external interrupt (#int_ext)

[cambo]

I wrote a interrupt program as below. When input pulse is lower than 100KHz, output pulse is fine(as i expected) but when input pulse is over than 100KHz, output pulse become slow... i dont know why. Please help me if u know it. Thanks

#include <16f873a.h>
#fuses HS,NOWDT,NOPROTECT,BROWNOUT,PUT,NOLVP

#define SECMAX 4


int sec;

#INT_EXT

void ext_isr(void)
{
sec--;
if(sec==0)
{
sec = SECMAX;
output_low(PIN_B1);
}
else{
output_high(PIN_B1);
}

}

void main (void)
{
EXT_INT_EDGE(L_TO_H);
sec = SECMAX;
enable_interrupts(INT_EXT);
enable_interrupts(GLOBAL);
while(1)
{
}
}

[Ken Johnson]

Not sure about the output pulse become slow - do you mean it just doesn't get faster as expected? Anyway . . .

You're probably reaching the pic speed limit. The standard isr pre/post processing added by ccs saves/restores a lot of stuff. Use the FAST keyword on your isr (doesn't save/restore anything). Look at the generated asm code, and add code (if needed) to save/restore only the regs used in the isr.

Ken

[cambo]

Yes, as input pulse frequency is faster(over100KHz), output pulse should also become faster too. But it is not.....

I dont make it yet, thanks for ur help anyway... i'm trying

[cambo]

Can u give me some examples?
Thanks

[gjs_rsdi]

if the controller works at 20MHz with 200ns=PC, you have 50 pc between interrupts, 100khz will be more or less the limit in C, with FAST maybe more. In asembler, your program will be able to handle arround 400khz.
joseph