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volcane
Joined: 23 Feb 2008
Posts: 29
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| measure frequency prescaler |
 Posted: Wed Mar 25, 2009 3:29 pm |
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Hi!
I need a pin-out (example b5) on a 16F877 the exact frequency of timer1 prescaler? I would like to measure with frequency meter the frequency of the prescaler.
I recommended that code? |
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PCM programmer
Joined: 06 Sep 2003
Posts: 21708
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| Posted: Wed Mar 25, 2009 3:47 pm |
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There is no way to bring the Timer1 Prescaler clock to an i/o pin.
There is no option in the 16F877 hardware to do this. |
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volcane
Joined: 23 Feb 2008
Posts: 29
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| Posted: Wed Mar 25, 2009 3:55 pm |
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| PCM programmer wrote: | There is no way to bring the Timer1 Prescaler clock to an i/o pin.
There is no option in the 16F877 hardware to do this. |
you can do with software? I would like to know the error of the oscillator |
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PCM programmer
Joined: 06 Sep 2003
Posts: 21708
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| Posted: Wed Mar 25, 2009 4:16 pm |
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You have full control over the application program in the PIC.
You can create a loop that toggles a pin. Example:
| Code: | #include <16F877.H>
#fuses XT, NOWDT, NOPROTECT, BROWNOUT, PUT, NOLVP
#use delay(clock=4000000)
//======================================
void main()
{
output_low(PIN_B5);
#use fast_io(b)
while(1)
{
output_high(PIN_B5);
output_low(PIN_B5);
}
} |
Then look at the .LST file to see what ASM code is generated by the
compiler:
| Code: | .................... while(1)
.................... {
.................... output_high(PIN_B5);
0010: BSF PORTB.5
.................... output_low(PIN_B5);
0011: BCF PORTB.5
.................... }
0012: GOTO 010 |
That's a total of 4 instruction clocks. The BSF and BCF each take 1 clock.
The GOTO takes 2 clocks. So the signal on pin B5 will be the instruction
clock divided by 4. The instruction clock is 1/4 of the crystal frequency,
so it's 1 MHz. Then the loop above divides it by 4, to give 250 KHz on
pin B5. You can measure that with an oscillocope or frequency meter. |
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volcane
Joined: 23 Feb 2008
Posts: 29
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| Posted: Thu Mar 26, 2009 2:35 pm |
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| Thanks works perfectly |
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