AVR Freaks

The problem I am having is that I am attempting to generate a delay on a waveform referenced from the leading edge.

It appears on the scope as if sometimes it will not see anything at all for some time, and then it will work correctly for a time. Below is the code I am using. The values for the delay function are being varied frequently and then recompiled, and it will work correctly with some values for the delay function but not with others.

If someone can determine why sometimes it fails to react to the trigger and generate an interrupt, (or at least appears to) I would appreciate it.

Thanks,

Daniel

// External Interrupt 0 service routine
interrupt [EXT_INT0] void ext_int0_isr(void)
{
// This rising edge triggered interrupt function
//outputs a waveform on PORTA by using delay function
// after generating delay it looks to be sure that the input is not presently high
// clears possible accumulated interrupt and then exits

//delay_ms(3); delay after trigger but before turning on portA

PORTA=0xff; //turn on porta
delay_us(600); //pulse high period in microseconds
//delay_ms(6); //pulse high period in milliseconds
PORTA=0x00; //turn off porta
delay_us(600); //delay until function can exit in microseconds
delay_ms(20); // delay until function can exit in milliseconds

//make sure that wave is not currently high before exiting

while(PIND.2==1 );

//clear possible accumulated interrupt
//if delay is as long as two input pulses, then interrupt needs to be cleared so that the stored one does
//not set another trigger.
#asm
push r16
push r17

in r16,0x3a //0x3a is the address of GIFR
ldi r17,64 /this doesn't quite seem right, but this is
or r16,r17
out 0x3a, r16

pop r17
pop r16
#endasm

}