Motorola PowerQUICC II MPC8280 Series Reference Manual page 662

Freescale Semiconductor, Inc.
BRG Configuration Registers 1–8 (BRGCx)
Each BRG clock source can be BRGCLK, or a choice of two external clocks (selected in
BRGCx[EXTC]). The BRGCLK is an internal signal generated in the MPC8280 clock
2
synthesizer specifically for the BRGs, the SPI, and the I C internal BRG. Alternatively,
external clock pins can be configured as clock sources. The external source option allows
flexible baud-rate frequency generation, independent of the system frequency.
Additionally, the external source option allows a single external frequency to be the source
for multiple BRGs. The external source signals are not synchronized internally before
being used by the BRG.
The BRG provides a divide-by-16 option (BRGCx[DIV16]) and a 12-bit prescaler
(BRGCx[CD]) to divide the source clock frequency. The combined source-clock divide
factor can be changed on-the-fly; however, two changes should not occur within two source
clock periods.
The prescaler output is sent internally to the bank of clocks and can also be output
externally on BRGOn through the parallel I/O ports. If the BRG divides the clock by an
even value, the transitions of BRGOn always occur on the falling edge of the source clock.
If the divide factor is odd, the transitions alternate between the falling and rising edges of
the source clock. Additionally, the output of the BRG can be sent to the autobaud control
block.
17.1 BRG Configuration Registers 1–8 (BRGCx)
The BRG configuration registers (BRGCx) are shown in Figure 17-2. A reset disables the
BRG and drives the BRGO output clock high. The BRGC can be written at any time with
no need to disable the SCCs or external devices that are connected to BRGO. Configuration
changes occur at the end of the next BRG clock cycle (no spikes occur on the BRGO output
clock). BRGC can be changed on-the-fly; however, two changes should not occur within a
time equal to two source clock periods.
17-2 MPC8280 PowerQUICC II Family Reference Manual MOTOROLA
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