SRAM (Static RAM) Chip Pins:
In addition to the two power pins to supply the electrical power, SRAM chips
(like any RAM chips) have a set of address and data pins which constitute the
address bus and data bus. Beyond that, however, there are only three other
pins on most SRAM chips:
CS: Chip Select
OE: Output Enable
WE (or WR): Write Enable
CS selects a particular chip. When CS is active on a chip, that chip will
respond to read or write requests.
OE is the "read" signal to the chip. It indicates that you want the chip to
output data. Upon receiving an OE signal, the SRAM chip will check the
address bus, and send out whatever is at the selected address over the data
bus.
WE is the "write" signal to the chip. Upon receiving the WE signal, the SRAM
chip will check the address and data buses, and write the data to the
selected address.
Note that all of these three pins are usually "active low" on most SRAM
chips, meaning that to activate them, you send them a low voltage, and you
de-activate them by sending a high voltage.
(Interestingly, these pin functions are exactly the same as on most EEPROM
chips. In fact, an EEPROM's pin list looks identical to that of an SRAM,
except that many EEPROMs have an extra READY/BUSY output pin which turns BUSY
to indicate the EEPROM is busy writing to itself, and back to READY when the
write operation is complete, signifying the chip is available for use again.)
To Read From SRAM: Place whatever address you want to read on the address
bus. Then, simply make sure that WE is off so you won't accidentally write to
the address, then activate the CS and OE lines, and the chip will output
whatever is at the selected address onto the data bus.
To Write To SRAM: Select the desired address via the address bus, and also
send the chip the data to be written to that address via the data bus. Then,
all that should be required is to activate the CS and WE pins; However, it
seems that many SRAM chips actually need OE to be activated while the chip is
being written to, for some odd reason. Therefore, once you have set both the
address and data buses, activate ALL THREE control pins (CS, OE, and WE), and
the data should be stored at the desired address. After that, disable the WE
and OE signals, and stop sending the data to the data bus.
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