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Interrupt service routine in assembly

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dtyh

Sun Sep 14 2008, 09:50AM

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Hi, can any1 teach me how to setup an interrupt service routine in assembly code?

pdi33

Sun Sep 14 2008, 10:08AM

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hi dtyh,
the procedure is quite simple:
first determine the vector address of the interrupt service routine u want to process.
e.g. service routine for external interrupt1 is 0013h. now u should add a statement at this origin to jump to the service routine like this

org 13h
sjmp INT1_SR
.
.
.
.

INT1_SR:
.
.
;add ur service routine code here
.
.
reti

note u have to add the reti instruction at the end of the code to make the uC differentiate between the standard subroutines and the interrupt service routines.

* inspired to develop,developing to inspire *

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dtyh

Arun Kumar V

Mon Sep 15 2008, 09:53AM

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and don't forget to protect your registers and Accumulator using PUSH ACC and POP ACC.

when switching between register banks use : SETB PSW.3 and/or SETB PSW.4

use one bank for main program and other bank/s for interrupt service routines

Arun

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dtyh

dtyh

Tue Sep 16 2008, 05:34PM

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Thanks pdi33 and Arun. Do i need to protect register or Accumulator if i'm not using it in ISR? I'm still learning 8051 and dont know how to switch register bank.

Arun Kumar V

Tue Sep 16 2008, 07:03PM

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hello dtyh,

even if you are not using ACC or R registers in your ISR, its always a safe practice to protect them. may be you are not using them in your current program but you may be using them in some other future codes.

Switching between Register banks for Main and ISRs is advisable.

ISRs can make or break a embedded design, when then micro executes ISR and jumps back to main loop or the address where it was before interrupt occured, the programmer should ensure the status of important regs and Acc are undisturbed.

also keep in mind to move SP (stack pointer) to a safe location above Register banks and other RAM variables.

MOV SP,#50H should be your first instruction in the code ( 50H can be changed to some other value according to usage)

Arun

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nischay kumar

nischay kumar

Wed Sep 17 2008, 06:18AM

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yep, unprotected Acc. and registers create a havoc, i experienced this when i was working on my Digital clock project.

i forgot to move SP above R banks and my variables, this resulted in uncommon behavior.

further i used same registers of bank 1 for main and timer ISR, and after a lot of head scratching i realized my mistake.

Freddy

Wed Sep 17 2008, 11:59AM

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yeah, this is good. I use this too

and bits like TR0 & TR1 should be protected too to avoid havoc

pdi33 wrote ...

[ Edited Wed Sep 17 2008, 12:00PM ]

Build target 'Target 1''
linking...
Program Size: data=8.0 xdata=0 code=709
creating hex file from "Final Year Project"...
"Final Year Project" - 0 Error(s), 0 Warning(s).

dtyh

Thu Sep 18 2008, 11:02PM

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Hi, one question i want to ask...what address is safe for me to start my main code? As ISRs occupies some vector address.

pdi33

Fri Sep 19 2008, 12:02PM

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each vector address has three bytes free till u encounter the next vector. So the code can safely start three bytes after the last interrupt vector address. If u do not want to bother with searching for it, use
org 50h

* inspired to develop,developing to inspire *

funa

Wed Sep 24 2008, 08:19AM

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Hi , since you are still learning 8051 and you don't know how to switch register bank I would like to suggest to try with Pascal language.

Here you have an example with Turbo51.
http://turbo51.com/documentation/interrupts

Check compiled code in assembler file and you will see what you would have to do in assembly language.

Turbo51 - Free Pascal compiler for 8051
http://turbo51.com