ADCIN Problem

[JavPar]

Hello everyone,

I am using a PIC18F4620 to read voltages but I am having aproblem where if I read anything in between 0 or 5 volts, the voltages readings are erratic or oscillate. I have tried using resistors or capacitors on the ADCIN pin but nothing seems to work. I have been trying to solve these for weeks now I have done a lot of research but can't find anything that gives light to what the problem is. I am powering my project with a USB port.

I also noticed that if I disconnect the cable that goes to the ADCIN pin, I still get erratic numbers instead of reading 0. I have tried with code to get an average reading but this makes the code run slow.

Any help on this will be highly appreciated.

Following is an exctract of my code that am using separately to solve the problem with the ADCIN:

DEFINE LOADER_USED 1 'Bootloader Used
@ __CONFIG _CONFIG1H, _OSC_HS_1H
DEFINE OSC 8
' OSCCON=%01110000
Define ADC_BITS 10
Define ADC_CLOCK 3
Define ADC_SAMPLEUS 50
adval var word
TRISA = %11111111
ADCON1 = %00000010
ADCON2.7 = 1
'ADCON0 = %11000001
Pause 500
Serout2 PORTD.0,16468,[$1b, $2a, $ff]
loop:
ADCIN 1, adval
pause 50
Serout2 PORTB.7,16468,[dec adval] ' to PC
Serout2 PORTD.0,16468,[$fe, 1]
Serout2 PORTD.0,16468,[dec adval] ' to LCD
Goto loop
End

Javier.

[Pic_User]

Hi Javier,

Hello everyone,

I am using a PIC18F4620 to read voltages but I am having a problem where if I read anything in between 0 or 5 volts, the voltages readings are erratic or oscillate. I have tried using resistors or capacitors on the ADCIN pin but nothing seems to work. I have been trying to solve these for weeks now I have done a lot of research but can't find anything that gives light to what the problem is. I am powering my project with a USB port.

I also noticed that if I disconnect the cable that goes to the ADCIN pin, I still get erratic numbers instead of reading 0. I have tried with code to get an average reading but this makes the code run slow.

Any help on this will be highly appreciated. ....

Javier.

Don’t forget that the A/D module relies on an accurate, known, reference voltage, to compare to an unknown voltage. If your reference is your supply voltage then your supply voltage has to be rock solid. If you are switching loads attached to the supply / reference then the supply has to switch these loads without changing voltage.

You still can use A/D input filters but if the reference voltage is not stable your readings will reflect the variations.

Edited to add: Be sure your DEFINES are in Upper Case
-Adam-

[JavPar]

Hey Adam,

Thank you for your help. I am scratching my head over the reference voltage. From all the ADCIN samples I have seen, I haven't seen one that says anything about a reference voltage. Is this done by setting a register?
I am reading the datasheet right now to see if I find something about that. My defines are in capital letter in my program.

Javier.

[JavPar]

From the datasheet, I am using:

ADCON1 = %00001101

which specifyes that I am using the project's voltage internally from VDD and VSS for voltage reference. I also tried using external voltage for reference but I get the exact same results.

I tried putting a .1uF cap at the voltage in pin but no change.

I am going to tried with another sorce of 5v and not use the USB to see what happens.

Any suggestions will be appreciated.

Javier.

[JavPar]

I tried using a 5V power supply, but I still get the erratic voltage readings.

[skimask]

WHAT are you measuring and how have you got it connected?

Have you tried using a plain old meter?

What is this 5v power supply you speak of? A wall-plug type supply? Benchtop supply?

How much other stuff do you have connected to your '4620?

How about posting a bit of a schematic...