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PICList Thread
'Halted execution ?'
1995\03\27@162728 by Ran Talbott

>MCLR shorted with Vcc.

I've been running into a lot of trouble with this,  and it could
be your problem,  too.

I built a "breadboard" prototype of my first PIC product with
the cheap 1-transistor "brown-out" circuit from the databook,
and it seemed to be working fine.  When we did the first
pre-production boards,  we decided to try to save a few cents
by just tying MCLR to Vcc,  and have had frequent problems
with the chips not resetting correctly.

Some of the symptoms have been bizarre,  suggesting that
*some* of the instructions in the reset routine are not being
executed correctly.  E.g.,  once we had a case of the system
appearing to run normally,  except that a couple of the
output bits on port B weren't being driven.  But a couple
were,  so it looks like the TRISB value got scrambled.

I know from putting a scope on the power supply that we're
not getting a clean turn-on,  mostly because we're using
an alligator clip for a switch at the moment  ;-).  There are
warnings in the databook about this,  as I recall.

Another fun problem showed up while we were testing the
system on the bench:  if we leave the pulse generator that
simulates tach running when the CPU is  switched off,
the protection diodes feed enough energy into the power
supply to prevent a proper reset.  Any input that can source
more than a couple of milliamps can probably do this.

We've also seen a few cases where it looked like maybe the
WDT wasn't being initialized properly (we have it off,  too),
but I can't be sure about that:  it just looked like the system
was resetting itself,  so we powered off and on,  and the
problem went away.

Try putting a better reset circuit on the board.  Something
like a Dallas DS1233 would probably be better than the
1-transistor circuit from the databook,  but anything is
probably better than nothing.


1995\03\28@072129 by Aaron Wohl

Check to be sure that you are initilizing the registers.  They don't
come up zero  on parts with cover that doesn't admit light.  In the FAQ
there is some code to start with to zero the registers, especialy the
page select bits/ pclath.  Otherwise the first jump your program takes
may be it's last.

As Ran Talbott mentioned the specs have a requirement for rise time of
the MCLR signal.   I have never had a problem with a slow rise time
(even with clip leads).   The specs also warn about the inpeadance of
the MCLR pin and latchup, that I have had a problem with.   Try using a
resistor in the MCLR pin, say 4.7 K ohms tied high.

Dallas Semi has a credit card sales dept for low volume.   I got some of
the econoresets Ran mentioned.  But I haven't started using them yet as
I haven't had reset rise time problem with any pic chip (5x, or c84).
Aaron Wohl / ham callsign N3LIW / 412-731-3691 / 412-268-5032

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