Timer interrupt does not work properly

[nicogorr]

Hello.
I`m trying to send samples to a PC over RS232 at maximum speed. The fastest sample rate for 10 bit samples is about 5760HZ (115200bps constraint). The problem occurs in the Timer1 ISR that set the desired sampling frequency. The: setup_timer_1 ( T1_INTERNAL | T1_DIV_BY_8 ); means that 1 count in timer1 represents 1.6us. So, when the timer1 interrupts the code is executed in about 72.8 us(Calculated using timers and then some corrections via ASM list), then the timer is set to interrupt every 64*1,6uS=102.4us....Summing that time to the 72.8us already elapsed it gives 175,2uS, thts gives a Fs of approx. 5707hz...
Here the things get messy...at this speed in the PC I`m only geting about 4 to 6 Bytes per second..the same it happens when i set the timer at -100 -150 -200(that's it decrasing the sample rate)..only when I reach about -270 I`m able to get all the samples, and -270 means a sample rate of 1/(270*1,6uS + 72.8us)=1980Hz that`s not even close to 5700hz...
If it's possible I want to be able to reach at least 5000Hz samples.
Thanks in advance.
Nicolas.


#include <16f877a.h>
#device adc=10
#fuses HS,NOWDT,PUT,NOPROTECT,BROWNOUT,NOWRT,LVP
#use delay (clock=20000000)

#use rs232 (baud=115200, xmit=PIN_C6, rcv=PIN_C7, PARITY=N, BITS=8)
#org 0x0F00, 0x0FFF void loader16F877(void) {}
unsigned long muestra;
char adc_ALTO, adc_BAJO;

#int_TIMER1
Timer_test(){

disable_interrupts(INT_TIMER1); |
delay_us(20); |
muestra=read_adc(); |
adc_ALTO = (char)(((muestra >> 5)& 0x1f)|0xe0);|
adc_BAJO = (char)(muestra & 0x1f); } This lines takes about 72.8uS to
putc(adc_ALTO); | complete
putc(adc_BAJO); |
delay_us(20); |
enable_interrupts(INT_TIMER1); |
set_timer1(-64);// set_timer1(-64) + 72.8uS(Already elapsed)=> Fs=5707Hz or Ts=175uS

}

void main(){

setup_adc_ports(RA0_ANALOG);
setup_adc(ADC_CLOCK_INTERNAL);
set_adc_channel(0);
enable_interrupts(GLOBAL);
enable_interrupts(INT_TIMER1);
setup_timer_1 ( T1_INTERNAL | T1_DIV_BY_8 );
set_timer1(0);
while(1){
output_high(PIN_D3);
delay_ms(1000);
output_low(PIND_D3);
delay_ms(500);
}
}

[rwyoung]

Why are you delaying 20us inside your interrupt? That doesn't seem like a good idea. Also, the enable/disable timer interrupt calls are probably not necessary inside the ISR.

Consider using the ADC interrupt to let you know when a sample is completed, use the timer interrupt to schedule the start of the next acquisition and use your main loop to push new data out the RS232 port.

Also, as displayed, your code will not compile as there are extraneious "|" (pipe) symbols and at least one extra closing bracket "}" in the interrupt code. Use the Code button when posting code.
_________________
Rob Young
The Screw-Up Fairy may just visit you but he has crashed on my couch for the last month!

[PCM programmer]

I see one possible bug. I think your intention is to load Timer1
with -64 so it will overflow and cause an interrrupt in 64 counts.
Here is the problem: Timer1 is a 16-bit timer, but you are loading
it with an 8-bit value. You need to put an "L" after the -64 to tell
the compiler that this is a 16-bit constant. Example: -64L

Notice the difference in the .LST file between the two methods:

[nicogorr]

Thanks rwyoung,

The interrupt code is this:

[nicogorr]

PCM programmer, your totally right!!. Thank you very much.
I have been looking for errors in the code for about 2 days "You can't see the forest for the trees"..
Well that fix it, Thanks again.