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Part IV. Communications Processor Module
Table 16-2. BRG External Clock Source Options
BRG
BRG1
BRG2
BRG3
BRG4
BRG5
BRG6
BRG7
BRG8
16.2 Autobaud Operation on a UART
During the autobaud process, a UART deduces the baud rate of its received character stream
by examining the received pattern and its timing. A built-in autobaud control function
automatically measures the length of a start bit and modiÞes the baud rate accordingly.
If the autobaud bit BRGCx[ATB] is set, the autobaud control function starts searching for
a low level on the corresponding RXDn input, which it assumes marks the beginning of a
start bit, and begins counting the start bit length. During this time, the BRG output clock
toggles for 16 BRG clock cycles at the BRG source clock rate and then stops with BRGOn
in the low state.
When RXDn goes high again, the autobaud control block rewrites BRGCx[CD, DIV16] to
the divide ratio found, which at high baud rates may not be exactly the Þnal rate desired (for
example, 56,600 may result rather than 57,600). An interrupt can be enabled in the UART
SCC event register to report that the autobaud controller rewrote BRGCx. The interrupt
handler can then adjust BRGCx[CD, DIV16] (see Table 16-3) for accuracy before the Þrst
character is fully received, ensuring that the UART recognizes all characters.
After a full character is received, the software can verify that the character matches a
predeÞned value (such as ÔaÕ or ÔAÕ). Software should then check for other characters (such
as ÔtÕ or ÔTÕ) and program the preferred parity mode in the UARTÕs protocol-speciÞc mode
register (PSMR).
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
BRGCx[ATB] and enable the BRG Rx clock to the highest frequency. Then, immediately
before the autobaud process starts (after device initialization), set BRGCx[ATB].
16-4
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MPC8260 PowerQUICC II UserÕs Manual
CLK
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MOTOROLA
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