SR latch

santhosh raju

Intellectual 305 points

The application intends to use a logic whose functional table is as follows,

INPUT 1 | INPUT 2 | OUTPUT

0 | 0 | X

1 | 0 | 0

0 | 1 | 1

1 | 1 | no change

This is typically the functional table of a SR latch. Is there any SR latch which will have a Propagation delay less than 10 ns (less than 6 ns will be still more appreciated)?

Or is there any other logic that can have the same function with the mentioned propagation delay?

over 10 years ago

0 Chris Cockrill over 10 years ago

TI__Mastermind 46480 points

They might use a dual nand gate to build one. TheAUC is the fastest but only operates at lower voltage while the LVC is slightly slower but has a wider voltage range.

We dont have any SR latches in newer faster teschnologies but these should work just as well..

SN74AUC2G00, SN74LVC2G00

0 santhosh raju over 10 years ago in reply to Chris Cockrill

Intellectual 305 points

Hi Chris,

I appreciate the suggestion that you have given. But is there any AUC NANDs that work with 3.3 V with the same propagation delay (1 ns ) ?

0 Chris Cockrill over 10 years ago in reply to santhosh raju

TI__Mastermind 46480 points

no but here is one option. Put a series diode in line with 3.3V Vcc. This would drop Vcc to 2.6V. on the AUC part.

The inputs on the AUC can axcept 3.3V. The only problem is that the ouput would be 2.6V, but that is usually good enough for the Vih of a 3.3V part.

The other option isa LVC whic would still keep you under the 10ns total TPD..

0 santhosh raju over 10 years ago in reply to Chris Cockrill

Intellectual 305 points

Thank you for the inputs. SN74AUC2G00 suits the Vih and V il levels. But since the SR latch is built using the NAND gates, the output enable (Tri state buffer switch) has to be connected externally. But this adds on to the propagation delay. Is there any tristate buffer with the propation delay lesser than 2 ns?

The SN74AUC2G00 describes that it has a maximum propagation delay of about 1ns. Since 2 are connected in feed-back, what could be the propagation delay? Will it be 2 ns directly summing them up or will there be any other factors coming into play?

0 Chris Cockrill over 10 years ago in reply to santhosh raju

TI__Mastermind 46480 points

Yes, you would sum the two prop delays. there should be no other factors other than load.

These are our best 2 options as far as speed goes. All others will be much slower.

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SR latch