ADCIN Problem

[JavPar]

I have made a separate project just to clean it. I am reading voltage from the project or voltage from a ac/dc wall power supply sending 3V. Also I have tried reading a angular rate sensor and last tried reading nothing and still get erratic readings.

I use a digital multimeter to read the voltages and the readings are 100% stable on the meter.

For supply I have been using the 5 volts from my PC's USB port. I also tried using a wall-plug type supply suppliying 5V.

I cleaned my project so now I only have an LCD, a 4.7K resistor at pin 1 MCLR/VPP, on/off switch, reset button, 8 Mhz crystal with two 15pF from the crystal to ground.

Even with the new clean project, I still get erratic readings.

This is crazy. Will continue trying tomorrow. Thanks for your help.

[mackrackit]

Try adding an LED and make it blink at a steady rate. Give the PIC a heart beat.

Does the LED blink as expected?

Also try the ADC at 8 bit resolution.

Let us know what happens.

When you tried a capacitor on the ADC, did you have it from the ADC pin to ground? Was the value at leat 1uf ? l use 22uf myself.

Like skimask said, post a schematic. A picture of you setup might help too.

[JavPar]

Hi guys, arriving late from work. I loaded a little schematic I hope is clear. On my project I have 4 LEDs. On this clean one I will use one. Will try 8 bits. I have used caps .1uF and 10uF connected to GND.

Let you know tomorrow about the 8 bits.

Javier.

[Archangel]

I would suggest bypass caps on the Vcc / Vss and a choke in series with the adcin line to kill off any RF which might affect your circuit. I would ask if you are working near RF sources like Fluorescent lights, microwave ovens, televisions etc . . .

[skimask]

Hi guys, arriving late from work. I loaded a little schematic I hope is clear. On my project I have 4 LEDs. On this clean one I will use one. Will try 8 bits. I have used caps .1uF and 10uF connected to GND.

Let you know tomorrow about the 8 bits.

Javier.

From the picture...
There's a Vdd and Vss on each side of the PIC, 11-12 and 31-32. I only see one of each hooked up. I don't see any cap's across Vdd and Vss on each side.

[ardhuru]

When you tried a capacitor on the ADC, did you have it from the ADC pin to ground? Was the value at leat 1uf ? l use 22uf myself.

I'd second that; except, I use 0.1uf, with a 100K resistor in parallel with the capacitor.

Regards,

Anand

[JavPar]

Ok guys,

1.) Changed to 8 bits.

2.) Put caps 1uF on each side of the pic on the Vdd and Vss.

3.) No microwaves, no TVs, no fluorescent lights close to my project. I scanned my desk for any possible interferences but there is nothing.

4.) Put a .1 uF parallel with a 100K resistor at the ADCIN pin. Tried with different caps.

5.) Added a LED and it blinks ok. Do not see any skips or changes on the blinking rate.

6.) The only thing is that I could not find RF chokes locally (I'm in Panama Central America). I put a ferrite on the cable to the ADCIN pin just in case.

All of the above and problem still persists. I have not seen a single change on the way the erratic readins of voltage behave. I tried to see if there is a pattern to the erratic readings but can not see one.

Is this problem with erratic voltage readings something usual or rare? I have continued searching the web and I don'yt see anything that really talks about this.

Could this be a problem with the PIC18F4620? Or maybe I should not be using a breadboard?

Javier.

[ardhuru]

With what you describe, I feel certain the issue is not of hardware. I have used the ADC on a breadboard, with a far cruder supply (no RF chokes/filters) without a problem.

I have never used a PIC18F4620, though. Which is why I suspect perhaps its to do with some settings specific to that chip.

Could you temporarily replicate that with another PIC, say a 16F877?

[JavPar]

Unfortunatelly, I only have the 4620. I can not find pics locally. I am already looking to order more different pics from the states.

[skimask]

How erratic are the readings? Could you post a 'stream' of the readings?

[Acetronics2]

Ok guys,

1.) Changed to 8 bits.

2.) Put caps 1uF on each side of the pic on the Vdd and Vss.

3.) No microwaves, no TVs, no fluorescent lights close to my project. I scanned my desk for any possible interferences but there is nothing.

4.) Put a .1 uF parallel with a 100K resistor at the ADCIN pin. Tried with different caps.

5.) Added a LED and it blinks ok. Do not see any skips or changes on the blinking rate.

6.) The only thing is that I could not find RF chokes locally (I'm in Panama Central America). I put a ferrite on the cable to the ADCIN pin just in case.

All of the above and problem still persists. I have not seen a single change on the way the erratic readins of voltage behave. I tried to see if there is a pattern to the erratic readings but can not see one.

Is this problem with erratic voltage readings something usual or rare? I have continued searching the web and I don'yt see anything that really talks about this.

Could this be a problem with the PIC18F4620? Or maybe I should not be using a breadboard?

Javier.

Hi, Javier

Nothing about config.

BUT Your supply filtering looks to me a bit weak ... ( USB voltage stability ... ahem !!! )

I have a board with '452 and another with '2520 ... 220µF at the card pow. input + 10 µF Tantalum + 0.1µF ( closest as possible from PIC pins ).

see scheme attached.

Some capacitor tank ( > 10µF Tant.) close to your LCD could be useful too ...

Alain

[JavPar]

Hey guys I really appreciate your help. Alain, that schematic that you posted has made me think. I used my multimeter to test input voltage right at AN0 pin and the voltage is steady at 2.432 with the last gidit sometimes changing to 1.

I also noticed that measuring voltage in the output pin to my LCD I get oscillating 123mV to 127mV. That makes me think that possibly my PIC is doing something during the A/D convertion that it is outputting oscillating readings? The oscillating readings go out not only to my LCD pin but also to my PC pin.

[Alektric]

Had a similar problem my self - turned out to be the inverter for my LCDs backlight. My voltmeter gave no indication of any fluctuations.