Freescale Semiconductor MC9S08PT60, Справочное руководство

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SCIx_D field descriptions (continued)
Field Description
15.4 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.
15.4.1 Baud rate generation
As shown in the figure found here, the clock source for the SCI baud rate generator is the
bus-rate clock.
SBR[12:0]
Baud Rate =
SBR[12:0] × 16
16
Modulo Divide By
(1 through 8191)
Baud Rate Generator
Off If [SBR12:SBR0] =0
Rx Sampling Clock
(16 × Baud Rate)
Tx Baud Rate
Divide By
16
SCI Module Clock
SCI Module Clock
Figure 15-35. 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 or 12-bittime character frame
so any mismatch in baud rate is accumulated for the whole character time. For a
Freescale 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
Chapter 15 Serial communications interface (SCI)
MC9S08PT60 Reference Manual, Rev. 4, 08/2014
Freescale Semiconductor, Inc. 429