Electronic – Inverting 3-to-8 decoder with all-active option

decoderreset

I have a requirement for an inverting 3-to-8 decoder circuit (i.e. all 8 output high except the selected one being low), but with an option to have all the outputs go low.

The 74HC137 or 74HC138 is close to what I need, but it's three reset conditions all have all outputs high.

Does anyone know of a real-estate-conserving solution (i.e. not a lot of components)?

EDIT: Desired truth table:

A B C R   7 6 5 4 3 2 1 0
0 0 0 0   H H H H H H H L
0 0 1 0   H H H H H H L H
0 1 0 0   H H H H H L H H
0 1 1 0   H H H H L H H H
...
1 1 1 0   L H H H H H H H
X X X 1   L L L L L L L L

R represents some sort of reset input that enables all the outputs (lines 0-7). A, B, and C are the address inputs.

Best Answer

Maybe you could use a combination of 74HC138 decoder and 74HC245 bus buffer. Switch the 74HC245 off (into z state) for the all zeros state and put a pulldown resistor array to outputs.

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