal RAM

memory address; any instruction with an address of 80h through FFh refers to an

SFR control register.

What Are SFRs?

The 8051 is a flexible microcontroller with a relatively large number of modes of

operations. Your program may inspect and/or change the operating mode of the

8051 by manipulating the values of the 8051's Special Function Registers (SFRs).

SFRs are accessed as if they were normal Internal RAM. The only difference is that

Internal RAM is from address 00h through 7Fh where as SFR registers exist in the

address range of 80h through FFh. Each SFR has an address (80h through FFh) and

a name. The following chart provides a graphical presentation of the 8051's SFRs,

their names, and their address. As you can see, although the address range of 80h

through FFh offer 128 possible addresses, there are only 21 SFRs in a

standard8051. All other addresses in the SFR range (80h through FFh) are

considered invalid. Writing to or reading from these registers may produce undefined

values or behavior.

SFR Types

As mentioned in the chart itself, the SFRs that have a blue background are SFRs

related to the I/O ports. The 8051 has four I/O ports of 8 bits, for a total of 32 I/O

lines. Whether a given I/O line is high or low and the value read from the line are

controlled by the SFRs in green. The SFRs with yellow background are SFRs which

in some way control the operation or the configuration of some aspect of the 8051.

For example, TCON controls the timers, SCON controls the serial portThe remaining

SFRs, with green backgrounds, are "other SFRs." These SFRs can be thought of as

auxiliary SFRs in the sense that they don't directly configure the 8051 but obviously

the 8051 cannot operate without them. For example, once the serial port has been

configured using SCON, the program may read or write to the serial port using the

SBUF register.


SFR Descriptions

This section will endeavor to quickly overview each of the standard SFRs found in

the above SFR chart map. It is not the intention of this section to fully explain the

functionality of each SFR--this information will be covered in separate chapters of the

tutorial. This section is to just give you a general idea of what each SFR does.

P0 (Port 0, Address 80h, Bit-Addressable): This is input/output port 0. Each bit of

this SFR corresponds to one of the pins on the microcontroller. For example, bit 0 of

port 0 is pin P0.0, bit 7 is pin P0.7. Writing a value of 1 to a bit of this SFR will send a

high level on the corresponding I/O pin whereas a value of 0 will bring it to a low

level.

Programming Tip: While the 8051 has four I/O port (P0, P1, P2, and P3), if your

hardware uses external RAM or external code memory (i.e., your program is stored

in an external ROM or EPROM chip or if you are using external RAM chips) you may

not use P0 or P2. This is because the 8051 uses ports P0 and P2 to address the

external memory. Thus if you are using external RAM or code memory you may only

use ports P1 and P3 for your own use.

SP (Stack Pointer, Address 81h): This is the stack pointer of the microcontroller.

This SFR indicates where the next value to be taken from the stack will be read from

in Internal RAM. If you push a value onto the stack, the value will be written to the

address of SP + 1. That is to say, if SP holds the value 07h, a PUSH instruction will

push the value onto the stack at address 08h. This SFR is modified by all instructions

which modify the stack, such as PUSH, POP, LCALL, RET, RETI, and whenever

interrupts are provoked by the microcontroller.

Programming Tip: The SP SFR, on startup, is initialized to 07h. This means the

stack will start at 08h and start expanding upward in internal RAM. Since alternate

register banks 1, 2, and 3 as well as the user bit variables occupy internal RAM from

addresses 08h through 2Fh, it is necessary to initialize SP in your program to some

other value if you will be using the alternate register banks and/or bit memory. It's not

a bad idea to initialize SP to 2Fh as the first instruction of every one of your programs

unless you are 100% sure you will not be using the register banks and bit variables.

DPL/DPH (Data Pointer Low/High, Addresses 82h/83h): The SFRs DPL and DPH

work together to represent a 16-bit value called the Data Pointer. The data pointer is

used in operations regarding external RAM and some instructions involving code


memory. Since it is an unsigned two-byte integer value, it can represent values from

0000h to FFFFh (0 through 65,535 decimal).

Programming Tip: DPTR is really DPH and DPL taken together as a 16-bit value. In

reality, you almost always have to deal with DPTR one byte at a time. For example,

to push DPTR onto the stack you must first push DPL and then DPH. You can't

simply plush DPTR onto the stack. Additionally, there is an instruction to "increment

DPTR." When you execute this instruction, the two bytes are operated upon as a 16-

bit value. However, there is no instruction that decrements DPTR. If you wish to

decrement the value of DPTR, you must write your own code to do so.

PCON (Power Control, Addresses 87h): The Power Control SFR is used to control

the 8051's power control modes. Certain operation modes of the 8051 allow the 8051

to go into a type of "sleep" mode which requires much less power. These modes of

operation are controlled through PCON. Additionally, one of the bits in PCON is used

to double the effective baud rate of the 8051's serial port.

TCON (Timer Control, Addresses 88h, Bit-Addressable): The Timer Control SFR

is used to configure and modify the way in which the 8051's two timers operate. This

SFR controls whether each of the two timers is running or stopped and contains a

flag to indicate that each timer has overflowed. Additionally, some non-timer related

bits are located in the TCON SFR. These bits are used to configure the way in which

the external interrupts are activated and also contain the external interrupt flags

which are set when an external interrupt has occurred.

TMOD (Timer Mode, Addresses 89h): The Timer Mode SFR is used to configure

the mode of operation of each of the two timers. Using this SFR your program may

configure each timer to be a 16-bit timer, an 8-bit auto reload timer, a 13-bit timer, or

two separate timers. Additionally, you may configure the timers to only count when

an external pin is activated or to count "events" that are indicated on an external pin.

TL0/TH0 (Timer 0 Low/High, Addresses 8Ah/8Bh): These two SFRs, taken

together, represent timer 0. Their exact behavior depends on how the timer is

configured in the TMOD SFR; however, these timers always count up. What is

configurable is how and when they increment in value.

