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HC or HCT ?

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Andre' e Cristina Malafaya Baptista

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Feb 15, 1997, 3:00:00 AM2/15/97
to

Is there any problem if instead of a 74HCT I use a 74HC with a power
supply of 5VDC ?
Can I drive TTL compatible ICs with the 74HCs (at 5VDC)?

A.C.Rochat

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Feb 17, 1997, 3:00:00 AM2/17/97
to Andre' e Cristina Malafaya Baptista

The difference between 74HC and 74HCT is the input circuitry.
The outputs of both series can drive LSTTL or CMOS,
but the 74HCT inputs have TTL level compatible tresholds.
If you drive a 74HC with a 74LS, the noise margin is less from optimal, but this
can be fixed with a pull-up resistor. This will increase the output high voltage level
of the 74LS.

--------
Arjan

Danny Sanders

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Feb 18, 1997, 3:00:00 AM2/18/97
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On Sat, 15 Feb 1997 04:36:22 +0100, Andre' e Cristina Malafaya Baptista
<andre...@mail.telepac.pt> wrote:

> Is there any problem if instead of a 74HCT I use a 74HC with a power
> supply of 5VDC ?
> Can I drive TTL compatible ICs with the 74HCs (at 5VDC)?

The only difference between HCT and HC is the input voltage levels.
For HC, anything below 1.5V is a low signal, anything above 3.5V is
seen as high (assuming 5V supply). For HCT, these levels have been
changed to 0.8V and 2.0V so that they can be driven by TLL outputs,
meaning that wether a HCT can be replaced with a HC or not depends
on what it is driven by.

As both types have (almost) rail-to-rail outputs, driving TLL is
no problem.


Danny Sanders www
(o o)
,-------------------.---------------------------------------oOO--(_)--OOo-.
| Danny Sanders | E-Mail: da...@stack.nl |
| Jongemastate 32 | WWW: http://www.stack.nl/~danny/home/index.html |
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Boris Mohar

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Feb 18, 1997, 3:00:00 AM2/18/97
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On Sat, 15 Feb 1997 04:36:22 +0100, Andre' e Cristina Malafaya
Baptista <andre...@mail.telepac.pt> wrote:

>Is there any problem if instead of a 74HCT I use a 74HC with a power
>supply of 5VDC ?
>Can I drive TTL compatible ICs with the 74HCs (at 5VDC)?

One big difference is that HCT has also TTL input impedance (low)
while HC has CMOS input inpedance (high). This is important when
implementing RC delays etc.

Boris Mohar

Spehro Pefhany

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Feb 19, 1997, 3:00:00 AM2/19/97
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Boris Mohar (bor...@interlog.com) wrote:

: One big difference is that HCT has also TTL input impedance (low)


: while HC has CMOS input inpedance (high). This is important when
: implementing RC delays etc.

This isn't right, they both have very high input impedance. The threshold
voltages are different, though, so that will affect any RC delays.

HCT will draw significant supply current (at DC) with the inputs at
normal _TTL_ levels, whereas HC (and HCT for that matter) draw much less
(essentially just leakage current) when the inputs are at normal _CMOS_
levels (and at DC).
--
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
Spehro Pefhany "The Journey is the reward"
sp...@interlog.com
Fax:(905) 332-4270 (small micro system devt hw/sw + mfg)
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=


Jim Potter

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Feb 19, 1997, 3:00:00 AM2/19/97
to

bor...@interlog.com (Boris Mohar) wrote:

> One big difference is that HCT has also TTL input impedance (low)
>while HC has CMOS input inpedance (high). This is important when
>implementing RC delays etc.

That's funny. My gated 1 second oscillator works just fine with a
74HCT132 and 470 k charging resistor. Doesn't sound like low impedance
input to me.

Jim Potter


James M. Potter, President TEL: (505) 662-5804
JP Accelerator Works, Inc. FAX: (505) 662-5210
2245 47th Street EMAIL: jpo...@jpaw.com
Los Alamos, NM 87544-1604 URL: http://www.jpaw.com


Boris Mohar

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Feb 20, 1997, 3:00:00 AM2/20/97
to

On Wed, 19 Feb 1997 05:13:21 GMT, jpo...@jpaw.com (Jim Potter) wrote:

>bor...@interlog.com (Boris Mohar) wrote:
>
>> One big difference is that HCT has also TTL input impedance (low)
>>while HC has CMOS input inpedance (high). This is important when
>>implementing RC delays etc.
>
>That's funny. My gated 1 second oscillator works just fine with a
>74HCT132 and 470 k charging resistor. Doesn't sound like low impedance
>input to me.
>
>Jim Potter
>

Looks like I will have to eat some words here..
( but I swear that when I replaced that HCT04 with HC04 the intended
RC delay changed signicficantly. Maybe it was due to different
treshold.)

Well I was wrong before.

Boris Mohar

Ken Yap

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Feb 20, 1997, 3:00:00 AM2/20/97
to

In article <330b2c9b...@news.interlog.com>:

|On Sat, 15 Feb 1997 04:36:22 +0100, Andre' e Cristina Malafaya
|Baptista <andre...@mail.telepac.pt> wrote:
|
|>Is there any problem if instead of a 74HCT I use a 74HC with a power
|>supply of 5VDC ?
|>Can I drive TTL compatible ICs with the 74HCs (at 5VDC)?
|
| One big difference is that HCT has also TTL input impedance (low)
|while HC has CMOS input inpedance (high). This is important when
|implementing RC delays etc.
|
| Boris Mohar

No, they both have high CMOS input impedance. HCT is designed to be
driven from TTL and has different input thresholds. Both HC and HCT can
drive TTL with no problem.

Michael Covington

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Feb 23, 1997, 3:00:00 AM2/23/97
to

Jim Potter (jpo...@jpaw.com) wrote:
: bor...@interlog.com (Boris Mohar) wrote:
:
: > One big difference is that HCT has also TTL input impedance (low)

: >while HC has CMOS input inpedance (high). This is important when
: >implementing RC delays etc.
:
: That's funny. My gated 1 second oscillator works just fine with a

: 74HCT132 and 470 k charging resistor. Doesn't sound like low impedance
: input to me.

Right. HCT inputs and outputs are CMOS, just like HC. The difference
is that HCT inputs have somewhat different threshold voltages in order
to make them compatible with TTL.

--
Michael A. Covington http://www.ai.uga.edu/faculty/covington/
Artificial Intelligence Center <><
The University of Georgia Unless otherwise indicated, I am not
Athens, GA 30602-7415 U.S.A. speaking officially for the University.

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