PIN DESCRIPTIONS AND DEVICE OPERATION
The address inputs select a 4-bit memory location
during a read or write operation.
Chip Select (CS)
The Chip Select input must be LOW to enable read or
write operations with the RAM array. CS HIGH will place
the I/O pins in the high impedance state.
Write Enable (WE)
The Write Enable input controls the I/O buffers, deter-
mining whether a RAM read or write operation is en-
abled. When CS is LOW and WE is HIGH, the I/O pins
will output data from the selected RAM address loca-
tions. When both CS and WE are LOW, data presented
at the I/O pins will be written to the selected address
Data In/Data Out (I/O1–I/O4)
Data is written to or read from the X22C12 through the
I/O pins. The I/O pins are placed in the high impedance
state when either CS is HIGH or during either a store or
The STORE input, when LOW, will initiate the transfer of
the entire contents of the RAM array to the E2PROM
array. The WE and RECALL inputs are inhibited during
the store cycle. The store operation is completed in 5ms
A store operation has priority over RAM read/write
operations. If STORE is asserted during a read opera-
tion, the read will be discontinued. If STORE is asserted
during a RAM write operation, the write will be immedi-
ately terminated and the store performed. The data at
the RAM address that was being written will be unknown
in both the RAM and E2PROM arrays.
The RECALL input, when LOW, will initiate the transfer
of the entire contents of the E2PROM array to the RAM
array. The transfer of data will be completed in 1µs or
An array recall has priority over RAM read/write opera-
tions and will terminate both operations when RECALL
is asserted. RECALL LOW will also inhibit the STORE
Upon power-up the X22C12 will automatically recall
data from the E2PROM array into the RAM array.
The X22C12 has three write protect features that are
employed to protect the contents of the nonvolatile
• VCC Sense—All functions are inhibited when VCC is
• Write Inhibit—Holding either STORE HIGH or
RECALL LOW during power-up or power-down will
prevent an inadvertent store operation and E2PROM
data integrity will be maintained.
• Noise Protection—A STORE pulse of typically less
than 20ns will not initiate a store cycle.
3817 PGM T01