TL1/TH1 (Timer 1 Low/High, Addresses 8Ch/8Dh): These two SFRs, taken

together, represent timer 1. Their exact behavior depends on how the timer is


configured in the TMOD SFR; however, these timers always count up. What is

configurable is how and when they increment in value.

P1 (Port 1, Address 90h, Bit-Addressable): This is input/output port 1. Each bit of




level.

SCON (Serial Control, Addresses 98h, Bit-Addressable): The Serial Control SFR

is used to configure the behavior of the 8051's on-board serial port. This SFR

controls the baud rate of the serial port, whether the serial port is activated to receive

data, and also contains flags that are set when a byte is successfully sent or

received.

Programming Tip: To use the 8051's on-board serial port, it is generally necessary

to initialize the following SFRs: SCON, TCON, and TMOD. This is because SCON

controls the serial port. However, in most cases the program will wish to use one of

the timers to establish the serial port's baud rate. In this case, it is necessary to

configure timer 1 by initializing TCON and TMOD.

SBUF (Serial Control, Addresses 99h): The Serial Buffer SFR is used to send and

receive data via the on-board serial port. Any value written to SBUF will be sent out

the serial port's TXD pin. Likewise, any value which the 8051 receives via the serial

port's RXD pin will be delivered to the user program via SBUF. In other words, SBUF

serves as the output port when written to and as an input port when read from.

P2 (Port 2, Address A0h, Bit-Addressable): This is input/output port 2. Each bit of




level.

Programming Tip: While the 8051 has four I/O port (P0, P1, P2, and P3), if your

hardware uses external RAM or external code memory (i.e., your program is stored

in an external ROM or EPROM chip or if you are using external RAM chips) you may

not use P0 or P2. This is because the 8051 uses ports P0 and P2 to address the

external memory. Thus if you are using external RAM or code memory you may only

use ports P1 and P3 for your own use.


IE (Interrupt Enable, Addresses A8h): The Interrupt Enable SFR is used to enable

and disable specific interrupts. The low 7 bits of the SFR are used to enable/disable

the specific interrupts, where as the highest bit is used to enable or disable ALL

interrupts. Thus, if the high bit of IE is 0 all interrupts are disabled regardless of

whether an individual interrupt is enabled by setting a lower bit.

P3 (Port 3, Address B0h, Bit-Addressable): This is input/output port 3. Each bit of




level.

IP (Interrupt Priority, Addresses B8h, Bit-Addressable): The Interrupt Priority

SFR is used to specify the relative priority of each interrupt. On the 8051, an interrupt

may either be of low (0) priority or high (1) priority. An interrupt may only interrupt

interrupts of lower priority. For example, if we configure the 8051 so that all interrupts

are of low priority except the serial interrupt, the serial interrupt will always be able to

interrupt the system, even if another interrupt is currently executing. However, if a

serial interrupt is executing no other interrupt will be able to interrupt the serial

interrupt routine since the serial interrupt routine has the highest priority.

PSW (Program Status Word, Addresses D0h, Bit-Addressable): The Program

Status Word is used to store a number of important bits that are set and cleared by

8051 instructions. The PSW SFR contains the carry flag, the auxiliary carry flag, the

overflow flag, and the parity flag. Additionally, the PSW register contains the register

bank select flags which are used to select which of the "R" register banks are

currently selected.

Programming Tip: If you write an interrupt handler routine, it is a very good idea to

always save the PSW SFR on the stack and restore it when your interrupt is

complete. Many 8051 instructions modify the bits of PSW. If your interrupt routine

does not guarantee that PSW is the same upon exit as it was upon entry, your

program is bound to behave rather erratically and unpredictably--and it will be tricky

to debug since the behavior will tend not to make any sense.

ACC (Accumulator, Addresses E0h, Bit-Addressable): The Accumulator is one of

the mostused SFRs on the 8051 since it is involved in so many instructions. The

Accumulator resides as an SFR at E0h, which means the instruction MOV A,#20h is


really the same as MOV E0h,#20h. However, it is a good idea to use the first method

since it only requires two bytes whereas the second option requires three bytes.

B (B Register, Addresses F0h, Bit-Addressable): The "B" register is used in two

instructions: the multiply and divide operations. The B register is also commonly used

by programmers as an auxiliary register to temporarily store values.

8051 Basic Registers

The Accumulator .

The Accumulator, as its name suggests, is used as a general register to accumulate

the results of a large number of instructions. It can hold an 8-bit (1-byte) value and is

the most versatile register the 8051 has due to the sheer number of instructions that

make use of the accumulator. More than half of the 8051’s 255instructions

manipulate or use the accumulator in some way. For example, if you want to add the

number 10 and 20, the resulting 30 will be store in the Accumulator. Once you have a

value in the Accumulator you may continue processing the value or you may store it

in another register or in memory.

The "R" registers

The "R" registers are a set of eight registers that are named R0, R1, etc. up to and

including R7.These registers are used as auxiliary registers in many operations. To

continue with the above example, perhaps you are adding 10 and20. The original

number 10 may be stored in the Accumulator whereas the value 20 may be stored in,

say, register R4. To process the addition you would execute the command: ADD

A,R4 After executing this instruction the Accumulator will contain the value 30.You

may think of the "R" registers as very important auxiliary, or "helper", registers. The

Accumulator alone would not be very useful if it were not for these "R" registers. The

"R" registers are also used to temporarily store values. For example, let’s say you

want to add the values in R1 and R2 together and then subtract the values of R3 and

R4. One way to do this would be: MOV A, R3 ;Move the value of R3 into the

accumulator

ADD A, R4 ; Add the value of R4

MOV R5, A ; Store the resulting value temporarily in R5

MOV A, R1 ; Move the value of R1 into the accumulator


ADD A, R2 ; Add the value of R2

SUBB A, R5 ; Subtract the value of R5 (which now contains R3 + R4)

As you can see, we used R5 to temporarily hold the sum of R3 and R4. Of course,

this isn’t the most efficient way to calculate (R1+R2) - (R3 +R4) but it does illustrate

the use of the "R" registers as a way to store values temporarily.

