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Real Time Clock and LCD Display

Task:

Using UP3 Education Kit board implement a stop-watch timer. Initialize Real Time Clock

(RTC) device and read seconds value afterwards. Minutes may either be read from RCT as

well, or computed based on seconds value. Output seconds and minutes on LCD display.

Also incorporate a Reset mechanism, so the stop-watch may be halted and started over.

Refer to UP3 manual for pin numbers.

I2C Bus

I2C is a two-line, bi-directional serial bus most suitable for short distance communication.

It was developed by Phillips in the 1980s, but still remains a widely used mean for data

exchange between many devices.

Devices on the I2C bus are organized as Masters and Slaves. To begin communication

Master device issues a start sequence. To end transmission Master device sends a stop

sequence. Slave devices just respond to these requests. Slave device is selected using a 7-bit

address, which is unique for every device connected to the same I2C bus.

Data exchange is made using two wires SCL (clock line) and SDA (serial data and

address line). Master device drives the SCL line to clock each new bit on the SDA line in

either direction. Slave device can force a wait by driving the SCL line Low. Before each

new clock cycle Master device must ensure the wait condition is not present.

Both SCL and SDA lines are open-drain pulled up with resistors. It means that any device

attached to the bus can drive the line Low, but it cannot drive it High. The device releases

the line instead to let it float. To implement a bi-directional transmission on SDA line an

open-drain buffer must be used. It can be instantiated using the following VHDL code:

with data_in select data_out

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Real Time Clock

UP3 Education Kit board has a Real Time Clock circuit with a built-in 32.768 kHz

oscillator (external crystal controlled). Eight bytes of internal RAM are used for holding

clock/calendar information, which is stored in binary coded decimal (BCD) format. Data is

transferred serially via a two-line, bi-directional I2C bus. After each Read or Write

operation the Address Register increments automatically. Voltage Sense and Switch

Circuitry detects power failures and turns to the battery supply when the main power source

is disconnected. Typical data retention time for the 50 mA/h 3 V lithium cell (included with

UP3 board) is approximately 5 years.

RTC operates as a slave device. Access to the internal RAM (Table 1) is granted if a start

sequence is followed by the correct slave address (D0h). Note that some bytes hold control

bits along with clock/calendar information. Bits DB7 and DB6 of the Hours byte contain

the century enable bit (CEB) and the century bit (CB). When CEB is set to High, the CB

will toggle at the turn of the century. Setting CEB to Low disables CB toggle feature. Bit

DB7 of the Seconds byte contains the stop bit (ST). When ST is set to High, the internal

oscillator is stopped (which reduces the current drain). Setting ST to Low restarts the

oscillator within one second.

Table 1: Real Time Clock internal RAM map

AddressData

DB7 DB6 DB5 DB4 DB3 DB2 DB1 DB0Range

0 ST 10 Seconds Seconds 00-59

1 --- * 10 Minutes Minutes 00-59

2 CEB CB 10 Hours Hours 00-23

3 --- --- --- --- --- Day 01-07

4 --- --- 10 Date Date 01-31

5 --- --- --- 10 M. Month 01-12

6 10 Years Years 00-99

7 OUT FT S Calibration Control

*) --- denotes dont care value

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The seventh byte is used for calibration control. The oscillation rate of any crystal changes

with temperature. Real Time Clock circuit, provided on UP3 board, utilizes periodic

counter correction method. The calibration circuit adds (splits) or subtracts (blanks) counts

from the normal frequency of 32.768 kHz. Bit DB5 of the control byte contains the sign bit

(S). Setting S to High indicates positive calibration, while resetting it to Low indicates

negative calibration. Correction is made within a 64 minute cycle. Once per minute one

second is either shortened by 128 or lengthened by 256 oscillator cycles. Five lower bits

(DB4-DB0) of the control byte determine the rate, at which modification is made. If a

binary one (00001) is placed into the Calibration field of the control byte, only the first

two minutes in the 64 minute cycle will be adjusted. Placing binary two (00010) will

affect the first four minutes, and so on.

In order to test the correctness of the calibration settings, FT/OUT pin output (header

JP13.1 on the UP3 board) can be measured. When the frequency test bit (FT) is set to High,

the FT/OUT pin output should normally toggle at 512 Hz. The degree and direction of

oscillator frequency shift can be calculated based on deviation from the default value.

When FT is set to Low, the FT/OUT pin reflects the content of DB7 bit (OUT) of the

control byte.

RTC circuit exchanges data and address information with the FPGA device using two-line

I2C serial bus. One bi-directional line is used to transfer data signals (SDA), the second line

is for clock signal (SCL). The following bus conditions are defined:

Bus not busy -both data and clock lines remain High

Start data transfer - data line changes from High to Low, while the clock is High.

