PIC ADC analogue voltage reading

Hi just realised I should have posted this in the Micro controller forums.

Anyway here it is...

I am just writing some PIC code that will read a 0V-5V analogue value from its AN0 pin. The question I have is what value does the reading relate to in the code? E.g. 0V = 0 but is 5V = 255 and 1V = 51 (255/5)? Or is there some other values for each of these?

Thanks in advance,

Craig

richacm said:

I am just writing some PIC code that will read a 0V-5V analogue value from its AN0 pin. The question I have is what value does the reading relate to in the code? E.g. 0V = 0 but is 5V = 255 and 1V = 51 (255/5)? Or is there some other values for each of these?

Click to expand...

For an 8-bit A/D value, 5V would be 255 and 0V would be 0.
For a 10-bit A/D value, 5V would be 1023 and 0V would be 0.

Here's an easy calculation for it:
For 8-bit: ADRESH Value = (MeasuredVoltage/VREF)*256
For 8-bit: ADRESH/ADRESL Value = (MeasuredVoltage/VREF)*1024

VREF is your voltage reference, which is typically the PIC's internal 5V taken from VDD if you're not supplying your own.

Cool - makes perfect sense. Thanks.

futz said:

Here's an easy calculation for it:
For 8-bit: ADRESH Value = (MeasuredVoltage/VREF)*256
For 8-bit: ADRESH/ADRESL Value = (MeasuredVoltage/VREF)*1024

Click to expand...

Oops! Second line should be 10-bit.

If you're using a 10-bit A/D such as what many PICs have, but you want an 8-bit result, just set it to left justify, read ADRESH and ignore ADRESL.

If you want a 10-bit result, set it to right justify and read both ADRESH and ADRESL.