74139: Difference between revisions
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m (→Signal descriptions: we almost exclusively call the signal on the card edge "/ROMSEL", not "PRG /CE". Replace the latter with the former for consistency.) |
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The NES contains a 74LS139 to produce the <tt> | The NES contains a 74LS139 to produce the <tt>/ROMSEL</tt> signal from <tt>M2</tt> and <tt>A15</tt>. | ||
== References == | == References == | ||
Latest revision as of 21:20, 15 July 2019
The 74139 (common variants 74LS139, 74HC139) is a dual 2-to-4-line decoder/demultiplexer in the 7400 series. Each side of the chip is a decoder with an active low enable from a 2-bit address to four active low signals. The two decoders can be wired independently, or a /Y from one decoder can be wired to the /E on the other to calculate more complex combinational logic functions.
74139 Pinout
.---v---.
1/E -o|1 16|-- Vcc
1A0 --|2 15|o- 2/E
1A1 --|3 14|-- 2A0
1/Y0 -o|4 13|-- 2A1
1/Y1 -o|5 12|o- 2/Y0
1/Y2 -o|6 11|o- 2/Y1
1/Y3 -o|7 10|o- 2/Y2
GND --|8 9|o- 2/Y3
`-------'
Signal descriptions
- /E, A0, A1 are inputs
- /Y0, /Y1, /Y2, /Y3 are outputs
Each side computes the following function ("x" = don't care):
| /E | A1 | A0 | /Y0 | /Y1 | /Y2 | /Y3 |
|---|---|---|---|---|---|---|
| 0 | 0 | 0 | 0 | 1 | 1 | 1 |
| 0 | 0 | 1 | 1 | 0 | 1 | 1 |
| 0 | 1 | 0 | 1 | 1 | 0 | 1 |
| 0 | 1 | 1 | 1 | 1 | 1 | 0 |
| 1 | x | x | 1 | 1 | 1 | 1 |
The NES contains a 74LS139 to produce the /ROMSEL signal from M2 and A15.
