MPC8280 datasheet(635/1386 Pages) FREESCALE

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Baud-Rate Generators (BRGs)

MPC8280 PowerQUICC II Family Reference Manual, Rev. 1

Freescale Semiconductor

17-3

Table 17-1 describes the BRGCx fields.

Table 17-2 shows the possible external clock sources for the BRGs.

Table 17-1. BRGC

x Field Descriptions

Bits

Name

Description

0–13

—

Reserved, should be cleared.

RST

Reset BRG. Performs a software reset of the BRG identical to that of an external reset. A reset

disables the BRG and drives BRGO high. This is externally visible only if BRGO is connected to the

corresponding parallel I/O pin.

0 Enable the BRG.

1 Reset the BRG (software reset).

Enable BRG count. Used to dynamically stop the BRG from counting—useful for low-power modes.

0 Stop all clocks to the BRG.

1 Enable clocks to the BRG.

16–17

EXTC

External clock source. Selects the BRG input clock. See Table 17-2..

00 The BRG input clock comes from the BRGCLK (internal clock generated from the CPM clock);

see Section 10.4, “System Clock Control Register (SCCR).”

01 If BRG1, 2, 5, 6: The BRG input clock comes from the CLK3 pin.

If BRG3, 4, 7, 8: The BRG input clock comes from the CLK9 pin

10 If BRG1, 2, 5, 6: The BRG input clock comes from the CLK5 pin.

If BRG3, 4, 7, 8: The BRG input clock comes from the CLK15 pin

11 Reserved.

ATB

Autobaud. Selects autobaud operation of the BRG on the corresponding RXD. ATB must remain

zero until the SCC receives the three Rx clocks. Then the user must set ATB to obtain the correct

baud rate. After the baud rate is obtained and locked, it is indicated by setting AB in the UART event

0 Normal operation of the BRG.

1 When RXD goes low, the BRG determines the length of the start bit and synchronizes the BRG

to the actual baud rate.

19–30

Clock divider. CD presets an internal 12-bit counter that is decremented at the DIV16 output rate.

When the counter reaches zero, it is reloaded with CD. CD = 0xFFF produces the minimum clock

rate for BGRO (divide by 4,096); CD = 0x000 produces the maximum rate (divide by 1). When

dividing by an odd number, the counter ensures a 50% duty cycle by asserting the terminal count

once on clock low and next on clock high. The terminal count signals counter expiration and toggles

the clock. See Section 17.3, “UART Baud Rate Examples.”

DIV16

Divide-by-16. Selects a divide-by-1 or divide-by-16 prescaler before reaching the clock divider. See

Section 17.3, “UART Baud Rate Examples.”

0 Divide by 1.

1 Divide by 16.

MPC8280 Datasheet(PDF) 635 Page - Freescale Semiconductor, Inc

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