Assembly Program for PIC 16F84

Please give more detail - describe what you want to measure (range of values etc) and any circuit configuration which is relevant.

AFAIK, this device has no analog inputs - how do you propose to measure the resistance?

Thanks John

I have to find another PIC

Microchip provides a large library of appnotes and assembly listings on how to do just about anything with a PIC. Check out their site . You actually DON'T need one with an A2D input to do the mesurement you want.

It's really easy. You only need to use a capacitor that gets charged by the resistor and count time. Of course it isn't linear so you will need to allow for that and a look up table would be a simple way to implement that. Use output mode to discharge the cap, go to input mode and count until the bit toggles. Make sure you use an input with a schmidt trigger to get the best threshold repeatability. You can find sample code for this on the MC website as noted by another poster.

We did it before they offered A2D inputs. It has been a few years, but I'm pretty sure we used:

1) A reference capacitor connected to a 1st i/o
2) A reference resistor on a 2nd i/o for charge and discharge timing
3) The unknown resistor (thermistor in our case) on a 3rd i/o for charge and discharge timing.

I believe measuring both charge and discharge times gave the most accurate results, but review app-notes for best fit to your needs. Code example should be on Microchip site, but may be hard to find.

Thanks it seems good method

First, why do you want to use that chip! What you are asking is impractical. In engineering practice, we use what is the most simple approach to solving a problem with the shortest amount of man-hours(time) involved.

1. Find a PIC like the PIC16F526 that has a built in ADC

2. When employing the PIC, it's best to make sure you have an external oscillator for your PIC. Using the internal clock yields some noisy results.

3. When writing your code, implement a run-time averaging filter

pseudocode:

avg = sum( past_averages[] ) /MAX_PAST_AVERAGES

That seemed to work for me!

post more information to your problem and ask more questions