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'[PIC]Microchip ICSP _MCLR circuit help'
2005\03\10@083024 by Mcgee, Mark

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Hi

I'm looking at the Microchip mid-range document (DS31028A-page28-4), and this
shows a diagram of how to isolate VDD from VPP when the ICSP connector is
connected.

The diagram consists of VDD connected to a resistor (R1) and a capacitor (C1).
Between the capacitor is a diode(D1) which is connected (normal bias) to
_MCLR/VPP and the ICSP connector.  A second connection is made from VDD to the
VDD pin via resistor(R2) on the PIC and ICSPconnector.

(anybody care to show me some ASCII cct drawing tools?)

I understand that D1 protects/isolates the main cct from the +13v VPP when
programming.

My questions are;

1) D1 is referred to as a Schotky diode in the document.  Can somebody explain
what one is, and recommend a model number for me to use.
2) Also, no values are given for R1, R2 and C1.  I assume 'cos they can't
possibly know the capacitance of a third party cct, but can anybody recomend
good starting values for these?  I can give you a pointer to my circuit on the
web if you'd like to see the cct.  Should C1 be a tantalum cap or electrolytic
or what?
3) I don't use any of the other programming pins in my application, but my cct
has another component which fires off a data burst to my PIC periodically.  I
assume this won't affect the PIC while it's being programmed?  Or should I
isolate the entire cct from VDD via a dip switch or something to be sure?

Thanks
Mark

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2005\03\10@113243 by Harold Hallikainen

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It'd be GREAT if Microchip included this stuff in their datasheets. I
pulled something from one of their eval boards and have used it in several
18F based products. Here's what I use:

MCLR pin goes through 470 ohms and 220nF ceramic to ground.

MCLR has 10k pull-up to Vcc.

On a 2x5 header, pins are connected as follows:

1 - MCLR
2 - Vcc
3 - ground
4 - PgD (RB7)
5 - PgC (RB6)

I have little adapter cables that have a ribbon cable spliced to the RJ45
(that's it, isn't it?) that plugs into the ICD-2.

Additional pins on the 2x5 header handle I/O required on the product.

This has been used on several 18F products and there are now thousands in
the field. Seems to work...

Harold



--
FCC Rules Updated Daily at http://www.hallikainen.com

2005\03\10@115123 by Mcgee, Mark

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Thanks for the reply Harold

I should have stated that I'm using 16F628A.  Would that make any difference?
You don't use a diode of any description then?  It looked to me that the diode
was allowing a pull-up for _MCLR, but stopping the +13v programming voltage
b*ggering up the rest of the cct.  Will a 10k do the same job?

The cct I was discussing was in the Microchip Mid-Range document, but was
lacking in actual numbers sadly.

Cheers,
Mark

> {Original Message removed}

2005\03\11@125600 by Harold Hallikainen

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I've used the 16F628, but don't remember if I did any ICD stuff with it. I
don't believe a diode is necessary. With 8V across the 10k resistor
(13V-5V), we only get 800uA. I'm sure the remainder of the circuit can
sink 800uA without much trouble. What'll happen is that during the time
Vpp is present, your 5V regulator will just have to put out 800uA less.

Harold

{Quote hidden}

--
FCC Rules Updated Daily at http://www.hallikainen.com

2005\03\12@045305 by ThePicMan

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At 09.55 2005.03.11 -0800, you wrote:
>I've used the 16F628, but don't remember if I did any ICD stuff with it. I
>don't believe a diode is necessary. With 8V across the 10k resistor
>(13V-5V), we only get 800uA. I'm sure the remainder of the circuit can
>sink 800uA without much trouble. What'll happen is that during the time
>Vpp is present, your 5V regulator will just have to put out 800uA less.

In a ideal world where all PIC programmers can source 1 mA @ 13V.

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