Simple? logic function using 74HC logic

Mar 18, 2010 1 Replies

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Hello,

I'm struggling to find a simple schematic to implement this two-inputs (A B), one output (X) function, using 74HC building blocks.

In words:

- X is high whenever A and B are both high;

- if A is high when B goes low, X tracks B until A and B are both high;

- if A is low when B goes low, X remains high.

A-----------\__________________/--------\_______/---\___/--------------

B------\________/---\_____/-----------------\______________/-----------

X-------\________/---\_____/-------------------------------------------

The B to X delays must be low, in the order of 50 ns max @ 5V, for both transitions. A / B setup times are in the order of 200 ns min.

Any idea ? So far anything I can think of has some ugly delay/monostable.

TIA,

Francois Grieu

"Francois Grieu" schrieb im Newsbeitrag news: snipped-for-privacy@gmail.com...

Hello Francois,

You should use an FF to build a small state machine.

A solution with a theoretical positive edge clock input D-FF with positiv reset:

CLK = B\ D = A OR Q CLR = Q AND A AND B

X = (Q AND B) OR Q\

WIth a real FF 74HC74 with positive edge clock-input and low reset:

CLK = B\ -> Inverter 74HC14 D = A OR Q --> 1/4 74HC32 CLR\ = (Q AND A AND B)\ --> 1/3 74HC10

X = (Q AND B) OR Q\ --> (1/3 74HC10 +Inverter) + 1/4 74HC32

You should use a 74HC14 for the inverters to get fast edges, especially if your A and B inputs have rise times slower than 100ns.

Best regards, Helmut

PS: You would save the OR-Gate at the D-input if you use a JK-FF.

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