Uart Baud Rate Examples - Freescale Semiconductor MPC850 User Manual

Baud Rate Generators (BRGs)
performing the autobaud function must be connected to that SCC's autobaud 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, first clear
BRGCn[ATB] and enable the BRG receive clock to the highest frequency. Then,
immediately before the autobaud process starts (after device initialization), set
BRGCn[ATB].

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 CLK4) ÷ (1 or 16 according to BRGCx[DIV16])
÷ (clock divider + 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
GSMR_L[TDCR] = GSMR_L[RDCR] = 0b10.
Table 20-14. Typical Baud Rates for Asynchronous Communication
Baud
20 MHz
Rate
Div16 CD Actual Frequency
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
is assumed to
System Frequency
Div16 CD
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
MPC850 Family User's Manual
be 16× the baud
25 MHz
Actual Frequency Div16
50 1
75 1
150 1
300.5 1
600 0
1200 0
2400.1 0
4807.69 0
9585.9 0
19290 0
38109 0
rate; that is,
24.5760 MHz
CD Actual Frequency
1919 50
1279 75
639 150
319 300
2559 600
1279 1200
639 2400
319 4800
159 9600
79 19200
39 38400