Have you tied pin 10 (the output diode common ) to the supply voltage
for the motors? If not then maybe back emf from the motor coils have
killed the PIC.

Joe

-----Original Message-----
From: pic microcontroller discussion list
[mailto:PICLIST@MITVMA.MIT.EDU] On Behalf Of Gustaf J. Barkstrom
Sent: 27 February 2004 07:45
To: PICLIST@MITVMA.MIT.EDU
Subject: [EE:] Release of magic smoke


One of my 16F84A's just died after some high duty-cycle running of
stepper motors.  The circuit I built is basically the one found at the
bottom of this page: http://www.dakeng.com/u2.html Source code is also
available on that page.

However, I buffered the inputs of the PIC with a 74LS244 (octal line
driver), and used a Darlington array (ULN2803) on the outputs to drive
the steppers instead of the discretes shown in the schematic.  Note also
that I am not using a resistor between the PIC output pins and the
Darlington array input pins.

An ASCII block diagram of my circuit:

[parallel port data pin] -> [74LS244] -> [PIC] -> [ULN2803] -> [Stepper]

The entire circuit, including motors, is using the same 5+/GND regulated
power supply. PIC is RC clocked, and the power/gnd pins are not bypassed
on either the PIC or the line buffer/drivers.  Pin 3 of the PIC floats,
as the page linked above says "leave this pin floating if you like."

I ran two steppers (Vexta 2-phase, 5V/1A nominal) on this circuit for
several minutes, but no more than 30 seconds at a shot.  So, I got brave
and told my software to run the motors over a long distance, and they
ran continuously for about 4 minutes. After that, the chip froze (motors
stopped); held MCLR low and released it high again, still nothing.  I
touched the top of the Darlington arrays and the buffer/drivers, barely
warm.  I touched the PIC and about got a 2nd degree burn.

I turned everything off, went to bed, and turned it back on the next
day. Normally, the motors lock into an initial position at power-up, but
now no power is going to either of them.  Before I troubleshoot further,
I would like to get some input on the flaws of my circuit.  PCB art and
schematics available at your request, if the flaws of the circuit are
not already blatently obvious...

I think (in a sophomoric way) that my question is: why is so much
current going through the PIC?

By the way, the circuit worked very well until I turned up the
duty-cycle.

Thanks,

Gustaf

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