Larry's Blog

ADVICE ON CMOS IC's

THIS IS A LETTER I SENT
to George Gallant, Jim Morris
and Jim Fiske

hi
just realized something troubling
I was reading some hints for using the PIC
microcontrollers and found this:

"unconnected port bits should never
be set up as inputs"

"when floating, CMOS inputs can flip rapidly
between states and cause excessive current draw,
heating and even damaging the PIC"

So - set all unused bits of ports to outputs.

I went back to one of my books talking about
CMOS IC's and noticed this warning:

ALL unused inputs MUST be connected to Vdd(+) or Vss(gnd)
otherwise erratic chip behavior and
excessive current consumption will occur.

other rules for CMOS were NEVER connect
an input signal to a cmos circuit when
the power is off.

another curious warning was to
avoid slowly rising and falling input signals
since they cause excessive power
consumption. it recommended
rise times faster than 15 usecs

I think I have been lucky so far
but I'm certainly going to observe
these precautions.

Regards.

Larry Keegan in Stoneham, MA