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I have an old tire pressure monitoring system and was hoping the PIC16F886 in it doesn't have the code protect bit set so that I can modify the behavior of it (namely turn off the beeping from it in certain scenarios but not others.) It has one set of six unpopulated pins, most of them direct to the ICSP pins on the PIC. However, pins 1 and 2 on the board go to the two pins at the bottom of a PNP transistor, and the top single pin connects to pin 1 on the PIC, VPP. Pin 1 on the board is connected to pin 1 on the PIC via the transistor, while no power is supplied to the board.

Anyone have a guess of the purpose of the transistor in the context of factory programming? When I connect pins 1, 3, 4, 5, 6 on the board to a PICKit2, I get VPP voltage level errors. Here is an image:

enter image description here

K_T
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    from what I can see, it looks like a power up reset circuit. R8 and C12 on the base forming a RC time constant. It might also be part of a watchdog circuit. In terms of programming you might need to apply a voltage level to the pad labelled 'not connected directly' in order to disable its function and allow the programmer to control the MCLR pin. – Kartman Sep 23 '22 at 03:52
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    Looks to me like the S8550 just allows a lower value capacitor to be used in the reset circuit. Maybe it needs LVP mode. – Spehro Pefhany Sep 23 '22 at 04:26
  • Would you mind to add the circuit around the transistor ([edit] your question)? – the busybee Sep 23 '22 at 06:06

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