Uniformity of 'POT' accross pins?

[peterdeco1]

Hi Anand. I had the same problem measuring resistors. And yes, I found that portc gave me entirely different results than portb on a 16F73. The portb pins were very close but 2 of the 4 used gave me a slightly different result. So, I modified the automatic calibration program shown in the PBP manual. Perhaps you can do a similar routine to calibrate your capacitors. Scale1, scale2, scale3 and scale4 get calibrated individually and I am getting consistent results reading resistors from 470 ohms to 47K. The gosub makesound simply beeps a sounder letting you know as each scale gets calibrated. I also have a program that will beep the scale value to let you know what the actual number is. Let me know if you want that.

Clear 'ALL VARIABLES = 0
LET SCALE1 = 1 'START CALIBRATION @ 1

TEST1:
Pot PORTB.7,SCALE1,Y 'START ADVANCING SCALE UNTIL THE DEFAULT RESISTOR 47K = 253
IF (Y < 253) Then LET SCALE1 = (SCALE1 + 1): GoTo TEST1
GoSub MAKESOUND 'CALIBRATION1 COMPLETED


LET SCALE2 = 1 'START CALIBRATION @ 1

TEST2:
Pot PORTB.6,SCALE2,Y 'START ADVANCING SCALE UNTIL THE DEFAULT RESISTOR 47K = 253
IF (Y < 253) Then LET SCALE2 = (SCALE2 + 1): GoTo TEST2
GoSub MAKESOUND 'CALIBRATION2 COMPLETED

LET SCALE3 = 1 'START CALIBRATION @ 1

TEST3:
Pot PORTB.5,SCALE3,Y 'START ADVANCING SCALE UNTIL THE DEFAULT RESISTOR 47K = 253
IF (Y < 253) Then LET SCALE3 = (SCALE3 + 1): GoTo TEST3
GoSub MAKESOUND 'CALIBRATION3 COMPLETED

LET SCALE4 = 1 'START CALIBRATION @ 1

TEST4:
Pot PORTB.4,SCALE4,Y 'START ADVANCING SCALE UNTIL THE DEFAULT RESISTOR 47K = 253
IF (Y < 253) Then LET SCALE4 = (SCALE4 + 1): GoTo TEST4
GoSub MAKESOUND 'CALIBRATION4 COMPLETED

[ardhuru]

Bill, Peter, thanks for the suggestions.

Although I think the self-calibration idea is ingenious, in my app. it might be difficult/cumbersome to implement, as I have 4 channels, and when I deploy the instrument it might not be possible to sweep each channel from minima to maxima at the same time.

Also, I notice that the dissimilar readings on the different channels are happenning not because of the sensors (caps) themselves, but the inherent differences in the pic pins, even if they are on the same port.

I got around this by a technique that seems to be working very well; maybe it would helps others with a similar issue.

I am using just 1 pin to measure all 4 channels; the 4 sensors are multiplexed using a 4066 CMOS switch chip.

The outcome is that I now need to calibrate for just one channel, and as long as my sensors are uniform (which I found they are) I get a consistent reading for all 4 of them as I scan them one after the other.

Regards,

Anand