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SSPFSSOUT/
SSPFSSIN
SSPTXD
4 to 16 bits
output data
SSPCLKOUT/
SSPCLKIN
LSB
nSSPOE
LSB
0
LSB
MS
B
MSB
LSB
SSPRXD
8-bit control
LSB
MSB
LSB
MSB
Figure 66.
National Semiconductor Microwire frame format, continuous transfers
Master and Slave configurations
Figure 67 shows how a PrimeCell SSP (PL022) configured as master, interfaces to a Motorola
SPI slave. The SPI Slave Select (SS) signal is permanently tied LOW and configures it as a slave.
Similar to the above operation, the master can broadcast to the slave through the master
PrimeCell SSP SSPTXD line. In response, the slave drives its SPI MISO port onto the SSPRXD line
of the master.
0V
0V
0V
SSPTXD
nSSPOE
SSPRXD
SSPFSSIN
SSPCLKOUT
nSSPCTLOE
SSPCLKIN
MOSI
SCK
MISO
SS
SSPFSSOUT
Figure 67. PrimeCell SSP master coupled to an SPI slave
Figure 68 shows a Motorola SPI configured as a master and interfaced to an instance of a
PrimeCell SSP (PL022) configured as a slave. In this case, the slave Select Signal (SS) is
permanently tied HIGH to configure it as a master. The master can broadcast to the slave
through the master SPI MOSI line. In response, the slave drives its nSSPOE signal LOW.
This enables its SSPTXD data onto the MISO line of the master.