Datasheet

SCIx_D field descriptions (continued)
Field Description
0
R0T0
Read receive data buffer 0 or write transmit data buffer 0.
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 the following figure, the clock source for the SCI baud rate generator is the
bus-rate clock.
SBR[12:0]
Divide By
Tx Baud Rate
Rx Sampling Clock
(16 × Baud Rate)
Baud Rate Generator
Off If [SBR12:SBR0] = 0
SCI Module Clock
Baud Rate =
SCI Module Clock
SBR[12:0] × 16
16
Modulo Divide By
(1 through 8191)
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
Functional description
MC9S08PA60 Reference Manual, Rev. 1, 9/2012
416 Freescale Semiconductor, Inc.