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3633E–DFLASH–10/07

AT26DF321

5.

Device Operation

The AT26DF321 is controlled by a set of instructions that are sent from a host controller, com-

monly referred to as the SPI Master. The SPI Master communicates with the AT26DF321 via the

SPI bus which is comprised of four signal lines: Chip Select (CS), Serial Clock (SCK), Serial

Input (SI), and Serial Output (SO).

The SPI protocol defines a total of four modes of operation (mode 0, 1, 2, or 3) with each mode

differing in respect to the SCK polarity and phase and how the polarity and phase control the

flow of data on the SPI bus. The AT26DF321 supports the two most common modes, SPI

Modes 0 and 3. The only difference between SPI Modes 0 and 3 is the polarity of the SCK signal

when in the inactive state (when the SPI Master is in standby mode and not transferring any

data). With SPI Modes 0 and 3, data is always latched in on the rising edge of SCK and always

output on the falling edge of SCK.

Figure 5-1.

SPI Mode 0 and 3

6.

Commands and Addressing

A valid instruction or operation must always be started by first asserting the CS pin. After the CS

pin has been asserted, the SPI Master must then clock out a valid 8-bit opcode on the SPI bus.

Following the opcode, instruction dependent information such as address and data bytes would

then be clocked out by the SPI Master. All opcode, address, and data bytes are transferred with

the most significant bit (MSB) first. An operation is ended by deasserting the CS pin.

Opcodes not supported by the AT26DF321 will be ignored by the device and no operation will be

started. The device will continue to ignore any data presented on the SI pin until the start of the

next operation (CS pin being deasserted and then reasserted). In addition, if the CS pin is deas-

serted before complete opcode and address information is sent to the device, then no operation

will be performed and the device will simply return to the idle state and wait for the next

operation.

Addressing of the device requires a total of three bytes of information to be sent, representing

address bits A23-A0. Since the upper address limit of the AT26DF321 memory array is

3FFFFFh, address bits A23-A22 are always ignored by the device.

SCK

CS

SI

SO

MSB

LSB

MSB

LSB