MC68336/376
QUEUED SERIAL MODULE
MOTOROLA
USER’S MANUAL
Rev. 15 Oct 2000
9-21
Before QSPI operation is initiated, QSM register PQSPAR must be written to assign
necessary pins to the QSPI. The pins necessary for slave mode operation are MISO
and MOSI, SCK, and PCS0/SS. MISO is used for serial data output in slave mode, and
MOSI is used for serial data input. Either or both may be necessary, depending on the
particular application. SCK is the serial clock input in slave mode. Assertion of the
active-low slave select signal SS initiates slave mode operation.
Before slave mode operation is initiated, DDRQS must be written to direct data flow
on the QSPI pins used. Configure the MOSI, SCK and PCS0/SS pins as inputs. The
MISO pin must be configured as an output.
After pins are assigned and configured, write data to be transmitted into transmit RAM.
Command RAM is not used in slave mode, and does not need to be initialized. Set the
queue pointers, as appropriate.
When SPE is set and MSTR is clear, a low state on the slave select PCS0/SS pin
begins slave mode operation at the address indicated by NEWQP. Data that is
received is stored at the pointer address in receive RAM. Data is simultaneously
loaded into the data serializer from the pointer address in transmit RAM and transmit-
ted. Transfer is synchronized with the externally generated SCK. The CPHA and
CPOL bits determine upon which SCK edge to latch incoming data from the MISO pin
and to drive outgoing data from the MOSI pin.
Because the command RAM is not used in slave mode, the CONT, BITSE, DT, DSCK,
and peripheral chip-select bits have no effect. The PCS0/SS pin is used only as an
input.
The SPBR, DT and DSCKL fields in SPCR0 and SPCR1 bits are not used in slave
mode. The QSPI drives neither the clock nor the chip-select pins and thus cannot con-
trol clock rate or transfer delay.
Because the BITSE option is not available in slave mode, the BITS field in SPCR0
specifies the number of bits to be transferred for all transfers in the queue. When the
number of bits designated by BITS[3:0] has been transferred, the QSPI stores the
working queue pointer value in CPTQP, increments the working queue pointer, and
loads new transmit data from transmit RAM into the data serializer. The working queue
pointer address is used the next time PCS0/SS is asserted, unless the CPU32 writes
to NEWQP first.
The QSPI shifts one bit for each pulse of SCK until the slave select input goes high. If
SS goes high before the number of bits specified by the BITS field is transferred, the
QSPI resumes operation at the same pointer address the next time SS is asserted.
The maximum value that the BITS field can have is 16. If more than 16 bits are trans-
mitted before SS is negated, pointers are incremented and operation continues.
The QSPI transmits as many bits as it receives at each queue address, until the BITS
value is reached or SS is negated. SS does not need to go high between transfers as
the QSPI transfers data until reaching the end of the queue, whether SS remains low
or is toggled between transfers.