1. Home
  2. Electronics Design
  3. Simple? logic function using 74HC logic

Simple? logic function using 74HC logic

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

Reply to
Francois Grieu
Loading thread data ...

"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.

Reply to
Helmut Sennewald

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.