> > Returned to VCC, all the spikes from motor coil switching are dumped
onto
> > your VCC line. Further, if your motor voltage is more than a diode drop
> > higher than VCC, you will be running current from the motor supply onto
> your
> > VCC, which has serious consequences for any regulation you might be
> trying
> > to achieve on VCC.
>
> But would that have any negative consequences when the motor's
> Vcc is 12V and the PICs uses 5V derived from the 12V via a 7805?
> At least I used this setup for some time without noticing any
> problems. The impedance of my 12V is very low (nicads, big C).
>
> Wouter.

This is very similar to a printer I once looked at. They used similar
drivers for the printhead and stepper motors, with the diodes returned to
5V.

On measuring the 5V supply (7805) we discovered it was at 5.24V  This is
highly unusual for a 7805. Despite the specs, all I've ever seen were well
within 1% of 5V.
What was happening was his raw supply, which ran the motors and printhead,
was feeding through the motor and printhead resistances, and the diodes, and
into the 5V supply, pushing it dangerously close to the TTL supply limit. In
fact, the 7805 was completely shut down, until the printer began working the
motors and printhead, and thereby cutting off it's own "extra power supply".

We got a lot of these printers back with fried printheads, it seems that the
VCC transitions would occasionally crash the processor, and leave a
printhead element on for too long..