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PICList Thread
'mclr on pic16f84'
1998\01\25@050028 by Mark and Kim Penman

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hello

I am working on a project that involves controlling electric motors
through relays with a PIC16F84. The motors and relays have the
appropriate back EMF suppression circuits, but I find that the PIC
occasionally resets when the motor and/or relay swithes on/off. I suspect
that this has something to do with the MCLR pin on the PIC.

Can anybody advise a way of making the pic resistant to this type of
resetting, or possibly disabling the mclr pin?

Any assistance that you could give would be most appreciated.

Mark

1998\01\25@065306 by rtszib

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>

Check that you do not connect common ground of your PIC and motor. If so
you need to separate them. If they separated then the problem is in
relay's inductance.Check if you have proper decoupling capacitors near
to PIC (it was discussed in the List)
Tha problem may occure if  power rises above 7 V due to any reason. And
this can not be disabled. Check your power supply engineering - DOES IT
HAVE ENOUGH POWER TO SUPPRESS EMF?
The better idea is opto coupler with triac to control a motor that fully
decoupled motor circuit from control circuit.

Regards,
Vassili

1998\01\25@070344 by rtszib

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Check that you do not connect common ground of your PIC and motor. If so

you need to separate them. If they separated then the problem is in
relay's inductance.Check if you have proper decoupling capacitors near
to PIC (it was discussed in the List)
Tha problem may occure if  power rises above 7 V due to any reason. And
this can not be disabled. Check your power supply engineering - DOES IT
HAVE ENOUGH POWER TO SUPPRESS EMF?
The better idea is opto coupler with triac to control a motor that fully

decoupled motor circuit from control circuit.

Regards,
Vassili

1998\01\25@113210 by Morgan Olsson

picon face
If it don«t help tying MCLR to Vdd there is probably severe noise in chip
supply.

1) Are you certain that ni spikes etv are conming in return fron outputs,
or overshooting the inputs? Maybe try inserting resistors close to chip in
serial to the suspected pins?

2) Are the PIC supply stable and well decoupled?

3) Or, I dare say, there is a program error. We all do them... Have you
made a very simple test progran that make all output operations, and detect
reset?

PS We are all interested on avoiding problems of this kind, so please tell
if You find anything! DS

Regards
/Morgan

At 22:46 1998-01-25 -0800, you wrote:
{Quote hidden}

Morgan Olsson, MORGANS REGLERTEKNIK, Sweden, ph: +46 (0)414 70741; fax 70331
============================================================================

1998\01\26@120504 by David W. Duley

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In a message dated 98-01-25 05:02:18 EST, you write:

<<
hello

I am working on a project that involves controlling electric motors
through relays with a PIC16F84. The motors and relays have the
appropriate back EMF suppression circuits, but I find that the PIC
occasionally resets when the motor and/or relay swithes on/off. I suspect
that this has something to do with the MCLR pin on the PIC.

Can anybody advise a way of making the pic resistant to this type of
resetting, or possibly disabling the mclr pin?

Any assistance that you could give would be most appreciated.

Mark

>>
Hello Mark
In addition to the other reponses you will be getting about capcitance etc, make sure the diodes you are using for the back emf are fast enough. Most common diodes are fairly slow..

Dave

1998\01\27@220705 by Andrew Mayo

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The PIC chip is very sensitive to noise on MCLR. Keep the line to the
MCLR pullup very very short and well away from noise sources. Decouple
the PIC directly across power and ground with a 0.1 monolithic
capacitor. Ensure that power and signal grounds are separate and join
them at the edge of the board and go straight from there to ground.

{Quote hidden}

1998\01\28@203726 by Reginald Neale

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Andrew Mayo said:

>The PIC chip is very sensitive to noise on MCLR. Keep the line to the
>MCLR pullup very very short and well away from noise sources. Decouple
>the PIC directly across power and ground with a 0.1 monolithic
>capacitor. Ensure that power and signal grounds are separate and join
>them at the edge of the board and go straight from there to ground.
>

Good advice. Also, it is desirable to use a fairly low pull-up resistor. I
certainly wouldn't go any higher than 1K between Vdd and MCLR. You can tell
if this is contributing to the problem by trying different pull-up
resistors and seeing if higher values increase the frequency of undesired
resets.

Ferrite beads can be very helpful; wind a few turns around them and put
them in series with power and signal lines.

If the resets are only occasional, it can be difficult to tell whether a
particular modification has improved the situation. Connect a big relay in
"buzzer" mode and power it from the same line that powers the PIC circuit.
Hold it physically near the chip and the rest of the circuit. This often
broadcasts enough wide-band noise to expose marginal conditions. If you can
harden it against such "brute-force" attacks, it should improve the
situation.

Reg Neale

1998\01\30@174518 by Mark and Kim Penman

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Thanks to all who gave suggestions about how to fix the reset problems i
was having with the 16f84.

After trying out a number of ideas with mixed success, i ended up fitting
a dc-dc converter and optos on the i/o, which appears to have solved the
problem by isolating the pic completely.

thanks again

mark

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