MPC8280 datasheet(634/1386 Pages) FREESCALE

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Baud-Rate Generators (BRGs)

MPC8280 PowerQUICC II Family Reference Manual, Rev. 1

17-2

Freescale Semiconductor

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 (BRGC

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.

Field

—

RST

Reset

0000_0000_0000_0000

R/W

Addr

0x (BRGC1), 0x119F4 (BRGC2), 0x119F8 (BRGC3), 0x119FC (BRGC4),

0x115F0 (BRGC5), 0x115F4 (BRGC6), 0x1115F8 (BRGC7), 0x115FC (BRGC8)

Field

EXTC

ATB

DIV16

Reset

0000_0000_0000_0000

R/W

Addr

0x119F22 (BRGC1), 0x119F6 (BRGC2), 0x119FA (BRGC3), 0x119FE (BRGC4),

0x115F2 (BRGC5), 0x115F6 (BRGC6), 0x115FA (BRGC7), 0x115FE (BRGC8)

Figure 17-2. Baud-Rate Generator Configuration Registers (BRGC

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