11-8
DSP56853/854/855/857/858 User’s Manual
MOTOROLA
Preliminary
Operating Modes
11
Figure 11-3. Full-Duplex Master/Slave Connections
11.6.2 Slave Mode
The SPI operates in the Slave mode when the SPMSTR bit is clear. While in the Slave
mode, the SCLK pin is the input for the serial clock from the master DSP. Before a data
transmission occurs, the SS pin of the slave SPI must be at logic zero. SS must remain low
until the transmission is complete or a Mode Fault error occurs.
Note:
The SPI must be enabled (SPE = 1) for slave transmissions to be received.
Note:
Data in the transmitter shift register will be unaffected by SCLK transitions in
the event the SPI is operating as a slave but is deselected.
In a slave SPI module, data enters the Shift Register under the control of the serial clock,
SCLK, from the Master SPI module. After a full length data transmission enters the Shift
Register of a slave SPI, it transfers to the SPDRR and the SPRF bit in the SPSCR is set. If
the Receive Data (SPDRR) register, and the SPRF bit is set. in the SPSCR has been set, a
receive interrupt is also generated. To prevent an overflow condition, slave software must
read the SPDRR before another full length data transmission enters the Shift Register.
The maximum frequency of the SCLK for an SPI configured as a slave is the bus clock
speed. The bus clock speed is twice as fast as the fastest master SCLK potential
generation. Frequency of the SCLK for an SPI configured as a slave does not have to
correspond to any particular SPI baud rate. The baud rate only controls the speed of the
SCLK generated by an SPI configured as a master. Therefore, the frequency of the SCLK
for an SPI configured as a slave can be any frequency less than or equal to the bus speed.
When the master SPI starts a transmission, the data in the slave Shift Register begins
shifting out on the MISO pin. The slave can load its Shift Register with new data for the
next transmission by writing to its Transmit Data Register. The slave must write to its
Transmit Data Register at least one bus cycle before the master begins the next
Shift Register
Baud Rate
Generator
Master DSP
Slave DSP
VDD
MOSI
MISO
SCLK
SS