mc3 timing and projects

8/7/2019 MC3 Timing and projects

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DrTony Nicol - University of Central Lancashire Department of Computing. 108

Microcontrollers

Instruction timing

If the Microcontroller Unit (MCU) is to output results for the user to

see for example, it usually needs to be slowed down to our speed. A

simple way to do this is to waste processor cycles. But how many?

It is useful to have a delay function in multiples of milliseconds for

example so an accurate 1ms function is useful. To do this you need

to work out how many instruction cycles to waste then write the

code to waste them. An instruction cycle takes 4 clock cycles so at8MHz, the instruction cycle time is 0.5uS or 500ns. Dont forget

that although most instructions take 1 cycle, this is not always the

case. The debugger stopwatch feature is useful for measuring run-

time. For example, how long does it take the Wait loop to execute in

the following simple program?

#include

cblock 20hDelay

endc

org 0

movlw .200

movwf Delay

Wait

nop

nop

decfsz Delay

goto Wait

Here

goto Here

end


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Microcontrollers

200 iterations

x 5 cycles

x 0.5us cycle time

+ 0.5us last decfsz is 2 cycles

2x0.5us as last goto is not executed

= 499.5us


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Microcontrollers

Common constructs using 16x assembly language

Here are some examples of typical loops and conditional statements

with C equivalents.

Do while (x != 3)

#include

;

cblock 20h

X

endc

org 00h ; reset vector

Start

movlw 5

movwf X

DoWhile

; ... loop body code here

decf X,F ; Arbitrary instruction to mod X

; ... end loop body code

movf X,W ; Get X to check for end of loop

sublw 3 ; If Z=1 must be 3 so skip out of loop

btfss STATUS,C ; Loop ended so skip out

goto DoWhile ; Loop not ended so go roud again

goto Start ; Try DoWhile loop again

end


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MicrocontrollersNote: be careful not to use reserved words like else. If the label

stays blue then it is probably a reserved word so dont use it.

if (X == 0) y=3;

This example illustrates an easy way to test for a variable being zero.

#include

; if (X == 0) y=3;

cblock 20hX

y

endc


Start

movlw 1 ; Change this value to test

movwf X

; ... body code ...

; if (x == 0)

movf X,F ; move X to itself as it affects Z flag

btfss STATUS,Z ; Yes its 0 so skip

goto EndIfPart ; not 0 so skip if part

movlw 3 ; Do if part

movwf y

EndIfPart

goto Start ; Try If again

end


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Microcontrollersif (x == 5) y=0; else y=10;

#include

; if (x == 5) y=0; else y=10;

cblock 20h

X

y

endc


Startmovlw 5 ; Change this value to test

movwf X

; ... body code ...

movf X,W ; get X in w, subtract 5 and test for zero

sublw 5

btfss STATUS,Z ; If zero, skip and do if part

goto ElsePart ; Not zero so do the else part

clrf y ; If part y = 0

goto EndIfPart

ElsePart

movlw .10 ; y = 10

movwf y

EndIfPart

goto Start ; Try If Else again

end


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Microcontrollers

Accessing data in different modules

To enable you to develop functions that are reusable without having

to copy and past code from one program to another, you should

develop functions as individual modules then link into your program

the ones you want to use. For example, you may have written a

good delay routine that is useful to you in many of your programs

and may be usable to other members of a project team. You would

develop the delay function in a separate file then link it into your

main program. The following instructions show you how to create aproject in MPLAB.

The module from which a function is called (e.g. the main program

module) needs to declare the called functions EXTERN. This

informs the assembler that it will come across calls to functions for

which there is no source code as the source code is EXTERNal to

this file. For example, in your main program you may call two

functions called DelayMiliSecs and InitPorts which are declared indifferent files, so use:

EXTERN DelayMiliSecs

EXTERN InitPorts

Conversely, in the files DelayMiliSecs and InitPorts you need to add

Global directives. This tells the linker that the function is globally

available to be called from other files. Use:

GLOBAL DelayMiliSecs in the source file of the Delay routine

GLOBAL InitPorts in the source file of the Initialisation code


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MicrocontrollersOnce your main program is written (and the other files for your

project are available), create a new project file using Project | New.Fill in the project name and browse to the appropriate directory. On

clicking OK you would expect something to happenbut it doesnt,

so it looks like your file has not been createdbut it has. Select

View | Project and the project window displayed.

Right click on Source files and select Add Files. Browse for the

files you want in your project and add them.

The project needs a linker script to let the linker know where to put

the machine code in the output file. Either right click Linker Scripts

and point to

C:\Program Files\MPLAB IDE\MCHIP_Tools\LKR\16f8877A.lkr

(or wherever your default linker file is for the 16F877) or go to

Project | Build Options | Project and set the path

C:\Program Files\MPLAB IDE\MCHIP_Tools\LKR\

to the linker script path and manually enter 16f877.lkr into the linkerscript dialog box or copy the linker file to your functions directory

(this is what I have done).

Build the application and test it.

The project and source files are illustrated in Figure 24.

Note that org is no longer used as the code needs to be relocatablei.e. we dont want to have to worry about which addresses the linker

places each code module. Use the linker script directive STARTUP

to specify the startup code section then use CODE after that to allow

the linker to put the remainder of the code where it likes.


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Microcontrollers

Figure 24 MPLAB Project

Note the use of EXTERN and GLOBAL. Note also that the cblock

is no longer used. This too is unmaintainable in a project as you

would ned to keep track of all the addresses you allocate your

variables to. Use the UDATA directive to reserve memory and let

the linker choose the addresses to allocate.

If you want to see what the whole program looks like once linked,

open the .lst file. If you want to see where the linker has put your

variables, open the .map file.

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