MOTOROLA
Chapter 21. Serial Interface
21-39
Part V. The Communications Processor Module
21.4 Baud Rate Generators (BRGs)
The CPM contains four independent, identical baud rate generators (BRG) that can be used
with the SCCs and SMCs. The clocks produced by the BRGs are sent to the bank-of-clocks
selection logic, where they can be routed to the SCCs and/or SMCs. In addition, the output
of a BRG can be routed to a pin to be used externally. The following is a list of baud rate
generatorsÕ main features:
¥
Four independent and identical baud rate generators
¥
On-the-ßy changes allowed
¥
Each baud rate generator can be routed to one or more SCCs or SMCs
¥
A 16x divider option allows slow baud rates at high system frequencies
¥
Each BRG contains an autobaud support option
¥
Each BRG output can be routed to a pin (BRGOn)
Figure 21-29 shows a baud rate generator.
Figure 21-29. Baud Rate Generator (BRG) Block Diagram
The BRG clock source can be BRGCLK, CLK2, or CLK6 (selected in BRGCn[EXTC]).
The BRGCLK is generated in the MPC860 clock synthesizer as a source speciÞcally for
the BRGs, the SPI, and the I
2
C internal baud rate generator. Alternatively, the CLK2 and
CLK6 pins can be conÞgured as clock sources. These external source options allow ßexible
baud rate frequency generation, independent of the system frequency. Additionally, CLK2
and CLK6 allow a single external frequency to be the source for multiple BRGs. Note that
the CLK2 and CLK6 signals are not synchronized internally before being used by the BRG.
CLK2 Pin
Clock
Source
MUX
Divide by
1 or 16
Prescaler
12-Bit Counter
1 Ð 4,096
DIV 16
CD[0Ð11]
BRGO
n
Clock
BRGCLK
EXTC
ATB
Autobaud
Control
RXD
n
To Pin and/or
Bank of Clocks
BRG
n
CLK6 Pin
Содержание MPC860 PowerQUICC
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