Low-Frequency Counter

[Zeke]

Hopefully someone has some suggestions for me on this one.
I need to write a program to be able to count pulses that run from around 0.25Hz to 4Hz. The PULSEIN command would likely time-out long before it reads any pulses. I think I will need to use tmr1 or similar and advance a counter at each roll-over of the timer, and then convert this value to a time between my external pulses. Then, with some simple math, I can figure out the frequency.
My biggest problem is being able to setup the timer to run properly. I'm not sure how to do this. And, should I set-up my external pulse as an interrupt? I will likely be using a PIC16F627 as I have a couple of these handy.
Any help would be much appreciated.
Nick

[mister_e]

Something to start
On this forum
http://www.picbasic.co.uk/forum/showthread.php?t=1044

Melabs Website... choose counter.zip
http://www.melabs.com/resources/samples.htm#submitted

Both are the same, the second include the schematic

Sorry, based on a old and dirty F84 but should gives you some pointer.

[mister_e]

Beurk... no forget that..i just see your frequency range...

Maybe the use of PAUSE could be just enough.

0.25HZ => period =4Sec
4HZ => period = 0.25 Sec

Something like

While FreqInput=1 : Wend
While FreqInput=0 : wend
While FreqInput = 1
    Pause 10
    HalfPeriod=HalfPeriod+1
wend

Frequency = 10000/(HalfPeriod * 2)

In the above 4HZ will return => 400, 0.25HZ will return 25 as value....
then just to play with the DEC, DIG to display your frequency.

Should be enough to start.

[Zeke]

Steve,

Thanks for the info. I'm going to start testing this in the next few days. I'll make sure I post what finally works.

Nick

[Darrel Taylor]

Steve,

Playing Devil's advocate here.

What happens if the Duty Cycle of the pulses isn't 50% ?
<br>

[Zeke]

Darrel, in fact, the pulses will be coming from a sensor measuring Heart Rate, like an ECG signal, except that it will be run through a comparator so that I only pick-up the largest spike. Therefore, the signal will likely never have 50% duty cycle.
My plan is this...
1. Use Timer1 to count pulses (5,000 at 20MHz gives 1ms) starting at a pre-defined value, I think it works out to 60,535. In a tight loop I will check for the timer overflow bit, everytime this happens, I increment a variable as 1 ms and re-start the timer. This way I am counting milliseconds.
2. I will use RB0 as my interrupt signal (which is my pulse signal) and perform an interrupt routine that will calculate the heart rate based on the time (milliseconds from my variable above) between pulses. This routine will then reset my millisecond variable to check for the next time between pulses.
3. Another variable will check for 3 or 4 seconds to pass and then send the data via the built-in USART to a PC.

This is roughly how it should work. There are other issues that I need to look at, but that's easy. The hard part is keeping it accurate since at a heart rate of about 240 beats per minute (BPM), that's about 4 Hz, or 250ms between pulses. Therefore, in this range, a difference of 1 ms, is approximately 1 BPM. At lower heart rates, the resolution increases.
What do think??

BTW: I really appreciate the input so far.

Thanks,
Nick