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PICList Thread
'[PICLIST] [PIC] Reset Chips'
2002\07\11@125534 by John Dammeyer

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Hi guys,

I've been looking at various PIC projects that are published out on the
WEB and I find there is a major inconsistency with the reset circuits.
Some folks use reset chips and others just tie the MCLR to Vcc through a
resistor.  Given that the MCLR/VPP gets the 13V programming voltage are
there special reset chips that will handle that?  Is there even a need
with the PIC to have a reset chip?

Cheers,

John Dammeyer





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2002\07\11@132124 by Hazelwood Lyle

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John,
I had many of the same questions as I put
a board design together recently. The most
detailed info I could find on designing for
ICSP was buried in the help files of the
latest version of Picallw (http://www.picallw.com)

The need for a reset chip may be related to slow
rising VCC or requirement for some other
startup delays. "Normal" startups can usually
be handled by r/c at MCLR. If ICSP is planned,
a diode may also included in the circuit to prevent
VPP from driving VCC of the rest of the board.

I set mine up with R/C to the Anode, Cathode connected
to MCLR, MCLR straight to ICSP connector. So far, it has worked great.

Lyle Hazelwood

>{Original Message removed}

2002\07\11@134400 by Dwayne Reid

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At 09:54 AM 7/11/02 -0700, John Dammeyer wrote:
>Hi guys,
>
>I've been looking at various PIC projects that are published out on the
>WEB and I find there is a major inconsistency with the reset circuits.
>Some folks use reset chips and others just tie the MCLR to Vcc through a
>resistor.  Given that the MCLR/VPP gets the 13V programming voltage are
>there special reset chips that will handle that?  Is there even a need
>with the PIC to have a reset chip?

I *always* have some form of reset circuit in my products - usually a
combination of time and supply voltage operated.  I'm told that the higher
end PICs don't require external reset anymore but, so far, projects that
use higher end PICs require some form of fail-safe and/or eeprom and I wind
up adding an external watchdog/supervisor/reset controller/eeprom chip
anyways.  Simple PICs (12c508) absolutely require a proper reset circuit if
they are fed from a mains supply and you are shipping thousands of them all
over the world.

I don't do ICSP with the small PICs - the code does not change often enough
to justify it.  They get batch programmed before they are installed on the
PCB.  I therefore don't have to worry about protecting components connected
to MCLR.

Projects using an external watchdog can be programmed through the watchdog
chip's socket.  The watchdog I use is the Xicor X5043: 512 bytes of eeprom,
watchdog, reset controller, power supply supervisor.  I use RB6,7 to talk
to the Xicor chip (and all the shift registers on the card), MCLR is tied
to one pin of that chip.  It is socketed: I just plug the ICSP cable into
that socket.

dwayne

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