Programming a PIC in a 3V system.

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Anyone with PIC experience,

I have a 3.3V system that is using a PIC.

I am trying to design a circuit, so I can program the PIC in circuit.

The PIC requires 5V Vcc and 12V Vpp to program.

I have tried to add diodes to isolate the "high voltage" from the low
voltage, but I am not getting it.

Has anyone here solved this problem, and what did it look like.


Thanks


Donald

Re: Programming a PIC in a 3V system.

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Which PIC?

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Are you sure?  Many support LVP (low voltage programming), especially
the ones that operate at low voltages.

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Could you post your schematic and be more specific as to what you have
done so far?

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Probably, but you might also wish to check out www.piclist.com



Re: Programming a PIC in a 3V system.
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    10f200
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    Not this one.

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    I will try to find a way to post out of Protel.

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    Thanks, I'll try them.

Donald


Re: Programming a PIC in a 3V system.
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If you cannot use a low voltage programming PIC then you need to isolate the
PIC Vcc from the 3.3V system with a diode and use a pull up on Vpp to PIC
Vcc.

With this type of isolation the MCLR/VPP, PGC, and PGD are pretty much
dictated to the in circuit programming and become very hard to use as I/O
pins.


                   +---------------+
13.5V -----------+-: MCLR/Vpp  RB6 :----------- PGC
                 ! :           RB7 :----------- PGD
                 \ :               :
             22K / :               :
                 \ :  PIC16F876A   :
  5V ---------+  / :               :
      Shotkey !  \ :               :
       Diode  !  ! :               :
3.3V --->!----+--+-: Vcc       GND :
      A    C       +---------------+


The above ASCII art schematic has worked for me.

And yes the PIC16F876A does support low voltage programming.

My problem was that some of the parts I was getting had the LVP disabled.




Re: Programming a PIC in a 3V system.
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This is the circuit I use before.

However, the pins PGC and PGD are connected to 3V chips.

When I programmed the 10f200, my ICD2 put 5V on my 3V logic. The circuit
blew-up. ( My circuit did not work any longer and I relpaced the chip
and it worked again. )

So, more diodes ???


Donald


Re: Programming a PIC in a 3V system.


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Design a custom adapter that sits between the ICD and your circuit. The
adapter should disconnect the sensitive logic from those lines. For
example you can have a suitably biased transistor in the path to the
external logic, and run the base to your custom programming header.
WHen you attach your special custom programming adapter, it turns the
transistors off and isolates the pgming lines.


Re: Programming a PIC in a 3V system.
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You can try to clamp PGC and PGD to 3.0V with resistors and diodes. But this
tends to cause more ISP programming problems than it solves.

It's better to design your circuit such that PGC and PGD are connected to 5V
tolerant devices. For example, switches or buttons that are normally open
when you need to use the ISP and you enable on the internal weak pull up
resistors in the PIC for the buttons.



Re: Programming a PIC in a 3V system.

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You can usually isolate with series resistors.
Will your low-voltage parts actually suffer if they see 5V ?


Re: Programming a PIC in a 3V system.
5V is only needed to clear config bits. (security, WDT, etc)

I simply use a jumper (or SMT SPDT switch like Digikey CAS120GCT.. from
Copac Electronics) to switch the break out the PICVCC to 5V from the ICD2
-or- my custom flashing tool).

When done flashing the bootloader, I switch back to 3.3V and power up the
rest of my circuitry.

Holding the PIC in reset before switching in 5V as VCC doesn't damage any
3V circuitry attached to the PIC's I/O pins.

If you will be supplying 5V to 3V logic, ensure they have the Ioff
parametric as specified by Texas Inst.



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