Baud-Rate Generators (BRGs)
MPC8260 PowerQUICC II Family Reference Manual, Rev. 2
Freescale Semiconductor 17-5
17.3 UART Baud Rate Examples
For synchronous communication using the internal BRG, the BRGO must not exceed the BRG input clock
divided by 2. Therefore, with a BRG input clock of 66MHz (generated using an external clock source: refer
to BRGCx[EXTC]), the maximum BRGO rate is 33MHz. 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 66 MHz ÷ 16 = 4.125 Mbps. Keeping the above in mind, use the
following formula to calculate the bit rate based on a particular BRG configuration for a UART:
Table17-3 lists typical bit rates of asynchronous communication. Note that here the internal clock rate is
assumed to be 16× the baud rate; that is, GSMRx_L[TDCR] = GSMRx_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:
Table17 -3. Typical Baud Rates for Asynchronous Communication
Baud Rate
Using a 66-MHz BRG Input Clock
BRGC
x
[DIV16] BRGC
x
[CD] Actual Frequency (Hz)
75 1 3436 75.01
150 1 1718 149.98
300 1 858 300.13
600 1 429 599.56
1200 0 3436 1200.2
2400 0 1718 2399.7
4800 0 858 4802.1
9600 0 429 9593.0
19,200 0 214 19,186
38,400 0 106 38,551
57,600 0 71 57,292
115,200 0 35 114,583
460,000 0 8 458,333
Async Baud Rate BRGCLK or External Clock Source
Prescale Divider()Clock Divider + 1()Sampling Rate()
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=
BRGCx[EXTC]
BRGCx[DIV16]()BRGCx[CD] + 1()GSMRx_L[xDCR]()
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=
Sync Baud Rate BRGCLK or External Clock Source
Prescale Divider()Clock Divider + 1()
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