AM186ES Advanced Micro Devices, AM186ES Datasheet - Page 159

• Manufacturer Part Number: AM186ES
• Description: microcontrollers provide a low-cost/ high-performance solution for embedded system designers who wish to use the x86 architecture.
• Manufacturer: Advanced Micro Devices

10.2.5
Table 10-5
Serial Port 0/1 Baud Rate Divisor Registers
(SP0BAUD/SP1BAUD, Offset 88h/18h)
Each of the asynchronous serial ports has a baud rate divisor register, so the two ports can
operate at different rates.
These registers (Figure 10-7) specify a clock divisor for the generation of the serial clock
that controls the associated serial port. The baud rate divisor 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. Since power-save mode is automatically exited when an
interrupt is taken, serial port transmits and receives may be corrupted if the serial port is in
use and interrupts are enabled during power-save mode.
A general formula for the baud rate divisor is:
BAUDDIV =
(Processor Frequency
The maximum baud rate is 1/16 of the internal processor clock and is achieved by setting
BAUDDIV=0001h. This results in a baud rate of 2500 Kb at 20 MHz, 1562.5 Kb at 25MHz,
2062.5 Kb at 33 MHz, and 1250 Kb at 40 MHz. A BAUDDIV setting of zero results in no
transmission or reception of data.
The serial port receiver can tolerate a 3.0% overspeed and 2.5% underspeed baud rate
deviance.
Common Baud Rates
Baud
Rate
300
600
1050
1200
1800
2400
4800
7200
9600
19200
28800
38400
56000
57600
76800
115200
Divisor Based on CPU Clock Rate
20 MHz
4166
2083
1190
1041
694
520
260
173
130
65
43
33
22
22
16
10
25 MHz
5208
2604
1488
1302
868
651
325
217
162
81
54
40
28
27
20
13
Asynchronous Serial Port
(16 baud rate))
33 MHz
6875
3437
1964
1718
1145
859
429
286
214
107
71
53
36
35
26
18
40 MHz
8333
4166
2380
2083
1388
1041
520
347
260
130
86
65
45
43
32
22
10-13