Baud Rate Generators (BRGs)
7-108
MC68360 USER’S MANUAL
MOTOROLA
DIV16—BRG Clock Prescaler Divide by 16
The BRG clock prescaler bit selects a divide-by-1 or divide-by-16 prescaler for the clock
divider input.
7.9.3 UART Baud Rate Examples
For synchronous communication using internal baud rate generator, the BRGO output clock
must never be faster than the SyncCLK frequency divided by 2. Produced in the clock syn-
thesizer in the SIM60, SyncCLK is the frequency used internally by the synchronization cir-
cuitry in the SCCs, SMCs, and SPI. It defaults to the main system frequency (e.g., 25 MHz).
Thus, with a 25-MHz system where the SyncCLK is the same as the main system frequency,
the maximum BRGO output clock rate is 12.5 MHz.
The user should program the UART to 16
×
oversampling (RDCR and TDCR bits in the gen-
eral SCC mode register) when using the SCC as a UART. (On the QUICC, 8
×
and 32
×
options are also available.) Assuming 16
×
oversampling is chosen in the UART, a data rate
of 25MHz
÷
16 = 1.5625 Mbits/sec is the maximum possible UART speed.
Putting this together, the following formula for calculating the bit rate based on a particular
BRG configuration for a UART: async baud rate = (BRGCLK or CLK2 or CLK6)
÷
(clock
divider + 1)
÷
(1 or 16 depending on the DIV16 bit)
÷
(8 or 16 or 32 according to the RDCR
and TDCR bits in the general SCC mode register).
Table 7-3 lists examples of typical bit rates of asynchronous communication. Note that for
this mode, the internal clock rate is assumed to be 16
×
the baud rate.
NOTE: All values are decimal.
Table 7-4. Typical Baud Rates of Asynchronous Communication
QUICC System Frequency (MHz)
20
25
24.5760
Baud
Rates
Div16
Div
Actual
Frequency
Div16
Div
Actual
Frequency
Div16
Div
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