Rainbow Electronics AT25256A - Manual (Page 5)

5
AT25128A/256A [Preliminary]
3404A–SEEPR–10/03
Serial Interface
Description
MASTER: The device that generates the serial clock.
SLAVE: Because the Serial Clock pin (SCK) is always an input, the AT25128A/256A
always operates as a slave.
TRANSMITTER/RECEIVER: The AT25128A/256A has separate pins designated for
data transmission (SO) and reception (SI).
MSB: The Most Significant Bit (MSB) is the first bit transmitted and received.
SERIAL OP-CODE: After the device is selected with CS going low, the first byte will
be received. This byte contains the op-code that defines the operations to be performed.
INVALID OP-CODE: If an invalid op-code is received, no data will be shifted into the
AT25128A/256A, and the serial output pin (SO) will remain in a high impedance state
un til the fall ing edge of CS is de tected a gain. T his will reinitiali ze the seri al
communication.
CHIP SELECT: The AT25128A/256A is selected when the CS pin is low. When the
device is not selected, data will not be accepted via the SI pin, and the serial output pin
(SO) will remain in a high impedance state.
H O L D : T h e H O LD p i n i s u se d i n co n j un cti o n w it h the C S pi n to s e le ct th e
AT25128A/256A. When the device is selected and a serial sequence is underway,
HOLD can be used to pause the serial communication with the master device without
resetting the serial sequence. To pause, the HOLD pin must be brought low while the
SCK pin is low. To resume serial communication, the HOLD pin is brought high while the
SCK pin is low (SCK may still toggle during HOLD). Inputs to the SI pin will be ignored
while the SO pin is in the high impedance state.
WRITE PROTECT: The write protect pin (WP) will allow normal read/write operations
when held high. When the WP pin is brought low and WPEN bit is “1”, all write opera-
tions to the status register are inhibited. WP going low while CS is still low will interrupt a
write to the status register. If the internal write cycle has already been initiated, WP
going low will have no effect on any write operation to the status register. The WP pin
function is blocked when the WPEN bit in the status register is “0”. This will allow the
user to install the AT25128A/256A in a system with the WP pin tied to ground and still be
able to write to the status register. All WP pin functions are enabled when the WPEN bit
is set to “1”.