Rainbow Electronics AT26DF081A - Manual (Page 6)

6
3600H–DFLASH–11/2012
AT26DF081A
5. Device Operation
The AT26DF081A 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 AT26DF081A 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 AT26DF081A 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 AT26DF081A 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
deasserted before complete opcode and address information is sent to the device, then no oper-
ation 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 AT26DF081A memory array is
0FFFFFh, address bits A23 - A20 are always ignored by the device.
SCK
CS
SI
SO
MSB LSB
MSB LSB