ADCIN giving inconsistent results

[triton99]

I changed the code to reset V_total variable at the start of the V_read routine. I am not able to test until I get home. But i was under the assumption that if I did not reset V_Total it would then the last value would be reused in the next readings average.

Also when implementing the low-pass filter on the voltage in what values do you think would work for the resistor and capacitor??
What is a good cut-off frequency??



Thanks again for all the help.

[mister_e]

as-is: It really depend of the source....
assuming you want to use an opamp in between: it's a totally different thing.

You already use some averaging thing, Try the code modification, I'm pretty confident it will solve the problem.

You could also use the ADC in 8 bit mode instead of 10...

Anyways, have a look at this document
http://ww1.microchip.com/downloads/e...Doc/adn007.pdf

[Charles Linquis]

My guess is that if you clear V_total, you won't need anything else.

Consider:

If the A/D returns a value of 410

1st time through the loop

V_Total = 200

Second time through the loop

V_Total = 202

Third time through the loop

V_Total = 204

....

And it goes through those loops pretty fast. I can see how the results would look inconsistent

[HankMcSpank]

What I was wanting to know, is there a better way of reading the voltage using an ADC on the 16F675.

fwiw I've come to the conclusion that using the picbasic 'ADCIN' command = much badness (unstable ADC readings for me at least)

Nowadays, if I've a spare CCP module going free, I always use the PIC's special event trigger - it's a very tidy effiicient way of collecting ADC samples in the background - Once it's setup (it's not hard to do this), whenever you want a sample - it's already therefore you ...just go and grab the contents of ADRESH. Job done, eg...

v_in  = ADRESH
'now go do cool stuff with v_in

The 16f675 has a special event trigger option ...it uses CCP4 for it, so...if you're not intending utilising CCP4, then seriously consider dedicating it to the special event trigger....

[Charles Linquis]

Add one line and most of your problems will go away.

fans_off:
    low low_fan
    low high_fan
    return
    
read_v:

    V_Total = 0    <--------------------------------------------------------------

    for i = 1 to sample_size
       pauseus 50
       ADCIN 3, v_in                ' Read channel 3 to V_in (0-1023)
       V_Total = (V_In + V_total)
    next i
    
    V_total = V_total/sample

[triton99]

mister-e: Thanks for the document on reducing system noise. I dont know how I missed that one.

Charles Linquist: Thanks for pointing out reseting the V_Total variable. My brain sometimes gives me flawed logic...

HankMcSpank: I orginally tried to use the ADC directly using ADRESH. For some reason I could not get it to work. Once I get back home I will mess with it again. It seemed like a better approach.

Thanks again everyone, and I will be testing this again today on the car, so I will post the results when finished.

[triton99]

After a few weeks of work and family things, I finally found the time to complete the project. The ACDIN results were inconsistent at first, but I etched a new board and instead of SMD components I used thru-hole. The new board worked perfect on the bench but failed on the car...until I realized I had used a NPN transistor instead of an PNP. Before I realized this I made a small relay board that the transistor would switch, which would then turn on the cooling fans. I salvaged the relay from an old TRS-80 motherboard that was ruined. Once the transistor was swapped out and the relay board added, everything worked perfect. We took some test runs, and the cooling fans turned on/off as expected. When the transistors were switched I also had to reverse the high/low settings on the relay outputs on my board. My personal blog has a little more info. http://triton9.wordpress.com/
Thanks again to everyone for all the help!!