On Mon, 28 Oct 2002, Ian Forse wrote:

*>The 12F675 has a Vref input option on pin 6 (GP1), however this pin is also
*>one of the programming pins (ICSPCLK).  I want to use a TL431 as a fixed
*>voltage reference of 2.5V.  How can I also drive this pin for in-circuit
*>programming, or is it not possible?
*>Obviously for a one-off I could disconnect the Vref but this is for
*>production programming.

                       +-R2-*-R3-*
                       |    |    |
Vcc ----R1----*---AD1K-*--TL431--*-GND
              |                  |
              +---AD2K-*----R4---+
                       |
                       *-----o ICSP GP1
                       |
                     --O--
                     PIC GP1

R4 = 10 * R1, D1, D2 = universal diodes

This may or may not be accurate enough over temperature for your needs. A
simple analog switch or MOSFET would be better imho.

Peter

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