NXP Semiconductors KL25 Series - SPI Baud Rate Generation

SS OUT
SS IN
(SLAVE)
(MASTER)
(SLAVE OUT)
MISO
MSB FIRST
LSB FIRST
MOSI
(MASTER OUT)
(MISO OR MOSI)
SAMPLE IN
SPSCK
(CPOL = 1)
SPSCK
(CPOL = 0)
BIT TIME #
(REFERENCE)
BIT 7
BIT 0
BIT 6
BIT 1
BIT 2
BIT 5
BIT 0
BIT 7
BIT 1
BIT 6
1
2
6
7
8
...
...
...
Figure 37-24. SPI Clock Formats (CPHA = 0)
When CPHA = 0, the slave begins to drive its MISO output with the first data bit value
(MSB or LSB depending on LSBFE) when SS goes to active low. The first SPSCK edge
causes both the master and the slave to sample the data bit values on their MISO and
MOSI inputs, respectively. At the second SPSCK edge, the SPI shifter shifts one bit
position which shifts in the bit value that was just sampled and shifts the second data bit
value out the other end of the shifter to the MOSI and MISO outputs of the master and
slave, respectively. When CPHA = 0, the slave's SS input must go to its inactive high
level between transfers.
37.4.6 SPI Baud Rate Generation
As shown in the following figure, the clock source for the SPI baud rate generator is the
bus clock. The three prescale bits (SPPR2:SPPR1:SPPR0) choose a prescale divisor of 1,
2, 3, 4, 5, 6, 7, or 8. The three rate select bits (SPR3:SPR2:SPR1:SPR0) divide the output
of the prescaler stage by 2, 4, 8, 16, 32, 64, 128, 256, or 512 to get the internal SPI master
mode bit-rate clock.
Functional Description
KL25 Sub-Family Reference Manual, Rev. 3, September 2012
676 Freescale Semiconductor, Inc.