PGC (ICSPCLK) Conflicts with Vref ?


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  1. #1
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    Nov 2003
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    Default PGC (ICSPCLK) Conflicts with Vref ?

    on the 16F690
    i need to use the Vref pin for a reasonably accurate non-ratiometric AD conversion
    my plan was to attach to RA1 a low impedance voltage reference, type TBD

    but that same pin is the ICSPCLK (PGC) pin for programming the chip pin on board (ISP), what were they thinking ?

    can i just use a resistor to stand it off?,
    i don't want to pull and replace a jumper every time i burn and crash during development
    am using the MEL programmer

  2. #2
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    Default Re: PGC (ICSPCLK) Conflicts with Vref ?

    A resistor will isolate the two. 2K is probably a good value. Anything larger may compromise the accuracy of Vref, but check the input current on the Vref pin for your device to make certain.
    Charles Linquist

  3. #3
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    Default Re: PGC (ICSPCLK) Conflicts with Vref ?

    thanks for your reply

    i really don't understand the terrible choice of pin assignments here

    i checked the data sheet and i see 10uA Vref current max, at 2.5Vref that would be about 8 LSBs at 2K

    so i don't think that is acceptable

    and then there is a whopping 1mA during acq which i guess is off the table ?

    my thought at the moment (since i must have ISP and do not want to jump/unjump the ref during flash and crash development) is that i will use Vref = VDD and put a trimmer (yuck) on the VDD reg to get the tweaks right

    i ran into this same sort of conflict on the same pin a while back while using the ULPW cap on that same pin

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