measure the period of a signal

[zio_pecos]

hi,

I am working with a PIC16c745 and a oscilator of 24 MHz.

I need to measure the period of a square wave every 0.5 seconds

i captured it from rc2 pic


i know that i have to use ccp1 interrupt

but my code doesn't work...


can someone help me writing the correct code that i must use?

moreover explain me what i have to do...

i tried to use this code
http://www.ccsinfo.com/forum/viewtopic.php?t=20829

but i don't succed to make it work


thank you in advance

[Mark]

Post your code.

[Guest]

Depending on the speed and resolution you need your code could be really simple:

Set up a continuously running counter
Wait for 1/2 sec to elapse
Wait for starting edge of signal
Read the counter
Wait for ending edge of signal
Read counter again
Do math
Report result
Wait for next 1/2 sec.

This is just straight line code with no interrupts or subtleties. If you need to get more complex than this tell us more about your application so we can help you.

[Juan Pablo]

I finally used the next code in a PIC16F877 and it seems to work:

...
setup_ccp1(CCP_CAPTURE_RE);
setup_timer_1(T1_INTERNAL|T1_DIV_BY_4);
...
#int_ccp1 void velocidad() {
if(!second){
rise1 = CCP_1;
segundo=1;
}
else{
rise2 = CCP_1;
pulse_width = rise2 - rise1;
second=0;
show=1;
}
}
...

Using the RTCC I enable the capture every 500 ms and check the flag "show" to disable the capture in main when the period is obtained.
Anyway I began trying example code "ex_ccpmp.c", which uses CCP1 and CCP2.

Best Wishes.

[zio_pecos]

here is my code:

i have 3 questions:
1)
i have to transmit the value of freq or the period through usb
2)
how can i improve the resolution of the measurement
3)
what changes if i set
setup_timer_1(T1_INTERNAL|T1_DIV_BY_4) instead of
setup_timer_1(T1_INTERNAL|T1_DIV_BY_8);???

thank you

//--------------------------------------------------------------------------
// CCP1 Data
//
int16 CCP1Value;
int16 CCP1OldValue;
BOOLEAN CCP1Captured;

//--------------------------------------------------------------------------
#int_CCP1
CCP1_isr()
{
CCP1Value = CCP_1 - CCP1OldValue;
CCP1OldValue = CCP_1;
CCP1Captured = TRUE;
}
//------------


setup_counters(RTCC_INTERNAL,RTCC_DIV_256);
setup_ccp1(CCP_CAPTURE_RE);
setup_timer_1(T1_INTERNAL|T1_DIV_BY_8);

enable_interrupts(INT_CCP1);
enable_interrupts(global);

CCP1Value = 0;
CCP1OldValue = 0;
CCP1Captured = TRUE;


if (CCP1Captured) {

//---
// F = 1/T
// Timer1 prescaler DIV_BY_8
// Pic16C745 at 24Mz -> 1.66666e-7* 8=1.33333e-6
//

periodo=((float)CCP1Value*1.33333e-6)*550000;

//per=(long)periodo;
per=periodo;



CCP1Captured = FALSE;
}

[zio_pecos]

and if i set

setup_counters(RTCC_INTERNAL,RTCC_DIV_128);??

or

setup_counters(RTCC_INTERNAL,RTCC_DIV_64);?


etc???


what do i change in my project doing this??

thank you

[zio_pecos]

i can change the frequency of the signal connected to the ccp1 pin and i use this to control the resolution of my measurement because i also use an oscilloscope...

i only want to improve the resolution of the frequency measurement:

if the signal changes from 11k Hz to 10800Hz (this is controlled by me) i don't reach to read this changement. if the frequency changes over 250Hz i read a correct measure


why?


how can i do?

thanks

[zio_pecos]

anybody can help me???

[Mark]

With a 24MHz clock and a Timer1 divide by 8 the CCP count increments every 1.33333us

1/((24000000/4)/8) = 1.3333333333e-6.

So, say the CCP value is 68. 68*1.33us = 90.66us or 11.028KHz
So, say the CCP value is 69. 69*1.33us = 91.99us or 10.869KHz
So, say the CCP value is 70. 70*1.33us = 93.33us or 10.714KHz

Clear as mud?

[zio_pecos]

as i've understood this settings allow me to have a good resolution....


so why i can read a frequency changement only if is bigger than 250Hz??

maybe due to the break in 2 pieces i do to transmit through usb?

[Mark]

Did you not understand what I posted

The resolution is not high enough for timer1's increment rate. Do the math and you will see.

[zio_pecos]

[Mark]

Well you have Timer1 as a divide by 8. That will lower the resolution 8 times. Make it a divide by 1 and you will get 8 times the resolution.