bryan

Hi All:

Is it possible to use a PIC to generate low frequency square waves variable between 5 and 50hz with a 50% duty cycle or better off to use a 555 timer.

Thanks

monkeybiter

at last, i can say it.....
yes it's trivial.
it's very easy to output a square wave from a pic with full control over frequency and duty cycle... you just toggle the pin value when required by a preset delay. i used a spare 16f84 to do just this while debugging a zero cross iinterrupt routine on a 16f876a controlled dimmer.
of course a 555 is easy too, but you have to hang more bits and bobs off it to get the same result [assuming you have a pic programmer].
if you've got a spare low end pic my choice would be to use that.

hosh

what kind of frequency is possible, 60 Hz, 100 Hz, higher?

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monkeybiter

low end pics will run at 20MHz, ie instructiions are executed at 5MHz. If you simply toggled in an endless loop you could achieve over 1.6MHz [quick reckoning]. you could add controls with interupting inputs and still acheive hundreds of KHz.

eblc1388

Will you mess up the frequency output by servicing the interrupt request?

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bryan

Perhaps it was low frequency using PWM that was supposedly difficult using a PIC. Would you have any sample code from the 16f84 project you worked on that you could post to get me started? Thanks

monkeybiter

if you only interrupt on pin change [input from keypad etc. ] then you should suspend generated output during settings change [inputting frequency value, duty ratio etc.] otherwise generation should be unhindered.
i am talking about generating output dependant on a calculated delay loop based on a pair of variables [eg. on duration, off duration]
a better alternative would be to use Timer to generate a pin toggle interrupt. keypad input could then be monitored in a main loop. this should give a more continuous output.
when i did it i didn't implement any input to vary parameters, i incremented an eight bit port at a fixed rate, giving 50Hz on bit 0, 25Hz on 1, 12.5Hz on 3 etc. and manually selected the pin to suit my needs.

Mike, K8LH

What kind of resolution and tolerance do you need? 1.0 Hz ±1.0% or ±2.0% perhaps? And, how do you want to control or select the frequency? Thumbwheel switches? Serial input from a PC? A trim pot'?

donniedj

No, as long as you add the time to service a constant interrupt like a counter or timer into the frequency generation. 5 to 100 Hz is so incredibly slow that you can vector off to another subroutine irregularly and have no adverse effect on the frequency. 1% change of 100 Hz is about 100us. Using a 4MHz PIC grants you 100 cycles of error, an 8MHz PIC grants you 200 cycles of error, and a 20MHz PIC which will now require external crystal/resonator grants you 500 unplanned and unforseen cycles of error.

bryan

I was hoping for a resolution of 0.5 hz and would like to control by the use of two pushbuttons. Up and down.

Nigel Goodwin

Just as easy to do, it's really a trivial task for a PIC.

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