The "B" Register

The "B" register is very similar to the Accumulator in the sense that it may hold an 8-

bit(1-byte) value. The "B" register is only used by two 8051instructions: MUL AB and

DIV AB. Thus, if you want to quickly and easily multiply or divide A by another

number, you may store the other number in "B" and make use of these two

instruction Aside from the MUL and DIV instructions, the "B" register is often used as

yet another temporary storage register much like a ninth "R" register.

The Data Pointer (DPTR)

The Data Pointer (DPTR) is the 8051’sonly user- accessible 16-bit (2-byte) register.

The Accumulator, "R" registers, and "B" register are all1-byte values. DPTR, as the

name suggests, is used to point to data. It is used by a number of commands which

allow the 8051 to access external memory. When the 8051 accesses external

memory it will access external memory at the address indicated by DPTR. While

DPTR is most often used to point to data in external memory, many programmers

often take advantge of the fact that it’s the only true 16-bit register available. It is

often used to store 2-byte values which have nothing to do with memory locations.

The Program Counter (PC)

The Program Counter (PC) is a 2-byteaddress which tells the 8051 where the next

instruction to execute is found in memory. When the 8051 is initialized PC always

starts at 0000hand is incremented each time an instruction is executed. It is

important to note that PC isn’t always incremented by one. Since some instructions

require 2 or 3 bytes the PC will be incremented by 2 or 3 in these cases. The

Program Counter is special in that there is no way to directly modify it’s value. That is

to say, you can’t do something like PC=2430h. On the other hand, if you execute

LJMP 2340h you’ve effectively accomplished the same thing. It is also interesting to

note that while you may change the value of PC (by executing a jump instruction,

etc.) there is no way to read the value of PC. That is to say, there is no way to ask


the8051 "What address are you about to execute?"As it turns out, this is not

completely true: There isone trick that may be used to determine the current value of

PC

The Stack Pointer (SP)

The Stack Pointer, like all registers except DPTR and PC, may hold an 8-bit (1-byte)

value. The Stack Pointer is used to indicate where the next value to be removed from

the stack should be taken from. When you push a value onto the stack, the8051 first

increments the value of SP and then stores the value at the resulting memory

location. When you pop a value off the stack, the8051 returns the value from the

memory location indicated by SP and then decrements the value of SP. This order of

operation is important. When the 8051 is initialized SP will be initialized to 07h. If you

immediately push a value onto the stack, the value will be stored in Internal RAM

address 08h.This makes sense taking into account what was mentioned two

paragraphs above: First the 8051 will increment the value of SP (from 07h to 08h)and

then will store the pushed value at that memory address (08h).SP is modified directly

by the 8051 by six instructions: PUSH, POP, ACALL, LCALL, RET, and RETI. It is

also used intrinsically whenever an interrupt is triggered

2. Explain different operating modes of timer in 8051 microcontroller (AUC MAY 2012)

Same as Q6

3. Explain program memory interfacing in 8051 microcontroller (AUC MAY 2012)

Memory Organization

The 8051 has two types of memory and these are Program Memory and Data

Memory. Program Memory (ROM) is used to permanently save the program being

executed, while Data Memory (RAM) is used for temporarily storing data and

intermediate results created and used during the operation of the microcontroller.

Depending on the model in use (we are still talking about the 8051 microcontroller

family in general) at most a few Kb of ROM and 128 or 256 bytes of RAM is used.

However…

All 8051 microcontrollers have a 16-bit addressing bus and are capable of

addressing 64 kb memory. It is neither a mistake nor a big ambition of engineers who

were working on basic core development. It is a matter of smart memory


organization which makes these microcontrollers a real ―programmers‟ goody―.

Program Memory

The first models of the 8051 microcontroller family did not have internal program

memory. It was added as an external separate chip. These models are recognizable

by their label beginning with 803 (for example 8031 or 8032). All later models have a

few Kbyte ROM embedded. Even though such an amount of memory is sufficient for

writing most of the programs, there are situations when it is necessary to use

additional memory as well. A typical example are so called lookup tables. They are

used in cases when equations describing some processes are too complicated or

when there is no time for solving them. In such cases all necessary estimates and

approximates are executed in advance and the final results are put in the tables

(similar to logarithmic tables).

How does the microcontroller handle external memory depends on the EA pin logic

state


EA=0 In this case, the microcontroller completely ignores internal program memory

and executes only the program stored in external memory.

EA=1 In this case, the microcontroller executes first the program from built-in ROM,

then the program stored in external memory.

In both cases, P0 and P2 are not available for use since being used for data and

address transmission. Besides, the ALE and PSEN pins are also used.

Data Memory

As already mentioned, Data Memory is used for temporarily storing data and

intermediate results created and used during the operation of the microcontroller.

Besides, RAM memory built in the 8051 family includes many registers such as

hardware counters and timers, input/output ports, serial data buffers etc. The

previous models had 256 RAM locations, while for the later models this number was

incremented by additional 128 registers. However, the first 256 memory locations

(addresses 0-FFh) are the heart of memory common to all the models belonging to

the 8051 family. Locations available to the user occupy memory space with

addresses 0-7Fh, i.e. first 128 registers. This part of RAM is divided in several

blocks.

The first block consists of 4 banks each including 8 registers denoted by R0-R7. Prior

to accessing any of these registers, it is necessary to select the bank containing it.

The next memory block (address 20h-2Fh) is bit- addressable, which means that

each bit has its own address (0-7Fh). Since there are 16 such registers, this block

contains in total of 128 bits with separate addresses (address of bit 0 of the 20h byte

is 0, while address of bit 7 of the 2Fh byte is 7Fh). The third group of registers

occupy addresses 2Fh-7Fh, i.e. 80 locations, and does not have any special

functions or features.

