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How to set the pulse width of less than 500us of the 12f617

 
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longmenok



Joined: 23 Oct 2006
Posts: 12

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How to set the pulse width of less than 500us of the 12f617
PostPosted: Mon Jan 22, 2018 6:32 am     Reply with quote

There is a problem in my program.
If external oscillator 32768 is used, I can't get a pulse less than 500us.
I set pulse width to be 250us, but the output pulse width is 500us. No matter how I modify the pulse width which is less than 500us,output is always 500us.

If the internal 4M crystal oscillator is used, the output pulse is normal. That is to say, the set is 250us then the output is also 250us.

Why is this? I need a 250us pulse, but I can't know how to setup this code. How to solve this problem, I want to output a 250us pulse.


Thank you!
The version is 5.015

Code:

#include <12F617.H>

#fuses LP, NOWDT,NOPROTECT,BROWNOUT, NOPUT,NOMCLR// INTRC_IO
#use delay(clock = 32768, xtal) //minum delay 500us

//#fuses INTRC_IO, NOWDT?NOMCLR, NOPUT, NOPROTECT
//#use delay(internal=4000000)

while(TRUE)
   {
output_high(PIN_A1);
        delay_us(250);  //pulse width =250us     
   
     output_low(PIN_A1);  //600ppm delay=100ms, 400ppm delay=150ms
 
// 240ppm
       delay_ms(250); //240ppm delay=250ms


    }
Ttelmah



Joined: 11 Mar 2010
Posts: 19195

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PostPosted: Mon Jan 22, 2018 7:00 am     Reply with quote

The fastest you can get 244uSec, by selecting fast_io, and no delay.

At 32768Hz, each instruction of the chip itself takes 122uSec.
A pulse implies turning on, then off, so two instructions. 244uSec.

You were seeing 4 instructions, since using standard_io (the default), has the processor controlling the TRIS for every I/O operation. So 488uSec minimum.


Code:

#include <12F617.H>

#fuses LP, NOWDT,NOPROTECT,BROWNOUT, NOPUT,NOMCLR// INTRC_IO
#use delay(clock = 32768, xtal) //minum delay 500us

#use fast_io(a)

//Now inside the main, _you_ will need to control the TRIS
   output_drive(PIN_A1); //set A1 as an output

   while(TRUE)
   {
        output_low(PIN_A1); 
        output_high(PIN_A1);
   }
longmenok



Joined: 23 Oct 2006
Posts: 12

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PostPosted: Tue Jan 23, 2018 11:45 pm     Reply with quote

Thank you very much! Ttelmah.
It can work normally.
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