On Dec 30, 13:57, John Lawson wrote:
> On Mon, 30 Dec 2002, Sellam Ismail wrote:
>
> > Now, I seem to have a problem with CA1. If I have CA1 connected to
> > ground, it's stable. As soon as I remove it from ground and it's
> > floating, CA1's state starts to fluctuate wildly for a little over 2
> > seconds.
>
> Try tieing it to +5 via a 10K resistor, so it's either solidly a 0 or
> solidly a 1. Many digital chips get cranky when their pins 'float'.
Yes. Worst example I came across was a CMOS circuit which was designed by
an "expert". It worked on a breadboard, but when transferred to the final
PCB it misbehaved and eventually died of "dead chip". It turned out that a
couple of unused inputs were left unconnected and floating. On the
breadboard, that wasn't a problem, because it was dirty and there was
enough leakage (a hundred megohms or so) to keep them in a reasonable and
constant state. But the PCB was waterproofed, and they really did float,
eventually leading to excess dissipation in the chip, and destruction of
one of the gates. The reason is that a CMOS gate is basically two MOS
transistor switches, one connecting the output to the power rail, and the
other connecting it to ground. If the input floats halfway between logic
levels, both turn on (at least, in older devices), and the current destroys
one of the MOS transistors.
Another example was a TTL circuit that ran fine at low to medium
frequencies, but not at several megahertz. Floating input again; that sort
of thing has a drastic effect on the speed of the gate.
--
Pete Peter Turnbull
Network Manager
University of York
Received on Mon Dec 30 2002 - 13:28:00 GMT