74139: Difference between revisions
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(Created page with '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 fr…') |
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 '''74139''' (common variants 74LS139, 74HC139) is a dual 2-to-4-line decoder/demultiplexer in the 7400 series. | 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. | 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 | The two decoders can be wired independently, or a <tt>/Y</tt> from one decoder can be wired to the <tt>/E</tt> on the other to calculate more complex combinational logic functions. | ||
== 74139 Pinout == | |||
.---v---. | |||
1/E -o|1 16|-- Vcc | 1/E -o|1 16|-- Vcc | ||
1A0 --|2 15|o- 2/E | 1A0 --|2 15|o- 2/E | ||
| Line 14: | Line 15: | ||
`-------' | `-------' | ||
Each side computes the following function: | == Signal descriptions == | ||
* <tt>/E</tt>, <tt>A0</tt>, <tt>A1</tt> are inputs | |||
* <tt>/Y0</tt>, <tt>/Y1</tt>, <tt>/Y2</tt>, <tt>/Y3</tt> are outputs | |||
Each side computes the following function ("x" = don't care): | |||
{| class="tabular" | {| class="tabular" style="text-align:center;" | ||
! /E || A1 || A0 || /Y0 || /Y1 || /Y2 || /Y3 | ! <tt>/E</tt> || <tt>A1</tt> || <tt>A0</tt> || <tt>/Y0</tt> || <tt>/Y1</tt> || <tt>/Y2</tt> || <tt>/Y3</tt> | ||
|- | |- | ||
| 0 || 0 || 0 || 0 || 1 || 1 || 1 | | 0 || 0 || 0 || 0 || 1 || 1 || 1 | ||
|- | |- | ||
| 0 || 0 || | | 0 || 0 || 1 || 1 || 0 || 1 || 1 | ||
|- | |- | ||
| 0 || | | 0 || 1 || 0 || 1 || 1 || 0 || 1 | ||
|- | |- | ||
| 0 || | | 0 || 1 || 1 || 1 || 1 || 1 || 0 | ||
|- | |- | ||
| 1 || x || x || 1 || 1 || 1 || 1 | | 1 || x || x || 1 || 1 || 1 || 1 | ||
|} | |} | ||
The NES contains a 74LS139 to produce the | The NES contains a 74LS139 to produce the <tt>/ROMSEL</tt> signal from <tt>M2</tt> and <tt>A15</tt>. | ||
== References == | == References == | ||
*[http://www.datasheetcatalog.com/datasheets_pdf/7/4/H/C/74HC139.shtml Philips 74HC139 datasheet] | *[http://www.datasheetcatalog.com/datasheets_pdf/7/4/H/C/74HC139.shtml Philips 74HC139 datasheet] | ||
[[Category:7400 series|139]] | [[Category:7400 series|139]] | ||
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.
