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Communication Processor Module
16.8.3 UART Baud Rate Examples
For synchronous communication using the internal baud rate generator, the BRGOx output
clock must never be faster than the system frequency divided by 2. So, with a 25MHz
system frequency, the maximum BRGOx output clock rate is 12.5MHz. You must program
the UART to 16× oversampling when using a serial communication controller in UART
mode. On the MPC823e, 8× and 32× options are also available. Assuming 16×
oversampling is chosen in the UART, a data rate of 25MHz ÷ 16 = 1.5625Mb/sec is the
maximum possible UART speed. Putting this together, the following formula must be used
to calculate the bit rate based on a particular baud rate generator configuration for a UART:
async baud rate = (BRGCLK, CLK2 or CLK4) ÷ (clock divider + 1) ÷ (1 or 16
The following table lists typical bit rates of asynchronous communication. The internal clock
rate is assumed to be 16× the baud rate.
Table 16-23. Typical Baud Rates of Asynchronous Communication
BAUD
20
RATES
DIV16
DIV
50
1
1561
75
1
1040
150
1
520
300
1
259
600
0
2082
1200
0
1040
2400
0
520
4800
0
259
9600
0
129
19200
0
64
38400
0
32
57600
0
21
115200
0
10
NOTE: All values are decimal.
16-162
depending on the DIV16 bit) ÷ (8, 16, or 32 according to the
RDCR and TDCR fields in the GSMR_L register).
MPC823e SYSTEM FREQUENCY (MHz)
25
ACTUAL
DIV16
DIV
FREQUENCY
50.02
1
1952
75.05
1
1301
149.954
1
650
300.48
1
324
600.09
0
2603
1200.7
0
1301
2399.2
0
650
4807.7
0
324
9615.4
0
162
19231
0
80
37879
0
40
56818
0
26
113636
0
13
MPC823e REFERENCE MANUAL
24.5760
ACTUAL
DIV16
DIV
FREQUENCY
50
1
1919
75
1
1279
150
1
639
300.5
1
319
600
0
2559
1200
0
1279
2400.1
0
639
4807.69
0
319
9585.9
0
159
19290
0
79
38109
0
39
57870
0
26
111607
0
12
ACTUAL
FREQUENCY
50
75
150
300
600
1200
2400
4800
9600
19200
38400
56889
118154
MOTOROLA
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