Hi All,

I just want to bring up one point: while it seems reasonable that all
CMOS inputs should be tied in a final design (to minimise power
consumption due to pins changing state), and that in some very sensitive
circuits, it might cause problems, but I have been an electronics hobbist
in earnest for about 6 years, have played with lots of chips from many
logic families, and I have never had any problem with a circuit that was
cured by tieing an unused input! Have I just had good luck? Or, is this
similar to other's experience as well?

Sean


On Thu, 13 Aug 1998, John Haggins wrote:

> As a beginner, it took me a while to figure out that all UNUSED i/o lines
> MUST be tied to the power supply  (CMOS rule - all inputs must go
> somewhere). For weeks I was getting intermittent results because I didn't
> know this rule.
>
> Sounds like this may be your problem. Incidently, I found the answer in the
> book "Easy PIC'n" which gets my vote as best beginners book (and I've got
> them all :) Amazingly, none of the books highlight these symptoms or give
> any kind of bold text warning/tip - it must be a common cause of failure
> for beginners.
>
> John
>
>
> At 12:26 AM 8/13/98 -0400, you wrote:
> >Hi,
> >
> >I am having a problem with a protoype build of a PIC circuit.  I have Port
> >B connected to 2 LED's (2 mA each), 3 relays (13 mA each) and 3 to an MAX
> >232 chip.  The chip is a 16C72, windowed.
> >
> >I wrote a simple program in MicroEngineering Labs PIC Basic to see test the
> >operation of the PIC.  It goes as follows:
> >
> >High 0
> >Pause 1000
> >Low 0
> >Pause 1000
> >High 1
> >Pause 1000
> >Low 1
> >Pause 1000
> >
> >the rest goes on to do the same with the other lines conected to the relays.
> >
> >RB0 and RB1 operate the status LED's.  I energize the circuit and the LED
> >connected to RB0 Blinks on one second then off.  Then the LED connected to
> >RB1 doesn't light.  Or sometimes the RB0 LED lights goes off then RB1
> >lights and goes off.  Most of the time though the RB1 LED never lights.
> >
> >Why is the operation of this so intermittent?
> >
> >I set Watchdog timer OFF during compilation.  The MCLR is connected to a
> >DS1233 like Myke Predko recommends.  The crystal is 4 MhZ, with 22pF loads.
> > The current draw on Port B is less than 50mA max.  The circuit is powered
> >through a 5V regulator with a 9V battery.  The relays have protection
> >diodes on the coils to prevent surge currents from damaging the PIC.  The
> >PIC has a bypass cap at the Vdd and Vss.
> >
> >This has me stumped.
> >
> >
>