On Thu, Sep 05, 2013 at 10:34:09AM -0400, William Couture wrote:
> Quick question:  in a 74??? part, what is the difference between a
> 74HC??? and a 74HCT???

Trigger voltages on the inputs. The C in both indicates that the outputs
are CMOS. So they should be nearly rail to rail.

TTL inputs (which the HCT parts have) have a high trigger voltage of 2.4V
and IIRC a dead band of 0.8V to 2.0V.

CMOS inputs (which the HC parts have) have a high trigger voltage of 0.8Vdd
(which is 4V if Vdd is 5V) and a low trigger voltage of something like
0.8V (or some small perventage of Vdd. I cannot remember off the top of my
head what happens in the range inbetween.

You can mix and match in the interior of the circuit since both have CMOS
outputs and both recognize CMOS inputs properly. You need HCT parts when
you cannot guarantee a CMOS input to it.

I used HCT parts as a level shifter in my Trivial PIC programmer. The PC
parallel port had migrated to 3.3V outputs which are legal TTL, but would
not trigger CMOS inputs (such has the PIC's) properly. So I used an HCT
buffer to accept the TTL level from the parallel port and convert it to
CMOS outputs. Worked like a champ.

Hope this helps,

BAJ

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