Serial Interface
MPC885 PowerQUICC Family Reference Manual, Rev. 2
20-40
Freescale Semiconductor
20.4.3
UART Baud Rate Examples
For synchronous communication using the internal BRG, the BRGO output clock must not exceed the
system frequency divided by 2. So, with a 25-MHz system frequency, the maximum BRGO rate is 12.5
MHz. Program the UART to 16
×
oversampling when using the SCC as a UART. Rates of 8
×
and 32
×
are
also available. Assuming 16
×
oversampling is chosen in the UART, the maximum data rate is 25 MHz ÷
16 = 1.5625 Mbps. Keeping the above in mind, use the following formula to calculate the bit rate based
on a particular BRG configuration for a UART:
async baud rate = (BRGCLK or CLK2 or CLK6) ÷ (1 or 16 according to BRGC
x
[DIV16]) ÷ (clock
d 1) ÷ (8, 16, or 32 according to GSMR_L[TDCR, RDCR] in the general SCC mode register
low)
Table 20-14
lists typical bit rates of asynchronous communication. Notice that here the internal clock rate
is assumed to be 16
×
the baud rate; that is, GSMR_L[TDCR] = GSMR_L[RDCR] = 0b10.
For synchronous communication, the internal clock is identical to the baud rate output. To get the preferred
rate, select the system clock according to the following:
sync baud rate = (BRGCLK or CLK2 or CLK6) ÷ (1 or 16 according to BRGC
x
[DIV16]) ÷ (clock divider
+ 1)
For example, to get a rate of 64 kbps, the system clock can be 24.96 MHz, DIV16 = 0, and the clock
divider = 389.
Table 20-14. Typical Baud Rates for Asynchronous Communication
Baud
Rate
System Frequency
20 MHz
25 MHz
24.5760 MHz
Div16
CD
Actual
Frequency
Div16
CD
Actual
Frequency
Div16
CD
Actual
Frequency
50
1
1561
50.02
1
1952
50
1
1919
50
75
1
1040
75.05
1
1301
75
1
1279
75
150
1
520
149.954
1
650
150
1
639
150
300
1
259
300.48
1
324
300.5
1
319
300
600
0
2082
600.09
0
2603
600
0
2559
600
1200
0
1040
1200.7
0
1301
1200
0
1279
1200
2400
0
520
2399.2
0
650
2400.1
0
639
2400
4800
0
259
4807.7
0
324
4807.69
0
319
4800
9600
0
129
9615.4
0
162
9585.9
0
159
9600
19200
0
64
19231
0
80
19290
0
79
19200
38400
0
32
37879
0
40
38109
0
39
38400
57600
0
21
56818
0
26
57870
0
26
56889
115200
0
10
113636
0
13
111607
0
12
118154
Summary of Contents for PowerQUICC MPC870
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