Alt-BEAM Archive
Message #01058
To: "Feser, Jason" jfeser@ea.com
From: Steven Bolt sbolt@xs4all.nl
Date: Sat, 27 Feb 1999 12:18:54 +0100 (CET)
Subject: [alt-beam] Re: FW: Question about CMOS identifiers...
On Fri, 26 Feb 1999, Feser, Jason wrote:
> Probably basic stuff, but can someone define what the values HC, HCT, LS,
> ALS, AC and C are? I know that some are low speed / high speed, but I'm
> just not too clear on whats what....
This is something of a FAQ, and you should look up the family
specifications if you really want to know. But briefly:
You can forget about LS and ALS; those families are not cmos and
use too much power. Here is a bit of hopefully relevant info about
some other letter combinations:
74HC: cmos input switching levels 30% of Vcc and 70% of Vcc;
typical treshold 50% of Vcc. Supply: 2 to 6V
74HCT: TTL input switching levels 0.8V and 2V. Supply: 4.5 to 5.5V
74HCU: cmos switching levels 20% of Vcc and 80% of Vcc;
typical treshold 50% of Vcc. Supply: 2 to 6V. Unbuffered to allow
operation in linear mode.
That last HCU characteristic may sound attractive to some, but note
that these chips are more than usually sensitive to static charges.
74AC: Basically similar to 74HC, but much higher sink and source
output currents possible. Also a bit trickier in terms of pcb
layout and decoupling, as well as more expensive.
The best family for BEAM-type designs in general is probably 74HC.
At low frequencies, this family can be made to work at Vcc below 1V,
and will retain state (memory, if you will) at something like 0.5V
or even less. The outputs will sink and source enough to handle the
base current needed by driver transistors capable of dealing with
any motor you want to use for a small, mobile `bot. Even 74AC
outputs tend to get overloaded if you drive a motor directly.
Best,
Steve
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# sbolt@xs4all.nl # Steven Bolt # popular science monthly KIJK #
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