MC97F2664
192
April 11, 2014 Ver. 1.4
Figure 11.48 USI0/1 SPI Clock Formats when CPHAn=1 (where n = 0 and 1)
When CPHAn=1, the slave begins to drive its MISOn output when SSn goes active low, but the data is not
defined until the first SCKn edge. The first SCKn edge shifts the first bit of data from the shifter onto the MOSIn
output of the master and the MISOn output of the slave. The next SCKn edge causes both the master and slave
to sample the data bit value on their MISOn and MOSIn inputs, respectively. At the third SCKn edge, the USI0/1
shifts the second data bit value out to the MOSIn and MISOn output of the master and slave respectively. When
CPHAn
=1, the slave’s SSn input is not required to go to its inactive high level between transfers.
Because the SPI logic reuses the USI0/1 resources, SPI mode of operation is similar to that of synchronous or
asynchronous operation. An SPI transfer is initiated by checking for the USI0/1 Data Register Empty flag
(DREn=1) and then writing a byte of data to the USInDR Register. In master mode of operation, even if
transmission is not enabled (TXEn=0), writing data to the USInDR register is necessary because the clock SCKn
is generated from transmitter block.
SCKn
(CPOLn=1)
MISOn
MOSIn
SCKn
(CPOLn=0)
/SSn OUT
(MASTER)
BIT7
BIT0
/SSn IN
(SLAVE)
BIT6
BIT1
…
…
BIT2
BIT5
BIT0
BIT7
BIT1
BIT6
SAMPLE
MSB First
LSB First
Содержание MC97F2664
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