16-4
MPC8260 PowerQUICC II UserÕs Manual
MOTOROLA
Part IV. Communications Processor Module
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 BRGC
x
[ATB] is set, the autobaud control function starts searching for
a low level on the corresponding RXD
n
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 BRGO
n
in the low state.
When RXD
n
goes high again, the autobaud control block rewrites BRGC
x
[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 BRGC
x.
The interrupt
handler can then adjust BRGC
x
[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
BRGC
x
[ATB] and enable the BRG Rx clock to the highest frequency. Then, immediately
before the autobaud process starts (after device initialization), set BRGC
x
[ATB].
Table 16-2. BRG External Clock Source Options
BRG
CLK
1
2
3
4
5
6
7
8
9 10 11 12 13 14 15 16 17 18 19 20
BRG1
V
V
BRG2
V
V
BRG3
V
V
BRG4
V
V
BRG5
V
V
BRG6
V
V
BRG7
V
V
BRG8
V
V
Summary of Contents for MPC8260 PowerQUICC II
Page 1: ...MPC8260UM D 4 1999 Rev 0 MPC8260 PowerQUICC II UserÕs Manual ª ª ...
Page 66: ...lxvi MPC8260 PowerQUICC II UserÕs Manual MOTOROLA ...
Page 88: ...1 18 MPC8260 PowerQUICC II UserÕs Manual MOTOROLA Part I Overview ...
Page 120: ...2 32 MPC8260 PowerQUICC II UserÕs Manual MOTOROLA Part I Overview ...
Page 138: ...Part II iv MPC8260 PowerQUICC II UserÕs Manual MOTOROLA Part II Configuration and Reset ...
Page 184: ...4 46 MPC8260 PowerQUICC II UserÕs Manual MOTOROLA Part II ConÞguration and Reset ...
Page 202: ...Part III vi MPC8260 PowerQUICC II UserÕs Manual MOTOROLA Part III The Hardware Interface ...
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Page 382: ...10 106 MPC8260 PowerQUICC II UserÕs Manual MOTOROLA Part III The Hardware Interface ...
Page 392: ...11 10 MPC8260 PowerQUICC II UserÕs Manual MOTOROLA Part III The Hardware Interface ...
Page 430: ...Part IV viii MOTOROLA Part IV Communications Processor Module ...
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Page 980: ...A 4 MPC8260 PowerQUICC II UserÕs Manual MOTOROLA Appendixes ...
Page 1002: ...Index 22 MPC8260 PowerQUICC II UserÕs Manual MOTOROLA INDEX ...
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