decoderintegrated-circuitinverter
I apologize if my question sounds stupid, but I'm a novice and trying to explore all of my options. I currently have a 4/16 decoder, but I did not realize it was active-low and what I really need is an active-high decoder. How might I go about fixing this without putting an inverter on every output? Thanks!
(datasheet: https://datasheet.octopart.com/NTE74HC154-NTE-Electronics-datasheet-22129262.pdf)
The chip does not care, it reacts to voltage levels, not to logic signals.
This chip has two tables that describe the logic function it implements. The first one shows the functions when YOU define low = 1, high = 0:
The second table describes the same logical circuits, but now using the (common) convention high = 1, low = 0:
To illustrate the point, look at the last-but-one row, second column. In the first table (active low logic) it states F = AB, which is the math notation for the AND function. In the second table the same entry states F = A + B, which is the logical (inclusive) OR function. This all makes perfect sense: the common OR port IS AN AND PORT FOR 0's.
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.
Best Answer
Just invert the logic of the downstream circuitry.
To drive a LED, do this:
simulate this circuit – Schematic created using CircuitLab
(BUF1 is just any of your decoder's output pins) The resistor and LED can be interchanged.
That's the way TTL circuitry (good at pulling low) was always used.