At 10:14 AM 6/11/02 -0600, Micro Eng wrote: >Using the F877, I have PortA configured for analog inputs, and had 10K pots. >Seemed to work ok, I had a watch window set up, and a breakpoint right after >the read sequences (for 4 pots). > >I've changed the pots to 50K now, and upped the aquisition time, but the >values seem to be way off now, and are affected by each other. > >ie... > >if I read just one, it is close...but not exact. If I put the second pot in >the loop to read, the first one goes way off scale. I've read the actual >voltages on the pins, and they are not changing. >Anyone ever seen where just reading a second port affects the value of the >first one? You have 2 potential problems: input impedance and acquisition time. Microchip specifies that your source impedance should be no higher than 10K - you have a worst case source impedance of 25K (when the pot is in the exact middle). The cure is simple - add a 0.1 - 1 uF cap from the analog input to gnd. The cap supplies the charge required when the input is sampled and restores full accuracy to the a/d input. The 2nd problem is acquisition time. You have to allow a minimum setup time between when you select the desired analog input pin and when you start the a/d conversion. This time is specified fully in the data sheet but is typically 30 us or so. In other words, do this: select a/d pin, do something useful or waste time for at least 30 us, start a/d conversion. Note that when you are processing the result of a conversion that has already completed, you can select the next desired a/d channel BEFORE you actually start processing the data. Changing the a/d input does not corrupt the a/d result register (ADRES). Deal with both of those issues and see if the problems go away. dwayne -- Dwayne Reid Trinity Electronics Systems Ltd Edmonton, AB, CANADA (780) 489-3199 voice (780) 487-6397 fax Celebrating 18 years of Engineering Innovation (1984 - 2002) .-. .-. .-. .-. .-. .-. .-. .-. .-. .- `-' `-' `-' `-' `-' `-' `-' `-' `-' Do NOT send unsolicited commercial email to this email address. This message neither grants consent to receive unsolicited commercial email nor is intended to solicit commercial email. -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads