Measure Frequency

[Travin77]

Hello all,

I have been searching through the forum looking for a way to measure frequency. I found the Pulsin command but it may not be fast enough. I am using a 16f876a @ 20mhz. I will need to measure 2 seperate frequency streams ranging from 9490 hz to 8225 hz. It seems pulsin at 20 mhz can only see 1000 hz? Can someone suggest another solution or will pulsin work for me. Thanks

Travin

[dhouston]

Look at the COUNT command.

[mister_e]

Or use a internal counter... wich i tend to use now as it can work in background allowing to do something else in meantime.

[mister_e]

quick example of it

    '
    '    Pic Configuration
    '    =================
    DEFINE LOADER_USED 1
    DEFINE OSC         20
       
    '
    '    Hardware configuration
    '    ======================
    TRISA  = 255               ' PORTA : all input
    CMCON  = 7                 ' disable analog comparator
    ADCON1 = 7                 ' disable ADCs
    OPTION_REG = %11100001     ' TMR0 clock source = T0CKI (RA.4)
                               ' Source edge : low to high
                               ' Prescaller assign to TMR0
                               ' rate 1:4

    '
    '    Serial Communication definition
    '    ===============================
    DEFINE HSER_RCSTA 90h
    DEFINE HSER_TXSTA 24h
    DEFINE HSER_SPBRG 129      ' 9600 Bauds

    '   
    '    Variables definition 
    '    ===================
    Frequency var word
    TMR0IF    var INTCON.2

    '
    '    ------------------------[Program Start Here]--------------------------    
    '    
Start:
    TMR0=0                                                 ' clear TMR0
    IF PORTA.4=1 THEN START                                ' wait 'till T0CKI goes to low
    PAUSE 100                                              ' sampling time
    Frequency=TMR0<<2                                      ' multiply result to compensate the 1:4 rate
    if tmr0if then                                         ' Overflow? frequency over 256*4=1.024 Khz
       hserout ["Overflow Happened",13,10]                 '
       tmr0if=0                                            ' clear overflow flag
       else
           hserout ["Frequency:",dec Frequency dig 2,".",_ ' display results
                    dec Frequency dig 1,_
                    dec Frequency dig 0," KHZ",13,10]
       endif

    goto start

[Travin77]

Thanks for the help. I am going to try them both to see which is better for me. I appreciate it a lot.

Travin

[Brock]

How accurate does it have to be? If it can be off by a few Hz, then you could
try my method. I don't have the code on this computer, so I'll try to explain it.

Use an interupt input such as INT0 and INT1. Create a subroutine to measure
the frequency of each pin seperately and disable interupts, set the appropriate
edge type and configure a timer such as Timer 1.

First clear the interupt flag. Then, in a tight loop, poll the interupt flag until you
see it, then clear it again and start the timer. Then, in another tight loop, poll
the interupt flag until you see it. Then read the timer register and turn the
timer off. You should also add a count function in the loops that will time out
after a while and let the program continue if there is no pulse on the input. I'll
post an example when I get a chance.

Keep commands in the measuring loop to a minimum to reduce the latency. This
still may not be fast enough for the frequencies that you are talking about,
but it seems to work pretty good at lower frequencies.