6.3 Queued Serial Peripheral Interface
The queued serial peripheral interface (QSPI) communicates with external devices
through a synchronous serial bus. The QSPI is fully compatible with SPI systems
found on other Motorola products, but has enhanced capabilities. The QSPI can per-
form full duplex three-wire or half duplex two-wire transfers. A variety of transfer rate,
clocking, and interrupt-driven communication options are available.
Serial transfer of any number of bits from eight to sixteen can be specified. Program-
mable transfer length simplifies interfacing to a number of devices that require different
data lengths.
An inter-transfer delay of 17 to 8192 system clocks can be specified (default is 17 sys-
tem clocks). Programmable delay simplifies the interface to a number of devices that
require different delays between transfers.
A dedicated 80-byte RAM is used to store received data, data to be transmitted, and
a queue of commands. The CPU can access these locations directly. Serial peripher-
als can be treated like memory-mapped parallel devices.
The command queue allows the QSPI to perform up to 16 serial transfers without CPU
intervention. Each queue entry contains all the information needed by the QSPI to in-
dependently complete one serial transfer.
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A pointer identifies the queue location containing the command for the next serial
transfer. Normally, the pointer address is incremented after each serial transfer, but
the CPU can change the pointer value at any time. Multiple-task support can be pro-
vided by segmenting the queue.
The QSPI has four peripheral chip-select pins. Chip-select signals simplify interfacing
by reducing CPU intervention. If chip-select signals are externally decoded, 16 inde-
pendent select signals can be generated. Each chip-select pin can drive up to four in-
dependent peripherals, depending on loading.
Wraparound operating mode allows continuous execution of queued commands. In
wraparound mode, newly received data replaces previously received data in receive
RAM. Wraparound can simplify the interface with A/D converters by continuously up-
dating conversion values stored in the RAM.
Continuous transfer mode allows simultaneous transfer of an uninterrupted bit stream.
Any number of bits in a range from 8 to 256 can be transferred without CPU interven-
tion. Longer transfers are possible, but minimal CPU intervention is required to prevent
loss of data. A standard delay of 17 system clocks is inserted between each queue
entry transfer.
MC68331
QUEUED SERIAL MODULE
MOTOROLA
6-5
USER’S MANUAL
MOTOROLA [ MOTOROLA ]
