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'[PIC]: MCLR reset on f877'
2003\03\01@121757 by Ian McLean

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Resent with tag.  Whoops, I forgot the tag again first time around :(
(Slapping myself on wrist).

I cannot find any specific reference in the datasheet about this, but I
recall a previous post to this list, and would like someone to confirm this
for me.

Is it true that the MCLR cannot be used for reset if the LVP fuse is set ?

Also, is there any benefit to using a reset chip, or possibly a 555 timer in
inverted monostable mode to provide an inverted square pulse of known
length.  The datasheet says that MCLR has a filter built in to ignore small
glitches.  Every circuit I have seen simply momentarily ties MCLR low
through a reset button.

A have a circuit for a 555 timer to be used as a reset chip, published in
ETI Circuits No. 3, Pg. 77.  I have SPICE'd this circuit and it seems to do
a good job in simulation.

I know this topic has come up before, but I cannot recall getting a
definative answer on this.

Rgs
Ian

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2003\03\01@134524 by Dwayne Reid

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At 04:17 AM 3/2/03 +1100, Ian McLean wrote:

>Is it true that the MCLR cannot be used for reset if the LVP fuse is set ?

Not true.  RTFM - DS30292C pages 7, 123

>Also, is there any benefit to using a reset chip, or possibly a 555 timer in
>inverted monostable mode to provide an inverted square pulse of known
>length.  The datasheet says that MCLR has a filter built in to ignore small
>glitches.  Every circuit I have seen simply momentarily ties MCLR low
>through a reset button.

An external reset is useful in a number of circumstances: crappy power
supply (noisy or rise time too slow) or where you desire an external
watchdog (high reliability applications).

>A have a circuit for a 555 timer to be used as a reset chip, published in
>ETI Circuits No. 3, Pg. 77.  I have SPICE'd this circuit and it seems to do
>a good job in simulation.

If you are going to use an external reset controller, use something
designed for the job.  The 555 approach gives you nothing better than what
is already on-board the '877 - that design was appropriate for the micros
used in that time period but not for modern PICs.

Specifically: how does it behave in brownout?  Does it maintain reset
active all the way down to 0V supply?  How does it behave if the power
supply comes up very slowly?

Finally - search the archives.  Even a quick google search on  PIC 877
reset controller  will most likely provide the answers you are looking for.

dwayne

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2003\03\02@141340 by Wouter van Ooijen

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> Is it true that the MCLR cannot be used for reset if the LVP
> fuse is set ?

No

> Also, is there any benefit to using a reset chip

Depends a lot on how your power turns on, what excursions it might make
during operation, and whether you care a bout random eeprom writes (and
flash??) during shut-down.

Wouter van Ooijen

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