Additional RAM

In order to satisfy the programmers‟ constant hunger for Data Memory, the

manufacturers decided to embed an additional memory block of 128 locations into

the latest versions of the 8051 microcontrollers. However, it‟s not as simple as it

seems to be… The problem is that electronics performing addressing has 1 byte (8

bits) on disposal and is capable of reaching only the first 256 locations, therefore. In

order to keep already existing 8-bit architecture and compatibility with other existing


models a small trick was done.What does it mean? It means that additional memory

block shares the same addresses with locations intended for the SFRs (80h- FFh). In

order to differentiate between these two physically separated memory spaces,

different ways of addressing are used. The SFRs memory locations are accessed by

direct addressing, while additional RAM memory locations are accessed by indirect

addressing.

Memory expansion

In case memory (RAM or ROM) built in the microcontroller is not sufficient, it is

possible to add two external memory chips with capacity of 64Kb each. P2 and P3

I/O ports are used for their addressing and data transmission.


From the user‟s point of view, everything works quite simply when properly

connected because most operations are performed by the microcontroller itself. The

8051 microcontroller has two pins for data read RD#(P3.7) and PSEN#. The first one

is used for reading data from external data memory (RAM), while the other is used

for reading data from external program memory (ROM). Both pins are active low. A

typical example of memory expansion by adding RAM and ROM chips (Hardward

architecture), is shown in figure above.

Even though additional memory is rarely used with the latest versions of the

microcontrollers, we will describe in short what happens when memory chips are

connected according to the previous schematic. The whole process described below

is performed automatically. When the program during execution encounters an

instruction which resides in external memory (ROM), the microcontroller will activate

its control output ALE and set the first 8 bits of address (A0-A7) on P0. IC circuit

74HCT573 passes the first 8 bits to memory address pins.

A signal on the ALE pin latches the IC circuit 74HCT573 and immediately afterwards

8 higher bits of address (A8-A15) appear on the port. In this way, a desired location

of additional program memory is addressed. It is left over to read its content.

Port P0 pins are configured as inputs, the PSEN pin is activated and the

microcontroller reads from memory chip.


Similar occurs when it is necessary to read location from external RAM. Addressing

is performed in the same way, while read and write are performed via signals

appearing on the control outputs RD (is short for read) or WR (is short for write).

4. Explain the program memory & data memory structure of 8051 microcontroller. (AUC

NOV 2011) same as Q3

5. Draw the pin diagram of 8051 microcontroller & explain its port structure. (AUC NOV

2011)

Pinout Description

Pins 1-8: Port 1 Each of these pins can be configured as an input or an output.

Pin 9: RS A logic one on this pin disables the microcontroller and clears the contents

of most registers. In other words, the positive voltage on this pin resets the

microcontroller. By applying logic zero to this pin, the program starts execution from

the beginning.

Pins10-17: Port 3 Similar to port 1, each of these pins can serve as general input or

output. Besides, all of them have alternative functions:

Pin 10: RXD Serial asynchronous communication input or Serial synchronous

communication output.

Pin 11: TXD Serial asynchronous communication output or Serial synchronous

communication clock output.

Pin 12: INT0 Interrupt 0 input.


Pin 13: INT1 Interrupt 1 input.

Pin 14: T0 Counter 0 clock input.

Pin 15: T1 Counter 1 clock input.

Pin 16: WR Write to external (additional) RAM.

Pin 17: RD Read from external RAM.

Pin 18, 19: X2, X1 Internal oscillator input and output. A quartz crystal which

specifies operating frequency is usually connected to these pins. Instead of it,

miniature ceramics resonators can also be used for frequency stability. Later versions

of microcontrollers operate at a frequency of 0 Hz up to over 50 Hz.

Pin 20: GND Ground.

Pin 21-28: Port 2 If there is no intention to use external memory then these port pins

are configured as general inputs/outputs. In case external memory is used, the higher

address byte, i.e. addresses A8-A15 will appear on this port. Even though memory

with capacity of 64Kb is not used, which means that not all eight port bits are used for

its addressing, the rest of them are not available as inputs/outputs.

Pin 29: PSEN If external ROM is used for storing program then a logic zero (0)

appears on it every time the microcontroller reads a byte from memory.

Pin 30: ALE Prior to reading from external memory, the microcontroller puts the

lower address byte (A0-A7) on P0 and activates the ALE output. After receiving

signal from the ALE pin, the external register (usually 74HCT373 or 74HCT375 addon

chip) memorizes the state of P0 and uses it as a memory chip address. Immediately

after that, the ALU pin is returned its previous logic state and P0 is now used as a

Data Bus. As seen, port data multiplexing is performed by means of only one

additional (and cheap) integrated circuit. In other words, this port is used for both data

and address transmission.

Pin 31: EA By applying logic zero to this pin, P2 and P3 are used for data and address

transmission with no regard to whether there is internal memory or not. It means that

even there is a program written to the microcontroller, it will not be executed. Instead,

the program written to external ROM will be executed. By applying logic one to the

EA pin, the microcontroller will use both memories, first internal then external (if

exists).

Pin 32-39: Port 0 Similar to P2, if external memory is not used, these pins can be

used

as general inputs/outputs. Otherwise, P0 is configured as address output (A0-A7)

when the ALE pin is driven high (1) or as data output (Data Bus) when the ALE pin is

driven low (0).


Pin 40: VCC +5V power supply.

Input / Output Ports (I/O Ports)

All 8051 microcontrollers have 4 I/O ports each comprising 8 bits which can be

configured as inputs or outputs. Accordingly, in total of 32 input/output pins enabling

the microcontroller to be connected to peripheral devices are available for use.

Pin configuration, i.e. whether it is to be configured as an input (1) or an output (0),

depends on its logic state. In order to configure a microcontroller pin as an input, it is

necessary to apply a logic zero (0) to appropriate I/O port bit. In this case, voltage

level on appropriate pin will be 0.

Similarly, in order to configure a microcontroller pin as an input, it is necessary to

apply a logic one (1) to appropriate port. In this case, voltage level on appropriate pin

will be 5V (as is the case with any TTL input). This may seem confusing but don't

loose your patience. It all becomes clear after studying simple electronic circuits

connected to an I/O pin.

