I am trying to calculate the frequency of an incoming signal on the dspic30f4011 micro. I have setup a functioning pwm signal to test the incoming signal on. I believe my code is picking up the difference between the time captures of the signal, but I'm not sure how to calculate the frequency from there. I am running off the 120MHz primary FRC clock X16 multiplier which is futhur divided by 4 for the real clock. Any help is greatly appreciated.

I have found that PR2 of 58590 gives a period of timer 2 of 500ms which I verified with switching a simple led on and off from the timer2 interrupt.

(Please be aware that this code was copied and pasted below out of mutliple c files)

For the output on my lcd scree (diff) I am getting 5273.
The Pwm frequency was checked with pickit2 logic tool at 331Hz.

TRISD = 1; //Select pin IC1 (RD0) as input
IPC0 = IPC0 | 0x0010; //Interrupt priority level IC1IP<2:0> = 1
IEC0 = IEC0 | 0x0002; //Interrupt Input Compare module enable
PR2 = 58590; //ffffPR2 register at maximum, timer2 free-running
T2CON = 0x8030; //8030Timer 2 operates with prescaler 1:256 and internal clock
IC1CON = 0x0083; //2 orig Configuration of Input Capture module 1, selected TMR2,


unsigned int new,last,freq,diff = 0;

void _ISR _IC1Interrupt(){ //read freq


last=new;
new = IC1BUF;
if(new>last) {
diff = new - last;
}
else{
diff = (58590 - last) + new;
}

lcd_line1(0);
lcd_int(diff,5);



IFS0bits.IC1IF = 0; //Clear bit IC1IF (IFS<1>)
}


thanks for any help!

 

So I finally found out how to measure this frequency at 331 hz. When I start cranking up the frequency the accuracy starts diminishing fairly high at say 5khz. I am going to need to read frequencies at roughly 5khz-10khz with as good as accuracy as I can with what I got. What is the best way to improve accuracy? Also, if anyone is interested i can post the source code that works.

 

You need a frequency counter.

Here is some information from Microchip about using their PICs as frequency counters.
The ASM code is included.
http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=1824&appnote=en011033

 

Thanks! I'll definitely take a look. I'm using C but maybe I can pull out some info.