Stop data transfer - data line changes from Low to High, while the clock is High

Data valid - after the start condition the data line presents valid data during the

High period of the clock signal, data may be changed during the Low period of the

clock signal

Acknowledge after the reception of each byte the receiver device pulls data line

Low for one clock cycle, master receiver signals end of transmission by not sending

an acknowledge bit and slave transmitter leaves data line High

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To write data to the internal RAM master transmitter signals start data transfer condition

followed by slave receiver address and mode control bit. Master transmitter specifies slave

device address (1101000 in case of RTC) and Low mode control bit, which indicates

write operation. After RTC acknowledges this byte, master transmitter continues with the

eight bits of word address, which are placed to the address pointer. When address word is

acknowledged, master transmitter may begin sending data bytes one by one. RTC

acknowledges every data byte and automatically increments address pointer. When there is

no more data to send, master transmitter signals stop data transfer condition.

Reading from internal RAM starts out exactly the same as write operation. Master receiver

issues a start data transfer condition followed by slave device address and Low mode

control bit, RTC acknowledges first byte. Master receiver continues with word address,

which is also acknowledged by slave transmitter. Next, the start data transfer condition and

slave address are repeated, but this time mode control bit is High, which indicates read

operation. RTC sends an acknowledge bit and begins transferring the data bytes one by one.

Master receiver acknowledges every data byte, while slave transmitter automatically

increments address pointer. After the last byte of data master receiver does not issue an

acknowledge bit and generates stop data transfer condition.

LCD Display

LCD display is a very handy tool for showing information, debugging, etc. And it is no

wonder that many of todays FPGA boards are equipped with them. As a rule, LCDs require

fewer connections than LED displays and use less power, but they are controlled in a much

more complex way. While putting a few volts on the right pin would light a LED segment,

most of the LCDs have to be programmed.

Typically, LCD is equipped with a controller, which internally does all the functions.

Display data RAM (DDRAM), character generator ROM and RAM (CGROM and

CGRAM), liquid crystal display driver are also provided. Usually LCD display format

ranges from 8 to 80 characters. LCD modules have 14 or 16 (provided on UP3 board) pin

connections.

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There are two models of character generator: A00 (used in LCD provided with UP3 board)

and A02. Both have the standard ASCII character set, but the rest differs. The A00 model

has Japanese characters, plus a number of graphic, Greek and math symbols. The A02

model has the European characters plus a number of graphic, Greek and Cyrillic characters.

U1 (as LCD is marked on UP3 board) is a 16x2 character Liquid Crystal Display. Here

16x2 stands for two 16 character long lines. LCD can be configured to show only one

character line, though. Figure 1 shows LCD connected to the UP3 board. Note that LCD

module is connected facing away from the board, placing it otherwise would be illegal.

Figure 1: LCD display

The LCDs interface is standard. Connection is made through 16 pins. Eight pins form data

bus (DB7-DB0). Enable pin (E), register select pin (RS), read/write select pin (R/W) are

used for control. Finally, there are pins for power (Vss), ground (Vdo), for voltage to

control the contrast (Vo) and two more pins for the backlight (LED+ and LED-). Note that

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LCD shares data lines (DB7-DB0) with SRAM, SDRAM and FLASH. But controller

ignores incoming data while enable input signal (E) is set Low.

LCD module incorporates a controller, which performs all the functions using commands

sent by the user designed Finite State Machine or processor. It has two externally controlled

8-bit registers instruction register (IR) and data register (DR). IR stores instructions and

address information, DR data to be written or read. Write and read operation is selected

via R/W input signal, register is selected using RS input signal. Table 2 lists all available IR

and DR operations.

Table 2: IR and DR operations

RS R/W Operation

0 0 Instruction is written to IR

0 1 Busy flag (DB7) and address counter (AC) value (DB6-DB0) are read

1 0 Write data from DR to DDRAM or CGRAM

1 1 Read data from DDRAM or CGRAM to DR

Note that IR register can only be written. When IR is selected and R/W input is High

(indicating read operation) busy flag and address counter value are read instead.

Busy flag is set High while internal operation is being performed, so the next instruction

will not be accepted. Thus, new instruction must be sent only after busy flag is cleared (set

to Low). Many prefer to use appropriate delays instead of polling, though.

Address counter (AC) holds addresses to both DDRAM and CGRAM. Memory selection is

determined by the instruction. Note that after writing to (reading from) DDRAM or

CGRAM, AC is automatically incremented or decremented by one (direction is selected

using appropriate instruction).

The abbreviations DDRAM and CGRAM refer to the two types of memory used by LCD

module controller. DDRAM stands for display data RAM. It stores eighty 8-bit character

codes (although only 16 or 32 characters can be displayed at once). With shift instruction

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the viewable part of DDRAM can be altered. Figure 2 and Figure 3 illustrate relationships

between DDRAM addresses and the visible area of the LCD module for different display

modes.

