MC68HC05V12 Datasheet, PDF (105/246 Pages) Freescale Semiconductor, Inc – HCMOS Microcontreller Unit

Freescale Semiconductor, Inc.
Serial Peripheral Interface (SPI)
SPI Functional Description
10.5 SPI Functional Description
Figure 10-2 shows a block diagram of the serial peripheral interface
circuitry. When a master device transmits data to a slave via the MOSI
line, the slave device responds by sending data to the master device via
the master’s MISO line. This implies full duplex transmission with both
data out and data in synchronized with the same clock signal. Thus, the
byte transmitted is replaced by the byte received and eliminates the
need for separate transmit-empty and receive-full status bits. A single
status bit (SPIF) is used to signify that the I/O operation has been
completed.
The SPI is double buffered on read, but not on write. If a write is
performed during data transfer, the transfer occurs uninterrupted, and
the write will be unsuccessful. This condition will cause the write collision
(WCOL) status bit in the SPSR to be set. After a data byte is shifted, the
SPIF flag of the SPSR is set.
In the master mode, the SCK pin is an output. It idles high or low,
depending on the CPOL bit in the SPCR, until data is written to the shift
register, at which point eight clocks are generated to shift the eight bits
of data and then SCK goes idle again.
In a slave mode, the slave select start logic receives a logic low from the
SS pin and a clock at the SCK pin. Thus, the slave is synchronized with
the master. Data from the master is received serially at the MOSI line
and loads the 8-bit shift register. After the 8-bit shift register is loaded, its
data is parallel transferred to the read buffer. During a write cycle, data
is written into the shift register, then the slave waits for a clock train from
the master to shift the data out on the slave’s MISO line.
Figure 10-3 illustrates the MOSI, MISO, SCK, and SS master-slave
interconnections.
MC68HC05V12 — Rev. 2.0
Serial Peripheral Interface (SPI)
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Technical Data