MPC555
/
MPC556
QUEUED SERIAL MULTI-CHANNEL MODULE
MOTOROLA
USER’S MANUAL
Rev. 15 October 2000
14-35
Giving SPBR a value of zero or one disables the baud rate generator. SCK is disabled
and assumes its inactive state. At reset, the SCK baud rate is initialized to one eighth
of the IMB clock frequency.
provides some example SCK baud rates with a 40-MHz IMB clock.
14.7.5.3 Delay Before Transfer
The DSCK bit in each command RAM byte inserts either a standard (DSCK = 0) or
user-specified (DSCK = 1) delay from chip-select assertion until the leading edge of
the serial clock. The DSCKL field in SPCR1 determines the length of the user-defined
delay before the assertion of SCK. The following expression determines the actual de-
lay before SCK:
where DSCKL is in the range from 1 to 127.
NOTE
A zero value for DSCKL causes a delay of 128 IMB clocks, which
equals 3.2 µs for a 40-MHz IMB clock. Because of design limits, a
DSCKL value of one defaults to the same timing as a value of two.
When DSCK equals zero, DSCKL is not used. Instead, the PCS valid-to-SCK transi-
tion is one-half the SCK period.
14.7.5.4 Delay After Transfer
Delay after transfer can be used to provide a peripheral deselect interval. A delay can
also be inserted between consecutive transfers to allow serial A/D converters to com-
plete conversion. Writing a value to the DTL field in SPCR1 specifies a delay period.
The DT bit in each command RAM byte determines whether the standard delay period
Table 14-21 Example SCK Frequencies with a 40-MHz IMB Clock
Division Ratio
SPBR Value
SCK
Frequency
4
2
10.00 MHz
6
3
6.67 MHz
8
4
5.00 MHz
14
7
2.86 MHz
28
14
1.43 MHz
58
29
689 kHz
280
140
143 kHz
510
255
78.43 kHz
PCS to SCK Delay
DSCKL
f
SYS
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Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
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