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Appendix A: Supported Rabbit 4000 Baud Rates

This table contains divisors to put into TATxR registers. All frequencies that allow 57600 baud up to 30MHz are shown (as well as a few higher frequencies). All of the divisors listed here were calculated with the default equation given on the next page.

Crystal Freq. (MHz)
2400 baud
9600 baud
19200 baud
57600 baud
115200 baud
230400 baud
460800 baud
1.8432
23
5
2
0
-
-
3.6864
47
11
5
1
0
-
-
5.5296
71
17
8
2
-
-
-
7.3728
95
23
11
3
1
0
-
9.2160
119
29
14
4
-
-
-
11.0592
143
35
17
5
2
-
-
12.9024
167
41
20
6
-
-
-
14.7456
191
47
23
7
3
1
0
16.5888
215
53
26
8
-
-
-
18.4320
239
59
29
9
4
-
-
20.2752
65
32
10
-
-
-
22.1184
*
71
35
11
5
2
-
23.9616
*
77
38
12
-
-
-
25.8048
*
83
41
13
6
-
-
27.6480
*
89
44
14
-
-
-
29.4912
*
95
47
15
7
3
1
36.8640
*
119
59
19
9
4
-
44.2368
*
143
71
23
11
5
2
51.6096
*
167
83
27
13
6
-
58.9824
*
191
95
31
15
7
3

1

Baud rate is not available at given frequency.

2

Baud rate is available with further BIOS modification.

The default equation for the divisor is:

If the divisor is not an integer value, that baud rate is not available for that frequency (identified by a "-" in the table). If the divisor is above 255, that baud rate is not available without further BIOS modification (identified by a "*" in the table). To allow that baud rate, you need to clock the desired serial port via timer A1 (by default they run off the peripheral clock / 2), then scale down timer A to make the serial port divisor fall below 256.

Timer A can be clocked by the peripheral clock (PCLK) in addition to the default, which is the peripheral clock/2 (PCLK/2). Furthermore, the asynchronous serial port data rate can be 8x the clock in addition to the default of 16x the clock. Therefore, in addition to the equation above, the following equations may be used to find the asynchronous divisor for a given clock frequency.

Timer A clocked by PCLK/2, serial data rate = 16 x clock

Timer A clocked by PCLK, serial data rate = 16 x clock:

Timer A clocked by PCLK/2, serial data rate = 8 x clock:

Timer A clocked by PCLK, serial data rate = 8 x clock:

Rabbit 4000
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