11/23 clock issue

Eric Smith spacewar at gmail.com
Sun Feb 8 15:55:12 CST 2015

On Fri, Feb 6, 2015 at 12:44 AM, Holm Tiffe <holm at freibergnet.de> wrote:
> You don't ned no pullup for +5. All open TTL inputs are reading High w/o
> any pullup.

That's partially correct for "real" TTL, but not for an CMOS "TTL",
such as 74HC, HCT, AC, ACT, FC, FCT, BCT, LVC, etc.  In other words,
partially correct for "plain" 74xxx, 74L, 74H, 74S, 74LS, 74AS, 74ALS,
74F, but not many of the other "TTL" families which are actually CMOS.

The reason I say "partially" is that an open TTL input has very poor
noise immunity. From a DC perspective it is pulled high, but in
practice noise pulses can easily be coupled into it.  It has always
been considered poor practices to leave TTL inputs open.  And
*extremely* poor practice for CMOS.

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