MC908AZ32ACFUE Datasheet, PDF (171/324 Pages) Freescale Semiconductor, Inc – Microcontrollers

SCK CYCLE #
FOR REFERENCE
SCK CPOL = 0
Transmission Formats
1
2
3
4
5
6
7
8
SCK CPOL =1
MOSI
FROM MASTER
MISO
FROM SLAVE
MSB
BIT 6
BIT 5
BIT 4
BIT 3
BIT 2
BIT 1
LSB
MSB
BIT 6
BIT 5
BIT 4
BIT 3
BIT 2
BIT 1
LSB
SS TO SLAVE
CAPTURE STROBE
Figure 17-5. Transmission Format (CPHA = 1)
17.5.4 Transmission Initiation Latency
When the SPI is configured as a master (SPMSTR = 1), transmissions are started by a software write to
the SPDR ($0012). CPHA has no effect on the delay to the start of the transmission, but it does affect the
initial state of the SCK signal. When CPHA = 0, the SCK signal remains inactive for the first half of the
first SCK cycle. When CPHA = 1, the first SCK cycle begins with an edge on the SCK line from its inactive
to its active level. The SPI clock rate (selected by SPR1–SPR0) affects the delay from the write to SPDR
and the start of the SPI transmission. (See Figure 17-6). The internal SPI clock in the master is a
free-running derivative of the internal MCU clock. It is only enabled when both the SPE and SPMSTR bits
(SPCR) are set to conserve power. SCK edges occur half way through the low time of the internal MCU
clock. Since the SPI clock is free-running, it is uncertain where the write to the SPDR will occur relative
to the slower SCK. This uncertainty causes the variation in the initiation delay shown in Figure 17-6. This
delay will be no longer than a single SPI bit time. That is, the maximum delay between the write to SPDR
and the start of the SPI transmission is two MCU bus cycles for DIV2, eight MCU bus cycles for DIV8, 32
MCU bus cycles for DIV32, and 128 MCU bus cycles for DIV128.
MC68HC908AZ32A Data Sheet, Rev. 2
Freescale Semiconductor
171