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Marc
Joined: 11 Feb 2008
Posts: 5
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| Accuracy problem using set_timer0 function |
 Posted: Mon Feb 11, 2008 10:55 am |
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Hello, I'm using the code below to get a 2,5 seconds account, but instead of 2,5s I get 3,40. I need nClocks as int16 to have enough accuracy in my program.
Where is the problem? The code seems good and easy.
| Code: |
#include <16LF84A.h> //pic a utilizar
#fuses XT
#fuses NOWDT
#fuses PROTECT
#fuses PUT
#use delay (clock=4000000)
#use fast_io(a)
#use fast_io(b)
#define LED PIN_A1 //LED de funcionamiento
int16 nClocks=0;
#INT_TIMER0
void Interrupcion_Timer0()
{
set_timer0(156);
nClocks++;
if(nClocks==25000)
{
nClocks=0;
output_toggle(LED);
}
}
void main(void)
{
disable_interrupts(GLOBAL); //deshabilitar interrupciones
disable_interrupts(INT_EEPROM); //deshabilitar interrupcion EEPROM
disable_interrupts(INT_RB); //deshabilitar interrupcion portb
disable_interrupts(INT_EXT); //deshabilitar interrupcion externa
ext_int_edge(H_TO_L); //interrupcion en flanco de bajada
port_b_pullups(TRUE); //pullups portb activadas
setup_TIMER_0(RTCC_INTERNAL | RTCC_DIV_1);
set_tris_a(0b00000000); //porta todo como salida
set_tris_b(0b00000000); //portb todo como salida
enable_interrupts(INT_TIMER0); //interrupcion del contador
enable_interrupts(GLOBAL); //interrupciones activadas
do
{
}while(TRUE);
}
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Marc
Joined: 11 Feb 2008
Posts: 5
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| Posted: Mon Feb 11, 2008 10:58 am |
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I forgot to say that if I delete the set_timer0(156); line I get the correct time:
T=256*25000*(4/4ยท10^6)=6,4 seconds |
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Ttelmah
Guest
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| Posted: Mon Feb 11, 2008 11:08 am |
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How about searching the forum?....
Look for posts about the problems with setting a timer _to_ a value.
The problem is that timer0, is counting in instructions. By the time your code reaches the line to set the timer, typically something like _30_ instructions will have been executed since the last timeout. So instead of the 100 instructions to the 'next' interrupt, that you expect, the total, is more like 130.
There are a lot better approaches. What you are doing, results in a code, that will spend most of it's time in the interrupt. Have a look for code about RTC's, and in particular code posted (I think), by Neutone, which shows how to get accurate timings, without adjusting the timer value.
Best Wishes |
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Marc
Joined: 11 Feb 2008
Posts: 5
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| Posted: Mon Feb 11, 2008 11:26 am |
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| Thank you!! I'll look for for the Neutone's codes. |
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