18F452 Timer

[rhfarm]

I am trying to program a 40 Khz squarewave output on port B bit 2. I am using the timer interupt and timer1. The problem is that I can not get a frequence higher then around 31 KHz even when setting the 16 bit counter to start counting up from 0XFFE0. There seams to be around 16 us of overhead . I am using a 20 MHz clock and the timer should be incremented ever 200 ns. The code is listed below. Would be thankfull for any feed back.



#include <18f452.h>

#device *=16 adc=10 //icd=true
#use delay (clock=20000000)
#fuses HS,PUT,NOPROTECT,NOBROWNOUT,NOWDT,CCP2B3,NOSTVREN,NOLVP



int8 z=0;

// This interrupt is used to Drive LED at 40 KHz
// is automatically called ever 25us.
#INT_TIMER1
void wave_timer() {
int i;

set_timer1(0xFFF0);


if(z){
output_low(PIN_B2);
z=0;
}

else {
output_high(PIN_B2);
z=1;
}

}



main() {

setup_timer_1(T1_INTERNAL|T1_DIV_BY_1);
enable_interrupts(INT_TIMER1);
enable_interrupts(GLOBAL);

while(true){
}

}

[PCM programmer]