Output Enable Vs. Chip Select
Many memory devices designed to connect to a bus (such as RAM and ROM chips) have both CS (chip select) and OE (output enable) pins, which superficially appear to do the same thing. If CS is not asserted, the outputs are high impedance.
The difference lies in the time needed to output the signal. When chip select is deasserted, the chip does not operate internally, and there will be a significant delay between providing an address and receiving the data. (An advantage of course, is that the chip consumes minimal power in this case.)
When chip select is asserted, the chip internally performs the access, and only the final output drivers are disabled by deasserting output enable. This can be done while the bus is in use for other purposes, and when output enable is finally asserted, the data will appear with minimal delay. A ROM or static RAM chip with an output enable line will typically list two access times: one from chip select asserted and address valid, and a second, shorter time beginning when output enable is asserted.
Read more about this topic: Three-state Logic
Famous quotes containing the words output, enable, chip and/or select:
“Lizzie Borden took an axe
And gave her mother forty whacks;
When she saw what she had done,
She gave her father forty-one.”
—Anonymous. Late 19th century ballad.
The quatrain refers to the famous case of Lizzie Borden, tried for the murder of her father and stepmother on Aug. 4, 1892, in Fall River, Massachusetts. Though she was found innocent, there were many who contested the verdict, occasioning a prodigious output of articles and books, including, most recently, Frank Spiering’s Lizzie (1985)
“There is no prescribed method for resolving every specific conflict a mother has with her child, and there is certainly no method that will enable her to have exactly what she wants....There is, however, a larger goal, which is to establish and over-all climate of reasonableness, one in which she and her child can hear each other.”
—Elaine Heffner (20th century)
“Look Johnny, Spig just joined the Navy. I’m married to it. I run the mess hall. I swab the deck. I chip the rust. You’re afraid that they’ll kick Spig out of the Navy. I’m afraid that they won’t.”
—Frank Fenton, William Wister Haines, co-scenarist, and John Ford. Minne Wead (Maureen O’Hara)
“I don’t wish to give offense when I suggest that this country should select a king, or even a queen, rather than a president. One isn’t that quick to shoot a king or a queen—the majesty of royalty, you see.”
—David Webb Peoples, screenwriter. English Bob (Richard Harris)