ISR and Sleep

valemike · Guest

If i have the following code in my isr:

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

From section 12.13 of the 16F877 data sheet:
Power-down Mode (SLEEP):
"When the SLEEP instruction is being executed, the next instruction
(PC + 1) is pre-fetched." (This is the key point).
"If the GIE bit is set (enabled), the device executes the instruction
after the SLEEP instruction and then branches to the interrupt
address (0004h)." (When you pressed your switch, the GIE bit got set)
"In cases where the execution of the instruction following SLEEP is not
desirable, the user should have a NOP after the SLEEP instruction".

So, looking at the .LST file for your code, you see that the assembly
language instruction that updates the Z flag based on the value of 'flag'
is being pre-fetched and executed before 'flag' is actually updated in
your isr. The solution is to put a NOP after the Sleep() function.
You can use inline assembly for this, or define a macro and make the
nop look like a function call:

#define nop() #asm nop #endasm

sleep();
nop();

.................... while (1)
.................... {
.................... sleep();
*
0012: SLEEP
.................... if (flag)
.................... {
0013: MOVF flag,F // This instruction is pre-fetched !
0014: BTFSC STATUS.2
0015: GOTO 018
.................... flag = 0;
0016: CLRF flag
.................... }
.................... else
.................... {

MikeValencia · Joined: 04 Aug 2004 Posts: 238 Location: Chicago

Thanks PCM. You saved me a lot of time scratching my head.

What would happen would be, at the first wakeup from sleep, my else{} stuff would run, then at the next interrupt, the if{} stuff would run. It would happen every other interrupt. That's exactly the effect of what you described.

It should work fine now.
By the way, I don't think i need to use inline assembly to get a NOP. delay_cycles() should do the trick, i think.