
MOTOROLA
QUEUED SERIAL MODULE
MC68331
6-18
USER’S MANUAL
6
or
Giving SPBR a value of zero or one disables the baud rate generator. SCK is disabled
and assumes its inactive state value.
The DSCK field in command RAM determines the delay period from chip-select asser-
tion until the leading edge of the serial clock. The DSCKL field in SPCR1 determines
the period of delay before the assertion of SCK. The following expression determines
the actual delay before SCK:
where DSCKL equals {1, 2, 3,..., 127}.
When DSCK equals zero, DSCKL is not used. Instead, the PCS valid-to-SCK transi-
tion is one-half the DSCK period.
There are two transfer length options. The user can choose a default value of eight
bits, or a programmed value of eight to sixteen bits, inclusive. The programmed value
must be written into the BITS field in SPCR0. The BITSE field in command RAM de-
termines whether the default value (BITSE = 0) or the BITS value (BITSE = 1) is used.
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. There are two transfer delay options. The user can choose to delay
a standard period after serial transfer is complete or can specify a delay period. Writing
a value to the DTL field in SPCR1 specifies a delay period. The DT bit in command
RAM determines whether the standard delay period (DT = 0) or the specified delay pe-
Table 6-3 BITS Encoding
BITS
Bits per Transfer
0000
16
0001
Reserved
0010
Reserved
0011
Reserved
0100
Reserved
0101
Reserved
0110
Reserved
0111
Reserved
1000
8
1001
9
1010
10
1011
11
1100
12
1101
13
1110
14
1111
15
SPBR
System Clock
2
SCK
×
()
Baud Rate Desired
()
×
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PCS to SCK Delay
DSCKL
System Clock Frequency
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