> For the MOSFET driver, your suggested pull-up resistor is
> usually a bad idea because it will cause the solenoid to
> come on if the PIC output is high impedance for any reason
> (eg. during startup), or if the PIC is unplugged.

One technique I have used is to switch the pull-xxxx resistor (or
schottky diode clamp) based on an I/O that goes high impedance
during reset. The pin needs to be actively set to enable the output
driver transistors, etc.

> >Sorry to be picky, but newbies especially should be VERY
> >conservative with all their ratings.

Both newbies and oldbies need to pay attention to the reset state
and what happens when a watchdog occurs. I've seen so many
products where this is ignored and some of them are really scary.

A typical scenario would be a fet that turns on while the micro is in
reset, attached to an under-rated power supply. Power comes up
at the normal rate, fet turns on, adding larger load onto power
supply, power supply sags, fet turns off, light load on supply,
supply recovers, power comes up, fet turns on,......
Net result = 50% PWM on solenoid controlling hydraulic ram.
Ouch !

A similar situation occurs a lot of the time when outputs are turned
on by the micro under software control. You might have a reset
generator that monitors supply voltage and adds a few milliseconds
of delay but that's not long in power supply terms. If you turn on all
your loads straight away you get a similar situation to that above.

Also in that category is misuse of watchdogs. Wonderful things but
should be regarded like airbags - only used when all else has failed
and should deploy in a useful way.
Ones that restart the system and do the same thing that led to the
watchdog in the first place are of limited use in many situations.

Steve.

======================================================
Steve Baldwin                Electronic Product Design
TLA Microsystems Ltd         Microcontroller Specialists
PO Box 15-680, New Lynn      http://www.tla.co.nz
Auckland, New Zealand        ph  +64 9 820-2221
email: steveb@tla.co.nz      fax +64 9 820-1929
======================================================

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