Input/ Output (I/O) pin

Figure above illustrates a simplified schematic of all circuits within the microcontroller

connected one of its pins. It refers to all the pins except those of the P0 port which do

not have pull-up resistors built-in.


Output pin A logic zero (0) is applied to a bit of the P register. The output FE transistor is turned on, thus connecting the appropriate pin to ground. Input pin A logic one (1) is applied to a bit of the P register. The output FE transistor is turned off and the appropriate pin remains connected to the power supply voltage over a pull-up resistor of high resistance. Logic state (voltage) of any pin can be changed or read at any moment. A logic zero and logic one are not equal. A logic one represents a short circuit to ground. Such a pin acts as an output. A logic one is loosely connected to the power supply over a resistor of high resistance. Since this voltage can be easily reduced by an external signal, such pins acts as input.


Port 0 The P0 port is characterized by two functions. If external memory is used then the lower address byte (addresses A0-A7) is applied on it. Otherwise, all bits of this port are configured as inputs/outputs. The other function is expressed when it is configured as an output. Unlike other ports consisting of pins with built-in pull-up resistor connected by its end to 5 V power supply, pins of this port have this resistor left out. This apparently small difference has its consequences: If any pin of this port is configured as an input then it acts as if it ―floats‖. Such an input has unlimited input resistance and indetermined potential. When the pin is configured as an output, it acts as an ―open drain‖. By applying logic 0 to a port bit, the appropriate pin will be connected to ground (0V). By applying logic 1, the external output will keep on ―floating‖. In order to apply logic 1 (5V) on this output pin, it is necessary to built in an external pull-up resistor. Port 1 P1 is a true I/O port, because it doesn't have any alternative functions as is the case with P0, but can be cofigured as general I/O only. It has a pull-up resistor built-in and is completely compatible with TTL circuits. Port 2 P2 acts similarly to P0 when external memory is used. Pins of this port occupy addresses intended for external memory chip. This time it is about the higher address byte with addresses A8-A15. When no memory is added, this port can be used as a general input/output port showing features similar to P1. Port 3


All port pins can be used as general I/O, but they also have an alternative function. In order to use these alternative functions, a logic one (1) must be applied to appropriate bit of the P3 register. In terms of hardware, this port is similar to P0, with the difference that its pins have a pull-up resistor built-in.

6. Draw the TMOD register format & explain the different operating modes of timer in

8051 microcontroller. (AUC NOV 2011)

Counters and Timers

As you already know, the microcontroller oscillator uses quartz crystal for its operation. As the frequency of this oscillator is precisely defined and very stable, pulses it generates are always of the same width, which makes them ideal for time measurement. Such crystals are also used in quartz watches. In order to measure time between two events it is sufficient to count up pulses coming from this oscillator. That is exactly what the timer does. If the timer is properly programmed, the value stored in its register will be incremented (or decremented) with each coming pulse, i.e. once per each machine cycle. A single machine-cycle instruction lasts for 12 quartz oscillator periods, which means that by embedding quartz with oscillator frequency of 12MHz, a number stored in the timer register will be changed million times per second, i.e. each microsecond.

The 8051 microcontroller has 2 timers/counters called T0 and T1. As their names suggest, their main purpose is to measure time and count external events. Besides, they can be used for generating clock pulses to be used in serial communication, so called Baud Rate.

Timer T0 As seen in figure below, the timer T0 consists of two registers – TH0 and TL0 representing a low and a high byte of one 16-digit binary number.

Accordingly, if the content of the timer T0 is equal to 0 (T0=0) then both registers it

consists of will contain 0. If the timer contains for example number 1000 (decimal),

then the TH0 register (high byte) will contain the number 3, while the TL0 register

(low byte) will contain decimal number 232.


Since the timer T0 is virtually 16-bit register, the largest value it can store is 65 535.

In case of exceeding this value, the timer will be automatically cleared and counting

starts from 0. This condition is called an overflow. Two registers TMOD and TCON

are closely connected to this timer and control its operation.

TMOD Register (Timer Mode)

The TMOD register selects the operational mode of the timers T0 and T1. As seen in

figure below, the low 4 bits (bit0 - bit3) refer to the timer 0, while the high 4 bits (bit4 -

it7) refer to the timer 1. There are 4 operational modes and each of them is escribed

herein.

Bits of this register have the following function:

GATE1 enables and disables Timer 1 by means of a signal brought to the

INT1 pin (P3.3):

1 - Timer 1 operates only if the INT1 bit is set.

0 - Timer 1 operates regardless of the logic state of the INT1 bit.


C/T1 selects pulses to be counted up by the timer/counter 1:

1 - Timer counts pulses brought to the T1 pin (P3.5).

0 - Timer counts pulses from internal oscillator.

T1M1,T1M0 These two bits select the operational mode of the Timer 1

T1M1 T1MO MODE DESCRIPTION

0 0 0 13 bit timer

0 1 1 16 bit timer

1 0 2 8 bit auto reload timer

1 1 3 Split mode

GATE0 enables and disables Timer 1 using a signal brought to the

INT0 pin (P3.2): 1 - Timer 0 operates only if the INT0 bit is set, 0 - Timer

0 operates regardless of the logic state of the INT0 bit.

C/T0 selects pulses to be counted up by the timer/counter 0: 1 - Timer

counts pulses brought to the T0 pin (P3.4). 0 - Timer counts pulses from

internal oscillator.

T0M1,T0M0 These two bits select the operational mode of the Timer 0

T0M1 T0MO MODE DESCRIPTION

0 0 0 13 bit timer

0 1 1 16 bit timer

1 0 2 8 bit auto reload timer

1 1 3 Split mode

Timer 0 in mode 0 (13-bit timer)

This is one of the rarities being kept only for the purpose of compatibility with the

previuos versions of microcontrollers. This mode configures timer 0 as a 13-bit timer

which consists of all 8 bits of TH0 and the lower 5 bits of TL0. As a result, the Timer 0


uses only 13 of 16 bits. How does it operate? Each coming pulse causes the lower

register bits to change their states. After receiving 32 pulses, this register is loaded

and automatically cleared, while the higher byte (TH0) is incremented by 1. This

process is repeated until registers count up 8192 pulses. After that, both registers are

cleared and counting starts from 0.

