7402: Difference between revisions
From NESdev Wiki
Jump to navigationJump to search
(Might as well have this) |
Rainwarrior (talk | contribs) m (→Bus Conflict Suppression Implementation: does capital AND make its logical function clearer?) |
||
| (9 intermediate revisions by 4 users not shown) | |||
| Line 1: | Line 1: | ||
The '''7402''' (variants 74LS02, 74HC02) is a 74-series logic IC with four independent 2- | The '''7402''' (variants 74LS02, 74HC02) is a 74-series logic IC with four independent 2-input NOR gates. | ||
== | == 7402 Pinout == | ||
.--\/--. | |||
1Y <- |01 14| -- +5V | |||
1A -> |02 13| -> 4Y | |||
1B -> |03 12| <- 4A | |||
2Y <- |04 11| <- 4B | |||
2A -> |05 10| -> 3Y | |||
2B -> |06 09| <- 3A | |||
Gnd -- |07 08| <- 3B | |||
`------' | |||
== Signal descriptions == | == Signal descriptions == | ||
Y is low if either A or B is high, and is high if A and B are both low. | Y is low if either A or B is high, and is high if A and B are both low. | ||
[[Category:7400 series| | == Bus Conflict Suppression Implementation == | ||
74HC02 -- Connection | |||
---------------------------- | |||
+5V -- +5V | |||
02 -- CPU R/W | |||
03 -- CPU R/W | |||
01 (02+03) -- 12 = /(CPU R/W) | |||
11 -- /ROMSEL | |||
12 -- 12 | |||
13 (11+12) -- 08, 09 = ROMSEL AND CPU R/W | |||
08 -- 13 | |||
09 -- 13 | |||
10 (08+09) -- PRG /OE = /(ROMSEL AND CPU R/W) | |||
Others -- GND | |||
This is sometimes used in discrete logic mappers to prevent [[bus conflict]]s (e.g. [[ANROM]]). By combining '''/ROMSEL''' with '''CPU R/W''', the PRG chip can have its output enabled ('''/OE''') only during reads, preventing a conflict with its [[74161|latch]] writes. | |||
See: [[Cartridge connector]], [[74161]] | |||
[[Category:7400 series|002]] | |||
Latest revision as of 22:58, 14 December 2022
The 7402 (variants 74LS02, 74HC02) is a 74-series logic IC with four independent 2-input NOR gates.
7402 Pinout
.--\/--.
1Y <- |01 14| -- +5V
1A -> |02 13| -> 4Y
1B -> |03 12| <- 4A
2Y <- |04 11| <- 4B
2A -> |05 10| -> 3Y
2B -> |06 09| <- 3A
Gnd -- |07 08| <- 3B
`------'
Signal descriptions
Y is low if either A or B is high, and is high if A and B are both low.
Bus Conflict Suppression Implementation
74HC02 -- Connection ---------------------------- +5V -- +5V 02 -- CPU R/W 03 -- CPU R/W 01 (02+03) -- 12 = /(CPU R/W) 11 -- /ROMSEL 12 -- 12 13 (11+12) -- 08, 09 = ROMSEL AND CPU R/W 08 -- 13 09 -- 13 10 (08+09) -- PRG /OE = /(ROMSEL AND CPU R/W) Others -- GND
This is sometimes used in discrete logic mappers to prevent bus conflicts (e.g. ANROM). By combining /ROMSEL with CPU R/W, the PRG chip can have its output enabled (/OE) only during reads, preventing a conflict with its latch writes.
See: Cartridge connector, 74161
