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Functional description
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.
22.5.1 Baud rate generation
As shown in the figure found here, the clock source for the SCI baud rate generator is the
bus-rate clock.
SCI Module Clock
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 an NXP
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.
22.5.2 Transmitter functional description
This section describes the overall block diagram for the SCI transmitter, as well as
specialized functions for sending break and idle characters.
410
Modulo Divide By
(1 through 8191)
SBR[12:0]
Baud Rate Generator
Off If [SBR12:SBR0] =0
Figure 22-3. SCI baud rate generation
MC9S08SU16 Reference Manual, Rev. 5, 4/2017
Divide By
Tx Baud Rate
16
16
Rx Sampling Clock
(16 × Baud Rate)
SCI Module Clock
Baud Rate =
SBR[12:0] × 16
NXP Semiconductors
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