Timer 0 in mode 1 (16-bit timer)

Mode 1 configures timer 0 as a 16-bit timer comprising all the bits of both registers

TH0 and TL0. That's why this is one of the most commonly used modes. Timer

operates in the same way as in mode 0, with difference that the registers count up to

65 536 as allowable by the 16 bits.

Timer 0 in mode 2 (Auto-Reload Timer)

Mode 2 configures timer 0 as an 8-bit timer. Actually, timer 0 uses only one 8-bit

register for counting and never counts from 0, but from an arbitrary value (0-255)


stored in another (TH0) register.The following example shows the advantages of this

mode. Suppose it is necessary to constantly count up 55 pulses generated by the

clock.If mode 1 or mode 0 is used, It is necessary to write the number 200 to the

timer registers and constantly check whether an overflow has occured, i.e. whether

they reached the value 255. When it happens, it is necessary to rewrite the number

200 and repeat the whole procedure. The same procedure is automatically

performed by the microcontroller if set in mode 2. In fact, only the TL0 register

operates as a timer, while another (TH0) register stores the value from which the

counting starts. When the TL0 register is loaded, instead of being cleared, the

contents of TH0 will be reloaded to it. Referring to the previous example, in order to

register each 55th pulse, the best solution is to write the number 200 to the TH0

register and configure the timer to operate in mode 2.

Timer 0 in Mode 3 (Split Timer)

Mode 3 configures timer 0 so that registers TL0 and TH0 operate as separate 8-bit

timers. In other words, the 16-bit timer consisting of two registers TH0 and TL0 is

split into two independent 8-bit timers. This mode is provided for applications

requiring an additional 8-bit timer or counter. The TL0 timer turns into timer 0, while

the TH0 timer turns into timer 1. In addition, all the control bits of 16-bit Timer 1

(consisting of the TH1 and TL1 register), now control the 8-bit Timer 1. Even though

the 16-bit Timer 1 can still be configured to operate in any of modes (mode 1, 2 or 3),

it is no longer possible to disable it as there is no control bit to do it. Thus, its

operation is restricted when timer 0 is in mode 3.


The only application of this mode is when two timers are used and the 16-bit Timer 1

the operation of which is out of control is used as a baud rate generator.

Timer Control (TCON) Register

TCON register is also one of the registers whose bits are directly in control of timer

operation. Only 4 bits of this register are used for this purpose, while rest of them is

used for interrupt control to be discussed later.

TF1 bit is automatically set on the Timer 1 overflow.

TR1 bit enables the Timer 1. 1 - Timer 1 is enabled, 0 - Timer 1 is disabled.

TF0 bit is automatically set on the Timer 0

overflow. TR0 bit enables the timer 0. 1 - Timer 1 is enabled, 0 - Timer 1 is

disabled.

How to use the Timer 0 ?

In order to use timer 0, it is first necessary to select it and configure the mode of its

operation. Bits of the TMOD register are in control of it:


Referring to figure above, the timer 0 operates in mode 1 and counts pulses

generated by internal clock the frequency of which is equal to 1/12 the quartz

frequency. Turn on the timer:

The TR0 bit is set and the timer starts operation. If the quartz crystal with frequency

of 12MHz is embedded then its contents will be incremented every microsecond.

After 65.536 microseconds, the both registers the timer consists of will be loaded.

The microcontroller automatically clears them and the timer keeps on repeating

procedure from the beginning until the TR0 bit value is logic zero (0).

How to 'read' a timer?

Depending on application, it is necessary either to read a number stored in the timer

registers or to register the moment they have been cleared.

- It is extremely simple to read a timer by using only one register configured in mode 2

or 3. It is sufficient to read its state at any moment. That's all!

- It is somehow complicated to read a timer configured to operate in mode 2. Suppose

the lower byte is read first (TL0), then the higher byte (TH0). The result is:

TH0 = 15 TL0 = 255

Everything seems to be ok, but the current state of the register at the moment of


reading was:

TH0 = 14 TL0 = 255

In case of negligence, such an error in counting (255 pulses) may occur for not so

obvious but quite logical reason. The lower byte is correctly read (255), but at the

moment the program counter was about to read the higher byte TH0, an overflow

occurred and the contents of both registers have been changed (TH0: 14→15, TL0:

255→0). This problem has a simple solution. The higher byte should be read first,

then the lower byte and once again the higher byte. If the number stored in the

higher byte is different then this sequence should be repeated. It's about a short

loop consisting of only 3 instructions in the program.There is another solution as

well. It is sufficient to simply turn the timer off while reading is going on (the TR0 bit

of the TCON register should be cleared), and turn it on again after reading is

finished.

Timer 0 Overflow Detection

Usually, there is no need to constantly read timer registers. It is sufficient to register

the moment they are cleared, i.e. when counting starts from 0. This condition is

called an overflow. When it occurrs, the TF0 bit of the TCON register will be

automatically set. The state of this bit can be constantly checked from within the

program or by enabling an interrupt which will stop the main program execution

when this bit is set. Suppose it is necessary to provide a program delay of 0.05

seconds (50 000 machine cycles), i.e. time when the program seems to be stopped:

First a number to be written to the timer registers should be calculated:

Then it should be written to the timer registers TH0 and TL0:

When enabled, the timer will resume counting from this number. The state of the


TF0 bit, i.e. whether it is set, is checked from within the program. It happens at the

moment of overflow, i.e. after exactly 50.000 machine cycles or 0.05 seconds.

How to measure pulse duration?