Home position

00 01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E 0F

After shift left

01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E 0F 10

After shift right

4F 00 01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E

Figure 2: 16x1 display visible DDRAM area (addresses are presented in HEX format)

Home position

00 01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E 0F

40 41 42 43 44 45 46 47 48 49 4A 4B 4C 4D 4E 4F

After shift left

01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E 0F 10

41 42 43 44 45 46 47 48 49 4A 4B 4C 4D 4E 4F 50

After shift right

27 00 01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E

67 40 41 42 43 44 45 46 47 48 49 4A 4B 4C 4D 4E

Figure 3: 16x2 display visible DDRAM area (addresses are presented in HEX format)

Note that when in 16x2 mode the first line is represented by DDRAM address range

starting from 00h down to 27h (forty characters in total), but the second line starts from 40h

and ends with address 67h (also forty characters in total).

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Values stored in DDRAM are actually addresses for the character generator, which stores

5x7 dot and 5x10 dot character patterns. It consists of CGRAM and CGROM. CGROM

holds predefined character patterns, while CGRAM can be programmed to store custom

ones (either eight 5x7 dot or four 5x10 dot characters for LCD module provided with UP3

board). Figure 4 illustrates A00 model character generator patterns.

Figure 4: A00 model character generator patterns

The leftmost column on Figure 4 represents CGRAM. CGRAM custom character pattern is

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programmed line by line. Note that only five least significant bits would be represented in

character pattern. It is recommended to write empty lines as well as on power-up CGRAM

may not be totally empty. Note that custom characters get duplicated. For example, codes

00h and 08h represent one and the same custom character pattern.

Two rightmost columns on Figure 4 show 5x10 dot character patterns. Note that LCD

module provided with UP3 board is not best suited to display them. 5x10 dot font can be

activated only in 16x1 display mode, but three lower rows get separated as they are

displayed on the second line. Regardless of the mode, upper seven rows of the 5x10 dot

pattern will always be displayed.

LCD module uses eight 8-bit wide instructions. These commands are differentiated by the

leftmost non-zero bit. Bits to the right of the instruction bit represent additional parameters.

Table 3 lists all available instructions.

Table 3. LCD module instructions

Instruction D7 D6 D5 D4 D3 D2 D1 D0

Clear display 0 0 0 0 0 0 0 1

Cursor at home 0 0 0 0 0 0 1 ---*

Entry mode set 0 0 0 0 0 1 I/D S

Display control 0 0 0 0 1 D C B

Cursor or display shift 0 0 0 1 S/C R/L --- ---

Function set 0 0 1 DL N F --- ---

CGRAM address set 0 1 ADDRESS

DDRAM address set 1 ADDRESS

*) --- denotes dont care value

Clear display instruction writes 20h (space character code) to all DDRAM addresses and

writes 00h to AC, cursor is returned to home position and entry mode direction is set to

increment. Cursor at home instruction basically does the same, but DDRAM content and

entry mode direction are left unchanged.

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Entry mode instruction sets the way characters are written and read. I/D parameter defines

whether AC value would increment (when set to High) or decrement (when set to Low)

after data is written to or read from either DDRAM or CGRAM. With S bit enabled (set to

High) writing data to DDRAM will shift visible memory area right or left, visually leaving

cursor at the same position. I/D bit defines shift direction (left when High, right when

Low). When S bit is disabled (set to Low) visible memory area position is not changed,

thus cursor would visually move to the next position.

Display control instruction is used to manage LCD visual appearance. D bit turns display

ON/OFF (DDRAM content remains unchanged), C bit makes cursor visible or hides it and

B bit defines whether the character at the cursor position will blink or not. Setting

parameter High enables corresponding feature, Low value disables it. Note that cursor or

blinking position is determined by the address stored in AC. If AC stores CGRAM address,

then cursor or blinking would occur at the wrong place.

Cursor or display shift instruction either moves cursor one position or shifts entire display

left or right. S/C bit selects cursor (Low value) or display shift (High value), R/L

determines direction (left when Low, right when High). Note that in two-line mode cursor

jumps to the second line after the 40th character of the first line.

Function set instruction defines some hardware aspects of display. DL bit selects either

8-bit (High value) or 4-bit (Low value) interface. When using 4-bit interface only four data

bus lines are active (DB7-DB4), the rest is disabled. Firstly, four high order bits of data are

transferred, then four remaining low order bits. N bit sets the number of display lines (one

line when Low, two lines when High). Note that LCD supplied with UP3 board supports

both modes. F bit defines character size (5x10 dots when High, 5x7 dots when Low).

CGRAM address set instruction loads AC with the value specified in the address field and

causes subsequent data to be stored in the character generator RAM. DDRAM address set

instruction does exactly the same, but this time the display data RAM is selected.

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On power up LCD module performs internal reset itself. However, if power supply

problems occur, internal reset circuit would not function correctly. It may be necessary to

provide initialization by instruction. The initialization sequence is as follows:

POWER ON

15 ms delay

Function Set

4.1 ms delay

Function Set

100 s delay

Function Set

5 ms delay

Function Set

Display OFF

Display ON

Entry Mode Set