Freescale Semiconductor HCS08 Series, Reference Manual

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16-Bit Serial Peripheral Interface (S08SPI16V1)
MC9S08JS16 MCU Series Reference Manual, Rev. 4
Freescale Semiconductor 205
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.
13.4.6 SPI Baud Rate Generation
As shown in Figure 13-15 , 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 (SPR2:SPR1:SPR0) divide the output of the prescaler stage by 2, 4, 8, 16, 32, 64, 128, or 256
to get the internal SPI master mode bit-rate clock.
The baud rate generator is activated only when the SPI is in the master mode and a serial transfer is taking
place. In the other cases, the divider is disabled to decrease I
DD
current.
The baud rate divisor equation is as follows:
The baud rate can be calculated with the following equation:
Figure 13-15. SPI Baud Rate Generation
13.4.7 Special Features
13.4.7.1 SS Output
The SS output feature automatically drives the SS pin low during transmission to select external devices
and drives it high during idle to deselect external devices. When SS output is selected, the SS output pin
is connected to the SS input pin of the external device.
The SS output is available only in master mode during normal SPI operation by asserting the SSOE and
MODFEN bits as shown in
Table 13-2 .
The mode fault feature is disabled while SS output is enabled.
BaudRateDivisor SPPR 1
+
( )
2
•
SPR 1 +
( )
=
Baud Rate BusClock BaudRateDivisor
⁄
=
DIVIDE BY
2, 4, 8, 16, 32, 64, 128, or 256
DIVIDE BY
1, 2, 3, 4, 5, 6, 7, or 8
PRESCALER BAUD RATE DIVIDER
SPPR2:SPPR1:SPPR0 SPR2:SPR1:SPR0
BUS CLOCK
MASTER
SPI
BIT RATE