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Reading Timer problem

 
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art



Joined: 21 May 2015
Posts: 181

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Reading Timer problem
PostPosted: Thu Jan 12, 2017 12:06 am     Reply with quote
Hi,

I'm trying to read time between pulse. I've connect a signal below to pin B6.

Code:

IGNORE       1        0        0      0       0         1         0         1         0        1          0       0       0          1        0
_______  ________    ____    ____    ____    ____    ________    ____    ________    ____    ________    ____    ____    ____    ________    ____    __________ HIGH 
      I  I      I    I  I    I  I    I  I    I  I    I      I    I  I    I      I    I  I    I      I    I  I    I  I    I  I    I      I    I  I    I
      I  I      I    I  I    I  I    I  I    I  I    I      I    I  I    I      I    I  I    I      I    I  I    I  I    I  I    I      I    I  I    I
      I__I      I____I  I____I  I____I  I____I  I____I      I____I  I____I      I____I  I____I      I____I  I____I  I____I  I____I      I____I  I____I          LOW
     350us 1750us    700us 



I get a result :
" 46 -85 -84 65 15 -83 46 -79 47 67 47 -79 2 14 -49 -82 "
The given result is not stable and I would like to know why it give -value ?

Here is my code

Code:

#include <18F4550.h>

//#fuses HSPLL,NOWDT,PROTECT,NOLVP,NODEBUG,USBDIV,PLL5,CPUDIV1,VREGEN

#fuses none
#use delay(clock=48000000)
#use rs232(baud=9600, xmit=PIN_C6, rcv=PIN_C7,ERRORS)
#include <string.h>
#include <input.c>
#include <stdio.h>
#include <stdlib.h>
#include <usb_cdc.h>
#include <usb_bootloader.h>

void main()
   {
      char c;
      int8 i ,x;
      int  r2;

      unsigned char d;
      unsigned char key;

                                           
                                           

   usb_init_cs();
   


   while (TRUE)
         {
         usb_task();


            if (usb_cdc_kbhit())
               {
                 c=usb_cdc_getc();
                 if (c=='\n') {putc('\r'); putc('\n');}
                 if (c=='\r') {putc('\r'); putc('\n');}

                                       

                 while(true)
                 
                     {
                  ART:

                  d = usb_cdc_getc();

 

                         if(d=='R')   // push R for RECEIVER
                             {
                                while (key!=32) // push SPACE BAR to stop
                                   {

                                 if(usb_cdc_kbhit())
                                   {key=usb_cdc_getc();}
                                   
                                   
                                   
                             
                                 int a1;

                                 setup_timer_0(RTCC_DIV_2|RTCC_INTERNAL); // presacaler=2, Use INTERNAL clock
                                                                         //then every tick approximately = 0.167 Usecond
                                 while(1)
                                   {
                                    printf(usb_cdc_putc,"\r");
                                    while (input(PIN_B6));// wait for B6 to go low
                                         r2=1;


                                    for(i=0; i<16; i++)
                                    {
                                    while (!input(PIN_B6)); // wait for B6 to go high
                                         set_timer0(0); // init timer0 to 0
                                    while (input(PIN_B6)); // wait for B6 to go low
                                         a1=get_timer0();
                                         
                                     printf(usb_cdc_putc,"%d ", a1 );
                                    }   
                                         
                                     delay_ms(200);   
                                         
                                   }     
                                         
                                         





                                   } key=0; // to initialize 'key' not as SPACE BAR
                             }

                     } // For USB format
               }       // For USB format
         }             // For USB format
}                      // For USB format





Ttelmah



Joined: 11 Mar 2010
Posts: 19481

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PostPosted: Thu Jan 12, 2017 1:51 am     Reply with quote
Several things, and one 'big one'....
First, the chip may be doing other things. USB interrupts will occur at intervals, and the chip has to go off and service these. If one occurs while it is waiting to 'see' an edge, the code to handle this will then be delayed while the interrupt is serviced....

There is also a 'quantisation error'. Several instruction times round the loop, and the edge could be occurring at any point in this loop (however see below...)

This is why the CCP is such a powerful tool. It can record the value in the associated timer, when an edge occurs, and save it.

The big one though is you are not waiting for the edges.....

You arrive round the loop. Signal is sitting high. The first test then jumps straight through, though you may well be half way through the pulse.....

Also remember the timer is a 16bit value. You are only looking at the low byte.
Your timer is clocking off 6MHz.
350uSec, will be about 2100 cycles.
You are also confusing things by displaying the number as 'signed'. %u is the correct format to display an unsigned value.

Code:

#define WAIT_RISING() while(input(PIN_B6)); while(!input(PIN_B6))
//wait for signal to have been low and go high.

//parts of code only
    int16 a1; //not int8


    //Then wait for a rising edge
    WAIT_RISING();
    set_timer0(0);
    while(input(PIN_B6))
       ;
    a1=get_timer(0);
    printf(usb_cdc_putc,"%lu ", a1 );
art



Joined: 21 May 2015
Posts: 181

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PostPosted: Thu Jan 12, 2017 10:48 pm     Reply with quote
Dear Ttelmah,

I've tried your code, but it seem still cannot get a stable result. It give a positive number but with letter "l" behind. Why?
Ttelmah



Joined: 11 Mar 2010
Posts: 19481

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PostPosted: Fri Jan 13, 2017 2:35 am     Reply with quote
I suspect it doesn't like not having a field width. I've always specified one, so never seen this. So "%4Lu".
Unless it interprets the 'l' it won't correctly pull the high byte. Also if you have case significance enabled, then the 'l' must be upper case.
You will still get variation (because of the other reasons mentioned), but hopefully a much smaller amount relative to the numbers involved.
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