Suppose it is necessary to measure the duration of an operation, for example how

long a device has been turned on? Look again at the figure illustrating the timer and

pay attention to the function of the GATE0 bit of the TMOD register. If it is cleared

then the state of the P3.2 pin doesn't affect timer operation. If GATE0 = 1 the timer

will operate until the pin P3.2 is cleared. Accordingly, if this pin is supplied with 5V

through some external switch at the moment the device is being turned on, the timer

will measure duration of its operation, which actually was the objective.

How to count up pulses?

Similarly to the previous example, the answer to this question again lies in the

TCON register. This time it's about the C/T0 bit. If the bit is cleared the timer counts

pulses generated by the internal oscillator, i.e. measures the time passed. If the bit

is set, the timer input is provided with pulses from the P3.4 pin (T0). Since these

pulses are not always of the same width, the timer cannot be used for time

measurement and is turned into a counter, therefore. The highest frequency that

could be measured by such a counter is 1/24 frequency of used quartz-crystal.

Timer 1

imer 1 is identical to timer 0, except for mode 3 which is a hold-count mode. It

means that they have the same function, their operation is controlled by the same

registers TMOD and TCON and both of them can operate in one out of 4 different

modes.


Timer 1 is identical to timer 0, except for mode 3 which is a hold-count mode. It

means that they have the same function, their operation is controlled by the same

registers TMOD and TCON and both of them can operate in one out of 4 different

modes

6. Explain how serial communication is performed in 8051 microcontroller. (AUC NOV

2011)

One of the microcontroller features making it so powerful is an integrated UART,

better known as a serial port. It is a full-duplex port, thus being able to transmit and

receive data simultaneously and at different baud rates. Without it, serial data send


and receive would be an enormously complicated part of the program in which the

pin state is constantly changed and checked at regular intervals. When using UART,

all the programmer has to do is to simply select serial port mode and baud rate.

When it's done, serial data transmit is nothing but writing to the SBUF register, while

data receive represents reading the same register. The microcontroller takes care of

not making any error during data transmission.

Serial port must be configured prior to being used. In other words, it is necessary to

determine how many bits is contained in one serial ―word‖, baud rate and

synchronization clock source. The whole process is in control of the bits of the SCON

register (Serial Control).

Serial Port Control (SCON) Register

SM0 - Serial port mode bit 0 is used for serial port mode selection

SM1 - Serial port mode bit 1

SM2 - Serial port mode 2 bit, also known as multiprocessor communication

enable bit. When set, it enables multiprocessor communication in mode 2 and

3, and eventually mode 1. It should be cleared in mode 0

.

REN - Reception Enable bit enables serial reception when set. When cleared,

serial reception is disabled

TB8 - Transmitter bit 8. Since all registers are 8-bit wide, this bit solves the

problem of transmiting the 9th bit in modes 2 and 3. It is set to transmit a logic

1 in the 9th bit.

RB8 - Receiver bit 8 or the 9th bit received in modes 2 and 3. Cleared by

hardware if 9th bit received is a logic 0. Set by hardware if 9th bit received is

a logic 1.

TI - Transmit Interrupt flag is automatically set at the moment the last bit of

one byte is sent. It's a signal to the processor that the line is available for a

new byte transmite. It must be cleared from within the software.

RI - Receive Interrupt flag is automatically set upon one byte receive. It


signals that byte is received and should be read quickly prior to being

replaced by a new data. This bit is also cleared from within the software.

SM0 SM1 MODE DESCRIPTION BAUD RATE

0 0 0 8 bit Shift

Register 1/12 of the quartz frequency

0 1 1 8 bit UART Determined by timer1

1 0 2 9 bit UART 1/12 of the quartz frequency

1 1 3 9 bit UART Determined by timer1

In mode 0, serial data are transmitted and received through the RXD pin, while the

TXD pin output clocks. The bout rate is fixed at 1/12 the oscillator frequency. On

transmit, the least significant bit (LSB bit) is sent/received first.

TRANSMIT - Data transmit is initiated by writing data to the SBUF register. In fact,

this process starts after any instruction being performed upon this register. When all

8 bits have been sent, the TI bit of the SCON register is automatically set.


RECEIVE - Data receive through the RXD pin starts upon the two following

conditions are met: bit REN=1 and RI=0 (both of them are stored in the SCON

register). When all 8 bits have been received, the RI bit of the SCON register is

automatically set indicating that one byte receive is complete.

Since there are no START and STOP bits or any other bit except data sent from the

SBUF register in the pulse sequence, this mode is mainly used when the distance

between devices is short, noise is minimized and operating speed is of importance. A

typical example is I/O port expansion by adding a cheap IC (shift registers 74HC595,

74HC597 and similar).


In mode 1, 10 bits are transmitted through the TXD pin or received through the RXD

pin in the following manner: a START bit (always 0), 8 data bits (LSB first) and a

STOP bit (always 1). The START bit is only used to initiate data receive, while the

STOP bit is automatically written to the RB8 bit of the SCON register.

TRANSMIT - Data transmit is initiated by writing data to the SBUF register. End of

data transmission is indicated by setting the TI bit of the SCON register.

RECEIVE - The START bit (logic zero (0)) on the RXD pin initiates data receive. The

following two conditions must be met: bit REN=1 and bit RI=0. Both of them are

stored in the SCON register. The RI bit is automatically set upon data reception is

complete.


The Baud rate in this mode is determined by the timer 1 overflow.

In mode 2, 11 bits are transmitted through the TXD pin or received through the RXD

pin: a START bit (always 0), 8 data bits (LSB first), a programmable 9th data bit and

a STOP bit (always 1). On transmit, the 9th data bit is actually the TB8 bit of the

SCON register. This bit usually has a function of parity bit. On receive, the 9th data

bit goes into the RB8 bit of the same register (SCON).The baud rate is either 1/32 or

1/64 the oscillator frequency.

TRANSMIT - Data transmit is initiated by writing data to the SBUF register. End of

data transmission is indicated by setting the TI bit of the SCON register.


