Circuit Board Reference Manual

Functional Description
MCF548x Reference Manual, Rev. 3
Freescale Semiconductor 27-23
27.7.3.1 Baud Rate Generator
The baud rate is the frequency of the DSPI serial communication clock (DSPISCK). The system clock f
sys
is divided by a prescaler (PBR) and scaler (BR) to produce DSPISCK. The PBR and BR fields in the
DCTARn registers select the frequency of DSPISCK by the formula below:
Table 27-15 shows an example of how to compute the baud rate.
27.7.3.2 CS to SCK Delay (t
CSC
)
The CS to SCK delay is the length of time from assertion of the DSPICSn signal to the first DSPISCK
edge. See Figure 27-17 for an illustration of the CS to SCK delay. The PCSSCK and CSSCK fields in the
DCTARn registers select the CS to SCK delay by the formula below:
Eqn. 27-5
Table 27-16 shows an example of how to compute the CS to SCK delay.
27.7.3.3 After DSPISCK Delay (t
ASC
)
The after DSPISCK delay is the length of time between the last edge of DSPISCK and the negation of
DSPICSn. See Figure 27-15 and Figure 27-16 for illustrations of the after DSPISCK delay. The PASC and
ASC fields in the DCTARn registers select the after DSPISCK delay by the formula below:
Eqn. 27-6
Table 27-17 shows an example of how to compute the after DSPISCK delay.
27.7.3.4 Delay after Transfer (t
DT
)
The delay after transfer is the length of time between negation of the DSPICSn signal for a frame and the
assertion of the DSPICSn signal for the next frame. See Figure 27-15 for an illustration of the delay after
transfer. The PDT and DT fields in the DCTARn registers select the delay after transfer by the formula
below:
Table 27-15. Baud Rate Computation Example
PBR Prescaler BR Scaler Fsys Baud Rate
0b00 2 0b0000 2 100 MHz 25 Mb/s
Table 27-16. PCS to DSPISCK Delay Computation Example
PCSSCK Prescaler CSSCK Scaler Fsys
PCS to DSPISCK
Delay
0b01 3 0b0100 32 100 MHz 0.96 us
Table 27-17. After DSPISCK Delay Computation Example
PASC Prescaler ASC Scaler Fsys After DSPISCK Delay
0b01 3 0b0100 32 100 MHz 0.96 us
DSPISCK baud rate =
PBR × BR
f
sys
t
CSC
1
f
sys
--------
PCSSCK CSSCK××=
t
ASC
1
f
sys
--------
PASC× ASC×=