Baud Rate Generators (BRGs)
7-104
MC68360 USER’S MANUAL
MOTOROLA
Each BRG Output May Be Routed to a Pin (e.g., BRGO1)
Refer to Figure 7-36 for the BRG block diagram.
Figure 7-36. Baud Rate Generator Block Diagram
The clock input to the prescaler may be selected by the EXTC bits to come from one of three
sources: BRGCLK, CLK2, or CLK6. Each source is discussed in the following paragraphs.
The BRGCLK is generated in the QUICC clock synthesizer specifically for the four BRGs
(as well as a fifth BRG that is part of the SPI) and defaults to the system frequency (e.g., 25
MHz). However, the clock synthesizer in the SIM60 has an option to divide the BRGCLK by
1, 4, 16, or 64 before it leaves the clock synthesizer. Whatever the resulting frequency of
BRGCLK, the user may use that frequency as the input to the QUICC BRGs.
The ability to reduce the frequency of BRGCLK before it leaves the clock synthesizer is use-
ful in low-power applications. In a low-power mode, the BRG clocking could be a significant
factor in overall QUICC power consumption. Thus, if the BRGs do not need to generate high
frequencies or do not require a high resolution in the user application, a lower frequency
BRGCLK may be input to the BRGs. The user may wish to dynamically change the general
system clock frequency in the clock synthesizer (slow go mode) while still having the BRG
run at the original frequency. The BRGCLK allows this option also.
NOTE
The BRG configuration register may be written at any time, re-
gardless of the BRGCLK input frequency.
Alternatively, the user may choose the CLK2 or CLK6 pins to be the clock source. An exter-
nal pin allows flexible baud rate frequency generation, regardless of the system frequency.
Additionally, the CLK2 or CLK6 pins allow a single external frequency to become the input
CD11–CD0
CLK2
AUTOBAUD
CONTROL
ATB
EXTC
DIV16
MUX
PRESCALER
DIVIDE BY
1 OR 16
12-BIT
COUNTER
1–4096
CLOCK
BRGCLK
CLK6
RXD1
BRGO1
TO PIN
AND/OR
BANK
OF CLOCKS