CAT25C128/256
CS: Chip Select
FUNCTIONAL DESCRIPTION
CSistheChipselectpin.CSlowenablestheCAT25C128/
256andCS highdisablestheCAT25C128/256. CS high
takes the SO output pin to high impedance and forces
the device into a Standby Mode (unless an internal write
operation is underway) The CAT25C128/256 draws
ZERO current in the Standby mode. A high to low
transition on CS is required prior to any sequence being
initiated. A low to high transition on CS after a valid write
sequence is what initiates an internal write cycle.
The CAT25C128/256 supports the SPI bus data trans-
mission protocol. The synchronous Serial Peripheral
Interface (SPI) helps the CAT25C128/256 to interface
directly with many of today’s popular microcontrollers.
TheCAT25C128/256containsan8-bitinstructionregis-
ter. (The instruction set and the operation codes are
detailed in the instruction set table)
After the device is selected with CS going low, the first
byte will be received. The part is accessed via the SI pin,
with data being clocked in on the rising edge of SCK.
Thefirstbytecontainsoneofthesixop-codesthatdefine
the operation to be performed.
WP: Write Protect
WP is the Write Protect pin. The Write Protect pin will
allow normal read/write operations when held high.
When WP is tied low and the WPEN bit in the status
register is set to “1”, all write operations 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.TheWPpinfunctionisblockedwhentheWPEN
bit is set to 0.
PIN DESCRIPTION
SI: Serial Input
SI is the serial data input pin. This pin is used to input all
opcodes, byte addresses, and data to be written to the
25C128/256. Input data is latched on the rising edge of
the serial clock.
HOLD: Hold
SO: Serial Output
HOLD is the HOLD pin. The HOLD pin is used to pause
transmission to the CAT25C128/256 while in the middle
of a serial sequence without having to re-transmit entire
sequence at a later time. To pause, HOLD must be
brought low while SCK is low. The SO pin is in a high
impedance state during the time the part is paused, and
transitions on the SI pins will be ignored. To resume
communication,HOLDisbroughthigh,whileSCKislow.
(HOLD should be held high any time this function is not
beingused.)HOLDmaybetiedhighdirectlytoVcc ortied
to Vcc through a resistor. Figure 9 illustrates hold timing
sequence.
SO is the serial data output pin. This pin is used to
transferdataoutofthe25C128/256.Duringareadcycle,
data is shifted out on the falling edge of the serial clock.
SCK: Serial Clock
SCK is the serial clock pin. This pin is used to synchro-
nize the communication between the microcontroller
and the 25C128/256. Opcodes, byte addresses, or data
presentontheSIpinarelatchedontherisingedgeofthe
SCK. Data on the SO pin is updated on the falling edge
of the SCK.
INSTRUCTION SET
Instruction
WREN
WRDI
Opcode
Operation
0000 0110
0000 0100
0000 0101
0000 0001
0000 0011
0000 0010
Enable Write Operations
Disable Write Operations
Read Status Register
Write Status Register
Read Data from Memory
Write Data to Memory
RDSR
WRSR
READ
WRITE
Doc. No. 25088-00 1/01
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