74138
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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.
74138 Pinout
.---v---.
S0 --|1 16|-- Vcc
S1 --|2 15|o- /Y0
S2 --|3 14|o- /Y1
/G2A -o|4 13|o- /Y2
/G2B -o|5 12|o- /Y3
G1 --|6 11|o- /Y4
/Y7 -o|7 10|o- /Y5
GND --|8 9|o- /Y6
`-------'
Signal descriptions
It computes the following function:
- G1, /G2a, /G2b, S2, S1, S0 are inputs
- /Y0…/Y7 are outputs
| G1 | /G2a | /G2b | S2 | S1 | S0 | /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.
