For the ultimate in isolation, use totally separate power
supplies (no common ground), and optoisolators to couple
the PWM signals.

Fr. Tom McGahee

----- Original Message -----
From: Dale Botkin <dale@BOTKIN.ORG>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Saturday, February 10, 2001 10:01 PM
Subject: Re: [EE] Servo control w/PIC16F84


> Robert,
>
> You'll probably find the servo is causing enough power supply disruption
> to the PIC to cause it to fail.  If you don't have plenty of decoupling
> caps on the PIC and the servo, you'll need them.  A better approach is to
> keep the power supplies separate if you can.  If you're running on battery
> power this is especially true, and if you're using a DC power supply that
> can't keep up with the servo's current spikes you're bound to have
> trouble.
>
> A servo can draw a surprising amount of current.
>
> Dale
>
>
> On Sat, 10 Feb 2001, Robert Shady wrote:
>
> > Okay, I've gotten the code done, but now
> > I am seeing something very interesting.
> >
> > I'm running my code against a scope, and
> > everything looks beautiful... Then..
> >
> > If I power the circuit & the servo from
> > the same +5V power supply, things work
> > okay for a minute but then the 16F84
> > "locks" (sorry for the technical term,
> > but I'm really not sure what happens,
> > it just quits running my code).
> >
> > If I power the servo seperately from
> > the circuit, the servo continuously
> > rotates (still watching the scope, the
> > signal is perfect).
> >
> > If I power the servo seperately, but
> > connect the grounds of the two power
> > supplies together, things work okay
> > for a minute and then the 16F84 "locks".
> >
> > I've tried the servo signal wire directly
> > into PORTB/0, through a resistor to PORTB/0,
> > through a diode to PORTB/0, through a
> > resistor and a diode to PORTB/0, all with
> > the same results...
> >
> > Does anybody have any ideas??  This one is
> > really stumping me.
> >
> > I've checked power power supplies, voltages
> > are correct (+4.9V) and the signal looks
> > clean.  I have tied MCLR & VDD to +5V, and
> > VSS to GND.
> >
> > -- Rob
> >
> >
> >
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> ---
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> discoveries, is not "Eureka!" (I found it!) but "That's funny ..."
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