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74F or 74HC?

DavidMil

New Member
OK, I am on another site and there is a disagreement about which chip type is better/faster; 74F or 74HC. So I'm looking for some feed back from
better informed readers. Specifically: a circuit was designed with 74F chips and some of the readers on the other site said that you could substitute
74HC without any problems seeing as the circuit is only running at 9.7MHz. So, can I get some feedback, pleased!

Thank you,
David
 

rjenkinsgb

Well-Known Member
Most Helpful Member
74F is definitely far faster, something like 3 to 4 times, and generally has much higher output drive capability.
Typical gate propagation delays something like 3nS vs 12nS for 74HC.

Whether you could substitute 74HC depends on the overall circuit; if a signal has to pass through several stages per clock cycle or involves slower parts such as memories, the accumulated delay may be too high for operation at 9.7MHz.

If it's eg. just a counter IC, it would probably be OK speed-wise.

74ALS or 74ACT devices are the newer "fast" alternatives if you do need higher speed / lower propagation as with 74F.
 

DavidMil

New Member
This is very good info! I see this publication is from 2017. That's ok because I still refer to my TTL manuals from 1988.
Problem is that these 74LS chips are fast going out of date because of the faster speed requirement of todays circuitry.
But thank you for the info (I've saved and printed a copy for my dining entertainment).

David
 

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