74138
The 74138 (common variants 74LS138, 74HC138) is a 3-to-8-line decoder/demultiplexer in the 7400 series. The decoder has one active high and two active low enables and is often used to calculate complex boolean functions.
.---v---.
A0 --|1 16|-- VCC
A1 --|2 15|o- /Y0
A2 --|3 14|o- /Y1
/EN1 -o|4 13|o- /Y2
/EN2 -o|5 12|o- /Y3
EN3 --|6 11|o- /Y4
/Y7 -o|7 10|o- /Y5
GND --|8 9|o- /Y6
`-------'
It computes the following function:
- A0-A2 : select which /Yx line to pull low
- /EN1, /EN2, EN3 : if they aren’t respectively 0, 0 and 1, the all outputs of /Yx pins will be forced to high
- /Y0-/Y7 : output signals
- VCC, GND : power supply
| Truth Table | |||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| EN3 | /EN2 | /EN1 | A2 | A1 | A0 | /Y0 | /Y1 | /Y2 | /Y3 | /Y4 | /Y5 | /Y6 | /Y7 |
| 0 | x | x | x | x | x | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 |
| x | 1 | x | x | x | x | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 |
| x | x | 1 | x | x | x | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 |
| 1 | 0 | 0 | 0 | 0 | 0 | 0 | 1 | 1 | 1 | 1 | 1 | 1 | 1 |
| 1 | 0 | 0 | 0 | 0 | 1 | 1 | 0 | 1 | 1 | 1 | 1 | 1 | 1 |
| 1 | 0 | 0 | 0 | 1 | 0 | 1 | 1 | 0 | 1 | 1 | 1 | 1 | 1 |
| 1 | 0 | 0 | 0 | 1 | 1 | 1 | 1 | 1 | 0 | 1 | 1 | 1 | 1 |
| 1 | 0 | 0 | 1 | 0 | 0 | 1 | 1 | 1 | 1 | 0 | 1 | 1 | 1 |
| 1 | 0 | 0 | 1 | 0 | 1 | 1 | 1 | 1 | 1 | 1 | 0 | 1 | 1 |
| 1 | 0 | 0 | 1 | 1 | 0 | 1 | 1 | 1 | 1 | 1 | 1 | 0 | 1 |
| 1 | 0 | 0 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 0 |
The NINA-03 boards use a 74'138 for their peculiar register range mask.