Paul B. Webster VK2BZC wrote:
>
> If you consider the device for a moment, you will note it is based on
> a technology called CMOS where "C" stands in full for "complementary
> symmetry".  This means that every part of the circuit has symmetrical
> connections to both Vcc and Gnd.  Except for inputs with deliberate
> asymmetry to make them compatible to TTL (which is indeed ground-
> referenced) outputs, Vcc and Gnd are indistinguishable in the circuit;
> their functions can be exchanged using a negative supply and suitable
> alteration in logic notation!
If you mean exchanging Vdd and Vss, you'll most likely destroy the
device because of the parasitic diodes between inputs and Vss.  If you
mean running the HC part with a Vdd of 0V and Vss of -5V, that's
probably OK.  Don't forget that internal "resistors" are actually biased
FETs which may not like such an arrangement.
>
>   In other words, it doesn't have a ground reference, the threshold is
> halfway between Gnd and Vcc for the function in question, so it makes
> not a whit of difference to which rail you fit the load capacitors.
> Lack of bypassing would be expected to cause as much mayhem with the
> load caps on Gnd as on Vcc.
The reason the crystal capacitors can be tied to the 5V rail is because
they need to be tied to _any_ low impedance point.  A properly designed
power supply's 5V rail is a low impedance point.  Power rail noise is
another issue that has been mentioned already.

--Matt