21-42
MPC860 PowerQUICC UserÕs Manual
MOTOROLA
Part V. The Communications Processor Module
Note that the SCC associated with this BRG must be programmed to UART mode and
select the 16
´
option for TDCR and RDCR in the general SCC mode register low. Input
frequencies such as 1.8432, 3.68, 7.36, and 14.72 MHz should be used. The SCC
performing the autobaud function must be connected to that SCCÕs BRG; that is, SCC2
must be clocked by BRG2, and so on.
Also, to detect an autobaud lock and generate an interrupt, the SCC must receive three full
Rx clocks from the BRG before the autobaud process begins. To do this, Þrst clear
BRGCn[ATB] and enable the BRG Rx clock to the highest frequency. Then, immediately
before the autobaud process starts (after device initialization), set BRGCn[ATB].
21.4.3 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 conÞguration for a UART:
async baud rate = (BRGCLK or CLK2 or CLK6)
¸
(1 or 16 according to BRGCx[DIV16])
¸
(clock d 1)
¸
(8,
16, or 32 according to GSMR_L[TDCR, RDCR] in the general SCC mode register low)
Table 21-14 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.
Table 21-14. 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
Содержание MPC860 PowerQUICC
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Страница 226: ...8 32 MPC860 PowerQUICC UserÕs Manual MOTOROLA Part II PowerPC Microprocessor Module ...
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Страница 274: ...III iv MPC860 PowerQUICC UserÕs Manual MOTOROLA Part III Configuration ...
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Страница 325: ...MOTOROLA Part IV Hardware Interface IV v Part IV Hardware Interface ...
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