MPC5553/MPC5554 Microcontroller Reference Manual, Rev. 5
20-54
Freescale Semiconductor
20.4.6.5
Peripheral Chip Select Strobe Enable (PCSS)
The PCSS signal provides a delay to allow the PCS signals to settle after transitioning thereby avoiding
glitches. When the DSPI is in master mode and PCSSE bit is set in the DSPI
x
_MCR, PCSS provides a
signal for an external demultiplexer to decode the PCS[0:4] signals into as many as 32 glitch-free PCS
signals.
shows the timing of the PCSS signal relative to PCS signals.
Figure 20-35. Peripheral Chip Select Strobe Timing
The delay between the assertion of the PCS signals and the assertion of PCSS is selected by the PCSSCK
field in the DSPI
x
_CTAR based on the following formula:
At the end of the transfer the delay between PCSS negation and PCS negation is selected by the PASC field
in the DSPI
x
_CTAR based on the following formula:
shows an example of the computed t
PCSSCK
delay.
shows an example of the computed the t
PASC
delay
.
20.4.7
Transfer Formats
The SPI serial communication is controlled by the serial communications clock (SCK) signal and the PCS
signals. The SCK signal provided by the master device synchronizes shifting and sampling of the data by
the SIN and SOUT pins. The PCS signals serve as enable signals for the slave devices.
When the DSPI is the bus master, the CPOL and CPHA bits in the DSPI clock and transfer attributes
registers (DSPI
x
_CTAR
n
) select the polarity and phase of the serial clock, SCK. The polarity bit selects
Table 20-27. Peripheral Chip Select Strobe Assert Computation Example
PCSSCK
Prescaler
f
SYS
Delay before Transfer
0b11
7
100 MHz
70.0 ns
Table 20-28. Peripheral Chip Select Strobe Negate Computation Example
PASC
Prescaler
f
SYS
Delay after Transfer
0b11
7
100 MHz
70.0 ns
PCSS
PCSx
t
PCSSCK
t
PASC
tPCSSCK =
PCSSCK
f
SYS
1
tPASC =
PASC
f
SYS
1
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