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18F87K22 A/D

 
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gtx15



Joined: 19 May 2018
Posts: 27

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18F87K22 A/D
PostPosted: Wed Jun 26, 2019 5:18 am     Reply with quote

Do I have to disable something to get a/d inputs on PortA to work ? I’m getting erroneous readings on all PortA a/d inputs. Rest of a/d inputs ok, temp sensor, voltage ref, PortF all work fine. TrisA programmed as inputs! a/d inputs programmed as well.

Thanks in advance.
temtronic



Joined: 01 Jul 2010
Posts: 9081
Location: Greensville,Ontario

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PostPosted: Wed Jun 26, 2019 6:31 am     Reply with quote

You need to post a small, compilable program for us to see and run. It's probably something simple BUT until we see your code.....

Jay
PCM programmer



Joined: 06 Sep 2003
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PostPosted: Wed Jun 26, 2019 9:46 am     Reply with quote

Sample A/D program for the 18F87K22:
http://www.ccsinfo.com/forum/viewtopic.php?t=57941&start=18

If it doesn't work, post your CCS compiler version.
gtx15



Joined: 19 May 2018
Posts: 27

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18F87K22 A/D
PostPosted: Sat Jun 29, 2019 2:03 pm     Reply with quote

It appears that if my A/D input goes slightly below VSS (Vref-). My A/D results Go negative. I'm getting results like FFEF. Any solution besides better pcb layout?
temtronic



Joined: 01 Jul 2010
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PostPosted: Sun Jun 30, 2019 4:48 am     Reply with quote

Most ( if not all ?) PICs have ADCs that ONLY read positive voltages. So it s important to ensure no negative volts get to the ADC section. Hard to advise you what to do, other than general comments hardware... proper ground planes, shielded cabling, clamping inputs, etc. Software bodges would be multiple sampling, like 'Olympic' averaging as you should never rely upon one reading, especially for control purposes.
Microchip have several 'apnotes' about using the ADC section. Getting analog to work correctly, especially more than 8 bits is 1/2 design, 1/2 layout and 1/2 luck.
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