[sdiy] CMOS chip questions

[Florian Teply]

Am Saturday 18 July 2009 01:48:01 schrieb Scott Nordlund:
> 74HC is usually fine.  I prefer 74HCT since its inputs are TTL compatible. 
> If you need something faster, use 74AC.  The problem with diode logic (I
> guess you're using diode resistor logic?) is that you can't really chain
> stuff together.  The speed depends on your choice of diodes and resistors
> as well as whatever you're driving.  I believe TTL circuits prefer pull-up
> resistors rather than pull-down (doesn't matter with CMOS), so this may
> limit your options in some cases.
>
I'll agree totally here, 74HC instead of CD4xxx is usually just fine unless 
you really need the devices to withstand 15V Logic levels. And in the 74HC4xxx 
series you'll get the very same pinout as the original CD4xxx devices, so 
that is a non-issue as well. With most small-scale suppliers, 74HC has better 
availability than CD4xxx anyways. When it comes to single gates, there are 
such on the market as well. Okay, all i've seen are SMT devices, but when you 
need to save real estate, i'd go for that anyways. It ain't no black magic. A 
bit pricey in terms of $$/gate compared to the usual 4-gates-in-a-case stuff, 
but a lot easier to work with considering all those design questions popping 
up when working with discrete Diode-resistor logic. And: if you should happen 
to not use some gate in a case, you'll most likely be better of with spending 
a couple resistors and tieing those inputs to high or low.
Regarding 74HC vs. 74AHC vs. 74AC in most cases you're safe going for 74HC. 
Coz, when you actually need the additional speed of 74AC and AHC compared to 
plain 74HC, you'll know anyways. In all those cases with SDIY i've ever come 
across, 74HC had plenty of speed and still loads of margin. It's not like 
clock jitters of half a nanosecond would kill us here.

HTH,
Florian