AMD Am186TMER ユーザーズマニュアル
Asynchronous Serial Port
11-7
11.2.5
Serial Port Baud Rate Divisor Register (SPBAUD, Offset 88h)
This register (Figure 11-5) specifies a clock divisor for the generation of the serial clock
that controls the serial port. The serial clock rate is 16 times the baud rate of transmission
or reception of data. The SPBAUD register specifies the number of internal processor cycles
in one
that controls the serial port. The serial clock rate is 16 times the baud rate of transmission
or reception of data. The SPBAUD register specifies the number of internal processor cycles
in one
phase (half period) of the 16x serial clock.
If power-save mode is in effect, the baud rate divisor must be reprogrammed to reflect the
new processor clock frequency.
new processor clock frequency.
A general formula for the baud rate divisor is:
BAUDDIV=(Processor Frequency
÷
(32
⋅
Baud Rate))–1
The maximum baud rate is 1/32 of the internal processor clock and is achieved by setting
BAUDDIV=0000h. For a 40-MHz clock, a baud rate of 9600 can be achieved with
BAUDDIV=129 (81h). A 1% error applies.
BAUDDIV=0000h. For a 40-MHz clock, a baud rate of 9600 can be achieved with
BAUDDIV=129 (81h). A 1% error applies.
Figure 11-5
Serial Port Baud Rate Divisor Register (SPBAUD, offset 88h)
The value of SPBAUD at reset is undefined.
Bits 15–0: Baud Rate Divisor (BAUDDIV)—This field specifies the divisor for the internal
processor clock that generates one phase (half period) of the serial clock. The serial clock
operates at 16 times the data transmission or reception baud rate.
processor clock that generates one phase (half period) of the serial clock. The serial clock
operates at 16 times the data transmission or reception baud rate.
Table 11-3 shows baud rate divisors for a range of common baud rates and processor clock
rates.
rates.
Table 11-3
Serial Port Baud Rate Table
Baud Rate
Divisor Based on CPU Clock Rate
20 MHz
25 MHz
33 MHz
40 MHz
300
2082
2603
3471
4165
600
1040
1301
1735
2082
1200
519
650
867
1040
2400
259
324
433
519
4800
129
161
216
259
9600
64
80
107
129
14,400
42
53
71
85
19,200
31
39
53
64
625 Kbaud
0
N/A
N/A
1
781.25 Kbaud
N/A
0
N/A
N/A
1.041 Mbaud
N/A
N/A
0
N/A
1.25 Mbaud
N/A
N/A
N/A
0
15
7
0
BAUDDIV