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signals in CPU bus mode (including being active low). For a read access, all four of these bits should be 1.
At the same time, the slave transmits the byte enable values of the previous access on
SPI_MISO.
The next 24 bits the master transmits on
SPI_MOSI are address bits, starting from A24 (MSB) and ending
with A1 (LSB). At the same time, the slave transmits the address bits of the previous access on
SPI_MISO.
Next the master transmits 8 don’t care bits on
SPI_MOSI. During these clock periods the slave transmits 8
th bit is a status bit that indicates whether
the current read access was completed successfully (1) or is still in progress (0). Status=0 indicates that
the current operation has not yet completed and that the status command must follow (see section
Status=1 indicates that the current read was completed successfully and 32 bits of data follow on
SPI_MISO, starting from D31 (MSB) and ending with D0 (LSB). During these 32 clock cycles, the master
Status=0 indicates that the current read was not completed and that the status command must follow (see
section
10.3.4.3). During the next 32 clock cycles both the master and the slave must transmit don’t-care
bits to complete the read command. These 32 bits should be ignored.
The master ends the write access by deasserting
SPI_SEL_N.
The total number of
SPI_CLK cycles for a read command is 72.
Table 10-3. SPI_ Read Command Sequence
Bit Number
SPI_MOSI
SPI_MISO
1
Reserved
2–3
opcode 10 (read)
Previous access opcode
4
1
Previous access H_WR_BE3_N value
5
1
Previous access H_WR_BE2_N value
6
1
Previous access H_WR_BE1_N value
7
1
Previous access H_WR_BE0_N value
8
Reserved
9–32
Address [24 to 1]
Previous access Address [24 to 1]
33–39
Don’t care
Idle (7 bits)
40
Don’t care
Status bit: 1=access has finished, 0=access has not finished
41–72
Don’t care
Data (32 bits)
10.3.4.3 Status Command
The status command differs from read or write commands, since it does not initiate an internal access. Usually a
status command follows a read or write command that was not completed as described above.
The SPI status command proceeds as follows:
The SPI master (CPU) starts a status command by asserting
SPI_SEL_N (low).
Then, during each
SPI_CLK cycle a
SPI_MOSI data bit is transmitted by the master (CPU), while a
SPI_MISO bit is transmitted by the slave (the device).