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Woody
Joined: 11 Sep 2003
Posts: 83
Location: Warmenhuizen - NL
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| Problem comprehending timer0 in a 16F88 |
 Posted: Mon Nov 06, 2006 4:06 pm |
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Hi,
Normally all my questions are answered searching this forum but this time I seem to not be able to find a solution. Probably I acquired tunnel vision from looking too long at this.
Here's my problem:
The CPU is a 16F88. It runs on INTRC, 8MHz. I set timer 0 as using the internal clock, with a DIV_4 prescaler. TMR0 is preset to 6, getting a rollover (and an interrupt) in 250 timercounts. In the timer interrupt routine TMR0 is preset to 6 again. Then a pin is toggled.
The pulse I expected to see on the toggled pin would be:
8 MHz / ( 4 (instruction clock) * 4 (prescaler) * 250 (rollover) ) = 2000 or 1 interrupt every 500uS giving me a pulse with a period of 1 ms.
The pulse i actually see on a scope has a period of 2 ms or every 1 ms an interrupt.
I'm pretty sure my reasoning is flawed, but where?
Here's the code:
| Code: |
#case
#include "16F88.h"
#fuses PROTECT, NOMCLR, INTRC_IO, BROWNOUT, NOWDT, NOCPD, PUT, NOLVP
#use delay(clock=8000000)
#byte TMR0 =0x01
#byte OSCCON = 0x8F
#byte OSCTUNE = 0x90
#define RA3 PIN_A3
#define INT_TEST RA3
// Program and subroutines:
void init(void) {
// Internal oscillator 8 MHz
OSCCON = 0b11100000;
// Center frequency
OSCTUNE = 0b00000000;
// Set timer0
setup_timer_0(RTCC_INTERNAL|RTCC_DIV_4);
TMR0+=6;
enable_interrupts(INT_RTCC); // Enable timer0 interrupt
enable_interrupts(GLOBAL); //
}
#INT_RTCC
void clock_isr(void) {
// Eerst de timer weer ijken op 250 counts voor interrupt
TMR0+=6;
output_toggle(INT_TEST);
}
void main() {
#zero_ram
init();
while(TRUE) {
} // end while true
} // end main
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Compiler version is 3.248.
I hope somebody can set me straight about this. Thanks for your time.
Paul |
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jecottrell
Joined: 16 Jan 2005
Posts: 559
Location: Tucson, AZ
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Woody
Joined: 11 Sep 2003
Posts: 83
Location: Warmenhuizen - NL
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| Posted: Tue Nov 07, 2006 2:42 pm |
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I read that. Before. And it did not help.
But fortunately I figured it out on my own. As stated, I looked at this problem for some time, which turns out not to be too productive. Letting the project be for two days and then setting OSCCON to the right value (0b01110000 instead of 0b11100000) solved my problem. Sometimes I just fail to see the bits in the byte.
Thanks for your time,
Paul |
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