square wave input to pic!!!

ambivalent · Guest

hi everyone.

i want to apply a square wave input (0 -> 12v) to PIC 16f877a. i will then detect the no of waves per second. i have two questions:

1. should i just apply this 12 volts square wave directly to PIC and use A/D to determine when 12volts are present on a pin to keep count of no of pulses per second, or should i reduce the voltage to 5v through a variable resistor and then apply it to the pin and forgo the whole A/D procedure.

2. to count the no of square pulses per second, i will have to use timers, or is there a better way of doing this.

Best regards

SherpaDoug · Joined: 07 Sep 2003 Posts: 1640 Location: Cape Cod Mass USA

Do not run a 12V signal straight into any PIC pin whether it is an A/D input or not! You run a good chance of damaging the PIC.
If all you want to do is measure the frequency and you don't care about amplitude you do not need to use an A/D. Read Microchip App note AN521 for info on extracting timing data from high voltage waveforms.
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Ttelmah · Guest

Reduce it to 5v. Otherwise, exit one PIC...
The 'best' method of generating the input, is probably to use an external comparator, with some hysteresis present, to give a clean square wave (otherwise you are likely to have problems if there is any noise on the signal), and then feed the signal into a timer or CTC.
How you use this, depends on the frequency involved. if the frequency is high, directly count the signal with a timer module. Otherwise, if you use the CTC in 'capture' mode, to count the clocks on the internal oscillator, between the signal edges, this gives a value directly related to the period (reciprocal of frequency). At low frequencies this will give much better accuracy.

Best Wishes

kender · Joined: 09 Aug 2004 Posts: 768 Location: Silicon Valley

If your 0-12V input is just a square wave with a little noise, you might get away without a comparator. I think this topology will work for:

Guest · Posted: Sat Oct 15, 2005 9:05 pm

Guest · Posted: Sun Oct 16, 2005 5:51 am

Both the Zener method and diode to VCC are viable solutions.

For the zener method I would use a 4.7volt zener.

The diode to VCC method injects current into the VCC supply of the circuit. This would not normally be a problem however you will need to ensure that this does not compromise the regulation characteristics of the power supply circuitry in your design.