MOTOROLA
9-8
ASYNCHRONOUS SERIAL COMMUNICATIONS INTERFACE
M68HC11
REFERENCE MANUAL
actual 16 x baud rate will be a binary submultiple (
÷
1,
÷
2,
÷
4,...
÷
128) of this pres-
caler output as selected by the SCR[2:0] bits.
Table 9-1
shows the highest baud rates
that result for various combinations of crystal frequency and prescaler select control
bit values. The actual frequency coming out of this prescaler stage is 16 times the
baud rates listed. Since not all of these entries are of practical use, the ones of interest
have been highlighted in bold. This prescaler is set to its fastest rate by default out of
reset and may be changed at any time. Of course, it makes no sense to change the
baud rate while any SCI transfer is in progress.
Table 9-1 Baud-Rate Prescale Selects
RCKB — SCI Baud-Rate Clock Test (Test Modes Only)
This bit is disabled and remains low in any mode other than test or bootstrap modes.
Reset clears this bit. While in test or bootstrap modes, this bit may be written but not
read (reads always return a logic zero). Writing a one to this bit enables a baud-rate
counter test mode where the exclusive-OR of the receiver clock (16 x the baud rate)
and the transmit clock (1 x the baud rate) is driven out the PD1/TxD pin. This control
bit is intended only for factory testing of the MCU.
SCR[2:0] — SCI Baud-Rate Selects
These three bits are used in conjunction with the SCI prescaler bits (see
Table 9-1
) to
specify the SCI baud rate. The prescale bits, SCP[1:0], determine the highest baud
rate; whereas, the SCR[2:0] bits select an additional binary submultiple (
÷
1,
÷
2,
÷
4,
...
÷
128) of this highest baud rate. The result of these two dividers working in series
is the 16 x receiver baud-rate clock.
Table 9-2
shows the SCI baud rates that result
for various settings of SCR[2:0] and the highest baud rates from
Table 9-1
. Since not
all of these entries are of practical use, the ones of interest have been highlighted in
bold. The SCR[2:0] bits are not affected by reset and may be changed at any time. Of
course, it makes no sense to change the baud rate while any SCI transfer is in
progress.
SCP1
SCP0
Division
Factor
Crystal Frequency
4.9152 MHz
Highest Baud Rate
76.80K Baud
25.60K Baud
19.20K Baud
5.908K Baud
1.2288 MHz
Bus Frequency (E clock)
2
23
Hz
8 MHz
4 MHz
3.6864 MH
0
0
1
1
0
1
0
1
1
3
4
131.072K Baud
43.691K Baud
32.768K Baud
10.082K Baud
2.1 MHz
125.000K Baud
41.667K Baud
31.250K Baud
9600 Baud
2 MHz
62.50K Baud
20.833K Baud
15.625K Baud
4800 Baud
1 MHz
57.60K Baud
19.20K Baud
14.40K Baud
4431K Baud
921.6 kHz
13