RECEIVE - The START bit (logic zero (0)) on the RXD pin initiates data receive. The

following two conditions must be met: bit REN=1 and bit RI=0. Both of them are

stored in the SCON register. The RI bit is automatically set upon data reception is

complete.

Mode 3

Mode 3 is the same as Mode 2 in all respects except the baud rate. The baud rate in

Mode 3 is variable.

The parity bit is the P bit of the PSW register. The simplest way to check correctness

of the received byte is to add a parity bit to it. Simply, before initiating data transmit,

the byte to transmit is stored in the accumulator and the P bit goes into the TB8 bit in

order to be ―a part of the message‖. The procedure is opposite on receive, received

byte is stored in the accumulator and the P bit is compared with the RB8 bit. If they

are the same- everything is OK!

Baud Rate

Baud Rate is a number of sent/received bits per second. In case the UART is used,

baud rate depends on: selected mode, oscillator frequency and in some cases on the

state of the SMOD bit of the SCON register. All the necessary formulas are specified

in the table:


7. Discuss about the organization of internal RAM & Special Function Registers of 8051

microcontroller. (AUC MAY 2011)

Same as Q1

8. Explain about the interrupt structure with the associated registers in 8051

microprocessor. (AUC MAY 2011)

Interrupts

An interrupt is a special feature which allows the 8051 to provide the illusion of

"multitasking" although in reality the 8051 is only doing one thing at a time. The word

"interrupt" can often be substituted with the word "event."An interrupt is triggered

whenever a corresponding event occurs. When the event occurs, the 8051

temporarily puts "on hold" the normal execution of the program and executes a

special section of code referred to as an interrupt handler. The interrupt handler

performs whatever special functions are required to handle the event and then

returns control to the 8051 at which point program execution continues as if it had

never been interrupted.

There are five interrupt sources for the 8051, which means that they can recognize 5

different events that can interrupt regular program execution. Each interrupt can be

enabled or disabled by setting bits of the IE register. Likewise, the whole interrupt

system can be disabled by clearing the EA bit of the same register. Refer to figure

below.

Now, it is necessary to explain a few details referring to external interrupts- INT0 and

INT1. If the IT0 and IT1 bits of the TCON register are set, an interrupt will be

generated on high to low transition, i.e. on the falling pulse edge (only in that

moment). If these bits are cleared, an interrupt will be continuously executed as far

as the pins are held low.


Handling Interrupt

When an interrupt request arrives the following occurs:

1. Instruction in progress is ended.

2. The address of the next instruction to execute is pushed on the stack.

3. Depending on which interrupt is requested, one of 5 vectors (addresses) is written to

the program counter in accordance to the table below:

4.

Interrupt Source Vector (Address)

IE0 3h

TF0 Bh

TF1 1Bh

RI, TI 23h

5. These addresses store appropriate subroutines processing interrupts. Instead of

them, there are usually jump instructions specifying locations on which these

subroutines reside.

6. When an interrupt routine is executed, the address of the next instruction to execute

is poped from the stack to the program counter and interrupted program resumes

operation from where it left off.

From the moment the interrupt is enabled, the microcontroller is on alert all the

time. When an interrupt request arrives, the program execution is stopped, the

normal program execution is stopped & the control jumps to the appropriate

address as shown in the table above. This address usually stores a jump instruction

specifying the start of appropriate subroutine. After its execution the program

resumes operation from where it left off.


Reset

Reset occurs when the RS pin is supplied with a positive pulse in duration of at

least 2 machine cycles (24 clock cycles of crystal oscillator). After that, the

microcontroller generates an internal reset signal which clears all SFRs, except

SBUF registers, Stack Pointer and ports (the state of the first two ports is not

defined, while FF value is written to the ports configuring all their pins as inputs).

Depending on surrounding and purpose of device, the RS pin is usually connected

to a power-on reset push button or circuit or to both of them. Figure below illustrates

one of the simplest circuit providing safe power-on reset.

Basically, everything is very simple: after turning the power on, electrical capacitor

is being charged for several milliseconds through a resistor connected to the

ground. The pin is driven high during this process. When the capacitor is charged,

power supply voltage is already stable and the pin remains connected to the

ground, thus providing normal operation of the microcontroller. Pressing the reset

button causes the capacitor to be temporarily discharged and the microcontroller is

reset. When released, the whole process is repeated… Through the program- step

by step...

Microcontrollers normally operate at very high speed. The use of 12 Mhz quartz

crystal enables 1.000.000 instructions to be executed per second. Basically, there

is no need for higher operating rate. In case it is needed, it is easy to built in a

crystal for high frequency. The problem arises when it is necessary to slow down

the operation of the microcontroller. For example during testing in real environment

when it is necessary to execute several instructions step by step in order to check


I/O pins' logic state. Interrupt system of the 8051 microcontroller practically stops

operation of the microcontroller and enables instructions to be executed one after

another by pressing the button. Two interrupt features enable that:

Interrupt request is ignored if an interrupt of the same priority level is in

progress.

Upon interrupt routine execution, a new interrupt is not executed until at

least one instruction from the main program is executed.

In order to use this in practice, the following steps should be done:

1. External interrupt sensitive to the signal level should be enabled (for

example INT0).

2. Three following instructions should be inserted into the program (at the

03hex. address):

What is going on? As soon as the P3.2 pin is cleared (for example, by pressing the

button), the microcontroller will stop program execution and jump to the 03hex

address will be executed. This address stores a short interrupt routine consisting of

3 instructions.

The first instruction is executed until the push button is realized (logic one (1) on

the P3.2 pin). The second instruction is executed until the push button is pressed

again. Immediately after that, the RETI instruction is executed and the processor

resumes operation of the main program. Upon execution of any program

instruction, the interrupt INT0 is generated and the whole procedure is repeated

(push button is still pressed). In other words, one button press - one instruction.


9. Explain in detail the modes of operation of Timer unit in 8051 microcontroller. (AUC

MAY 2011) Same as Q6

unit 4- 8051 microcontrollermahalakshmiengineeringcollege.com/pdf/eee/visem/ee2354...to note that...

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