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'[PIC]: Questions about PIC in-circuit programming'
2001\03\14@071633 by Neil Gandler

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I have questions relating to in-circuit programming for the PIC16F77.

* From what I understand the VPP pin must be tied to 12VDC+ during programming.

How do I isolate the VDD pins from this high voltage?

Is it recommended to use a diode between the VDD pin and VPP pin?
Or should I use a resistor?
If a resistor is recommended, what value is recommended?
* Do I need to power the PIC with +5VDC while it is being programmed (VDD, VSS pins)?

* When a PIC is in program mode (+12VDC on the VPP pin), what is the state of the I/O

pins. High impedance or output low?



Neil Gandler
Founder, CTO
MAI Technology Inc.
P.O. Box 64072
Sunnyvale, CA 94088-4072
Phone & Fax: 408-904-5089
e-mail: spam_OUTneil_gandlerTakeThisOuTspamgetmai.com
http://www.getmai.com

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2001\03\14@073957 by Peter Betts

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> * From what I understand the VPP pin must be tied to 12VDC+
> during programming.

Yes (13V ish)


> How do I isolate the VDD pins from this high voltage?
I use a schottky diode in line with a 10K resistor between my 5V supply and
the MCLR/VPP pin.
The programmer is connected directly to VPP so the diode isolates the 13V
from affecting the 5V rail.

> Is it recommended to use a diode between the VDD pin and VPP pin?
I do.

> Or should I use a resistor?
Well both I guess. Resistor to current limit and diode to block.


> If a resistor is recommended, what value is recommended?
I use 10Kohm


> * Do I need to power the PIC with +5VDC while it is being
> programmed (VDD, VSS pins)?

Yes.
Make sure you power it from the same 5V and 0V supplies from the programmer.
I've had loads of grief only connecting the programming pins and using the
5V supply from my normal on-board voltage regulator. The ground imbalance or
something really screws it up however much I tried to couple the grounds
together.


> * When a PIC is in program mode (+12VDC on the VPP pin), what
> is the state of the I/O pins
All high impedance/inputs I think. You must make sure the RB6 and RB7
programming pins are not being affected by your circuit. Isolating them may
help but I normally work my circuit out to either not use them or have them
connected to high impedance inputs (driving a 74HCXXX device) so I don't
have to worry about it. It depends on your circuit.

I have no trouble with 16F84 programming but did have a lot of trouble with
my in-circuit programming of 16F87X's using the PICALL/PIC16PRO programmer.

Since then I've invested in one of the really cheap Microchip ICD
(In-Circuit Debuggers) which programs without fault and gives me some
debugging facilities as well. Best $99 I've spent.

Pete

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2001\03\14@091131 by mike

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On Wed, 14 Mar 2001 04:14:57 -0800, you wrote:

>I have questions relating to in-circuit programming for the PIC16F77.
>
>* From what I understand the VPP pin must be tied to 12VDC+ during programming.
Yes
>How do I isolate the VDD pins from this high voltage?
By not connecting MCLR to VDD !
>Is it recommended to use a diode between the VDD pin and VPP pin? No
>Or should I use a resistor? Yes, if you would otherwise be connecting MCLR/Vpp to Vdd
>If a resistor is recommended, what value is recommended? Needs to be low enough to not cause spurious noise to cause a reset,
and high enough not to load the programmer. 10K would be a good guess.
>* Do I need to power the PIC with +5VDC while it is being programmed (VDD, VSS pins)?
Yes.
>* When a PIC is in program mode (+12VDC on the VPP pin), what is the state of the I/O

>pins. High impedance or output low?
High Impedance
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2001\03\14@115417 by Roman Black

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Neil Gandler wrote:
>
> I have questions relating to in-circuit programming for the PIC16F77.
> How do I isolate the VDD pins from this high voltage?
> Is it recommended to use a diode between the VDD pin and VPP pin?
> Or should I use a resistor?

We use a 68k resistor and no diode. never had a problem.
Using the PicStart Plus. In programming the PIC uses
about 10mA, much more than the bleed through the resistor.
No diode is needed.


> * Do I need to power the PIC with +5VDC while it is being programmed (VDD, VSS pins)?

We power up the entire 5v PIC circuit board first, the PIC
is held in reset by the programmer. Then we click
"program" in MPLAB and the VPP is raised and the PIC
is programmed. Very simple.


> * When a PIC is in program mode (+12VDC on the VPP pin), what is the state of the I/O pins. High impedance or output low?


They are held as high impedance inputs, this is
covered clearly in the Microchip documents. :o)
-Roman

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