ADC problem

[EmbdFreak]

Hi,
I am reading my 0-5v voltage, after conversion from the ADC. The problem is the voltage i get as a result of conversion and the actual input voltage is not the same. e.g. if the actual voltage is 1.5v the program below shows 3v. similarly if actual is 3v, adc converted is 4.8v. Ive connected a 1k pot to vary it. I dont see anything wrong in the code or in the circuitry. Plz suggests possible solutions

[kender]

What's the value of your pot? A/D cnverter on the PIC wants to see the impedance under 2.5k. So, either the low side of your pot should be always less then 2.5k, or your pot should be buffered with an op-amp buffer.

[TIMT]

shouldn't :-

setup_port_a(all_analog)

be :-

setup_adc_ports(all_analog)

I could be wrong, but it may be worth trying

Regards, Tim
_________________
Tim

[Humberto]

I don't see any wrong in your code except that you define ADC=8 then the variable
value as a long integer, wich is inconsistent because value would never be longer than 256.
You can test the AD converter with a 10K pot without problems while keeping the
wires as short as possible.
First, I would check if the conversion is done properly with:

[Ttelmah]

One other thing I would add, is to turn off the comparators. These have a habit on some CCS versions, of being enabled at boot, and interfering with pins. setup_port_a, is an 'alias' for setup_adc_ports, and though it should work, I'd suggest switching to the standard format.
Ar you actually checking the voltage with a meter?. Otherwise, I'd be suspicious of a wiring problem somewhere.

Best Wishes

[rnielsen]

Make sure that the result, in registers ADRESH & ADRESL, are right or left justified correctly. ADCON1 contains bit ADFM which controls this. It defaults to right justified, using bits 6-7(ADRESL) & 0-7(ADRESH) which will really mess things up if you are expecting the data to be contained in bits 0-7(ADRESL) & 0-1(ADRESH). Make sure ADFM is set to a '1' if you want to use the lower bits of the result register.

Ronald

[EmbdFreak]

yes, im actually measuring the voltage with meter while checking my AtoD's output value. What ive noticed is, as my input voltage reaches 3v the AtoD's output reaches 255(max value for 8 bit) and the program does not respond as i higher the input voltage further (3v to 5v ). Similarly the program starts responding once i lower the value again below 3V. The lower value after which the program stops printing is about 0.7V(input voltage). Hence the program shows its output for 0.7v-3v only
Ive used 1k, 10k pots and my compiler version is 3.203

[PCM programmer]

I tested your program with a 16F877A on a PicDem2-Plus board, with
PCM vs. 3.242 and 3.203. I only changed the #use rs232() statement
to use pins C6 and C7, because the board is wired for that.

It worked.

The PicDem2-Plus has a trimpot on pin RA0, and the board runs at +5v,
and I got 255 when the pot was pegged in one direction and 0 when it
was turned all the way in the other direction.

So I think you have a hardware problem:

1. The RA0 input may be partially blown. Try another channel.

2. You may have some circuit that is interfering with the operation
of the A/D. This could be a large series resistor between your
trimpot and the PIC pin. There could also be some resistor which
causes a voltage divider to exist on that pin. Measure the pin
with a voltmeter. Does it go to +5v when you peg the trimpot ?

3. You could have a drooping power supply. What is your power
supply ?

[Ttelmah]

One thing that could give this behaviour, is a bad Vdd connection to the processor itself. If it was only running off effectively 3v, it would give this behaviour, with the pot actually 'pulling up' the supply through the internal protection, once the voltage rises about 3.6v. Though the 'F', (as opposed to the 'LF'), is not specified to work below 4v, many will. Remember that Vdd, must be fed to both pin 11, and pin 32.

Best Wishes

[EmbdFreak]

PCM Programmer, it has WORKED !!!
I changed the channel, so ur solution no. 1 worked right away.
Thank u all for all ur replies !!!