Using two AtoD channels - they're interacting?!

[HankMcSpank]

Have you ever had one of those evenings where everything falls to pieces? Having used AtoD without issue for quite a while, my 12F683 is laughing at me....and now I'm not sure what I know & what I don't know wrt AtoDing!

Previously, in my newbie ignorance (bliss?), when wanting to use more than one AtoD channel, I was simply doing this (on a 16f690)...



ADCIN 1, intensity
pause 2 '
ADCIN 3, Signal_In


never had a problem - my AtoD (Seemingly) worked!

Ok, now I've moved to a smaller (simpler?!) PIC - a 12f683. It's kicking my backside - two nights wasted on the seemingly very simple task of getting two AtoD channels to work correctly.

Firstly, can I get a really basic question out of the way? (you have to promise not to laugh!)

when moving between AtoD channels, should I be setting the ADCON register *every* time? (the PICBasic compiler suggests so, so I guess the answer is yes!)

ie, should I actually be doing this...



ADCON0 = %10000111 ' select AN1
pause 50 'wait a bit
ADCIN 1, intensity ' 'ok, let's Ato D.
pause 50 'wait a bit
ADCON0 = %10001111 'now move on to a different AtoD channel - AN3
pause 50 ' let things settle
ADCIN 3,Signal_In


in other word, using the ADCCON0 register to move between the different Analogue Channels?

If so, then how the hell was I getting away with it on my 16f690?

ok, now to my (latest) immediate problem - if I apply a variable DC level to say AN1 (just for testing to observe the results), moving the test voltage level up/down also affects the AN3 AtoD reading (& vice versa) - eh?

Now I know you have to wait sufficient time to let things settle when moving between channels, but I'm already pausing a good while before AtoD'ing the next channel - what's going on?

[Bruce]

Have a look at some of the MeLabs examples for the LAB-X4 board & 12F683 A/D.
http://melabs.com/resources/samples.htm#x4pbp

I'll bet between these and the data sheet you'll see what the difference is..;o)

[HankMcSpank]

Thanks for taking the time to reply.

I had a look, here's what I found...



' ADCIN1X4.BAS
' For PIC12F683 and melabs PICBASIC PRO Compiler
'
' Display result of 10-bit A/D conversion on LCD

Include "modedefs.bas" ' Mode definitions for Serout

' Define ADCIN parameters
Define ADC_BITS 10 ' Set number of bits in result
Define ADC_CLOCK 3 ' Set clock source (3=rc)
Define ADC_SAMPLEUS 50 ' Set sampling time in uS


LCD Var GPIO.1 ' LCD TX pin

adval Var Word ' Create adval to store result


ADCON0.7 = 1 ' Right justify result
ANSEL = %00111000 ' Set AN3 analog, rest digital
CMCON0 = 7 ' Analog comparators off

Pause 500 ' Wait .5 second for LCD to init

mainloop: ADCIN 3, adval ' Read channel 3 to adval

Serout LCD, T2400, [$fe, 1] ' Clear screen
Serout LCD, T2400, ["Value: ", #adval] ' Display the decimal value

Pause 100 ' Wait .1 second

Goto mainloop ' Do it forever

End


Unfortunately, that example only uses one channel.

Pretty much after I posted, I sorted my problem (how does that always work then?!).

Two things were the root of my issue...

1. I'm using a sig gen, rectifying a sine wave from it & feeding into & low pass filter (the resulting extracted DC level feeds into one AtoD channel) ....I was getting over zealous with the sig gen output amplitude - and the rectified DC level breached 5V - which sends the PICs AtoD results a bit whacky (which is hardly surprising) ...one channels starts seriously impacting the sample from a totally different channel! So Hank makes amental note - be careful with the input voltage magnitude fed into an AtoD pin!

2. The other problem was that I wasn't allowing sufficient time between using successive AtoD channels - I'm finding that in my setup, it needs at least 30ms between successive AtoD channels 'samples' - hey ho.

[tenaja]

I'm finding that in my setup, it needs at least 30ms between successive AtoD channels 'samples' - hey ho.

This is the case with every PIC, and for each and every a/d reading. It is called the "charge time" for the sampling capacitor, and is most often the bottleneck for sampling rates. (Some PICs charge it faster.) With a single channel it is often not noticed because the sampling cap is charged while doing other things, or because you get an "old" reading from the same channel.