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'[PICLIST] Program voltage on the MCLR pin of 16C87'
2002\03\18@003142 by John Dammeyer

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Hi all,

I've just joined the list after an hiatus of a few years.

I'm just laying out a board with a 16C877 on it and I've included the RJ
connector so I can use the PIC-ICD for programming and debugging.  I've
got a couple of MCP2510 CAN devices all attached to the MCLR line for a
common reset.  As I started the layout I realized that the programming
voltage used to put the device into High Voltage Program mode would be
passed onto the other devices.

What I've done is connected a 1K to the MCLR pin and a 47K pull-up to
Vdd on the other end of the 1K resistor.  I've also put a 5.1V Zener
there.  That line can now go out to the other CAN devices and a switch
to ground while the RJ 6 pin Vpp line goes directly to MCLR on the
processor.  This should allow the 13V signal to show up at the processor
for programming but drop it down to 5.1V max at the other end of the 1K
and therefore to the rest of the circuit.

I can't really think of any other way other than two Reset circuits
joined at the inputs by the pushbutton but each one driving either the
PIC or the rest of the RESET circuit.

Any other suggestions?

Thanks,

John

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2002\03\18@043647 by Alan B. Pearce

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>I can't really think of any other way other than two Reset circuits
>joined at the inputs by the pushbutton but each one driving either the
>PIC or the rest of the RESET circuit.

If you need to reset everything at once you may need to have a pair of
transistors pulling low, one on the F877, and one on the other devices, with
the switch pulling the bases high through a resistor.

On the other hand, what happens when you want to program the CAN devices in
circuit? You may need a separate transistor per device going to the reset
switch.

I would also ask do you really need to have a reset switch? To reset on the
bench during development can you not power cycle and rely on the internal
reset timer?

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2002\03\18@101935 by John Dammeyer

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Thanks for your input.  It's an activity board so I'd like to have a
reset button.  There is a SPI command to each device to reset it so it's
not mandatory to have the reset line tied to it but if I added someother
external device that did need this I'm in the same boat.

John


> {Original Message removed}

2002\03\18@112438 by Dwayne Reid

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At 09:23 PM 3/17/02 -0800, John Dammeyer wrote:
>Hi all,
>
>I'm just laying out a board with a 16C877 on it and I've included the RJ
>connector so I can use the PIC-ICD for programming and debugging.  I've
>got a couple of MCP2510 CAN devices all attached to the MCLR line for a
>common reset.  As I started the layout I realized that the programming
>voltage used to put the device into High Voltage Program mode would be
>passed onto the other devices.

I use a small diode (1n4148) between the PIC and the rest of the devices: K
to MCLR, A to other devices.  Needs 2 pullup resistors - one on each side
of the diode.  Put your reset controller right at MCLR; make sure it can
handle Vpp without damage.

Asserting MCLR (pulling it LO) will pull the !reset line of the other
devices LO via the diode.

I use this technique on several of my controller boards: it allows ICSP
without damaging the shift registers who's !reset lines are also controlled
by the watchdog/reset controller on the board and it also allows some of
those !reset lines to have a simple monostable timer on them (keep the SRs
held in reset if the watchdog keeps resetting the PIC).  The only caveat is
to make sure the forward drop of the diode does exceed the maximum allowed
voltage to be regarded as a logic LO.  That is not a problem for the SRs
that I use and should not be a problem for the MCP2510s - but check it!

dwayne


Dwayne Reid   <spam_OUTdwaynerTakeThisOuTspamplanet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
(780) 489-3199 voice          (780) 487-6397 fax

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