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MCF51QE128 MCU Series Reference Manual, Rev. 3
Freescale Semiconductor 305
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15.2.7 SCI Data Register (SCIxD)
This register is actually two separate registers. Reads return the contents of the read-only receive data
buffer and writes go to the write-only transmit data buffer. Reads and writes of this register are also
involved in the automatic flag clearing mechanisms for the SCI status flags.
15.3 Functional Description
The SCI allows full-duplex, asynchronous, NRZ serial communication among the MCU and remote
devices, including other MCUs. The SCI comprises a baud rate generator, transmitter, and receiver block.
The transmitter and receiver operate independently, although they use the same baud rate generator.
During normal operation, the MCU monitors the status of the SCI, writes the data to be transmitted, and
processes received data. The following describes each of the blocks of the SCI.
15.3.1 Baud Rate Generation
As shown in Figure 15-13, the clock source for the SCI baud rate generator is the bus-rate clock.
Figure 15-13. SCI Baud Rate Generation
SCI communications require the transmitter and receiver (which typically derive baud rates from
independent clock sources) to use the same baud rate. Allowed tolerance on this baud frequency depends
on the details of how the receiver synchronizes to the leading edge of the start bit and how bit sampling is
performed.
The MCU resynchronizes to bit boundaries on every high-to-low transition. In the worst case, there are no
such transitions in the full 10- or 11-bit time character frame so any mismatch in baud rate is accumulated
for the whole character time. For a Freescale Semiconductor SCI system whose bus frequency is driven
by a crystal, the allowed baud rate mismatch is about ±4.5 percent for 8-bit data format and about ±4
percent for 9-bit data format. Although baud rate modulo divider settings do not always produce baud rates
that exactly match standard rates, it is normally possible to get within a few percent, which is acceptable
for reliable communications.
76543210
RR7R6R5R4R3R2R1R0
WT7T6T5T4T3T2T1T0
Reset00000000
Figure 15-12. SCI Data Register (SCIxD)
SBR12:SBR0
DIVIDE BY
Tx BAUD RATE
Rx SAMPLING CLOCK
(16 × BAUD RATE)
BAUD RATE GENERATOR
OFF IF [SBR12:SBR0] = 0
BUSCLK
BAUD RATE =
BUSCLK
[SBR12:SBR0] × 16
16
MODULO DIVIDE BY
(1 THROUGH 8191)