MC9S08RG60 Datasheet, PDF (145/232 Pages) Motorola, Inc – Microcontrollers

SoC Guide — MC9S08RG60/D Rev 1.10
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 MC9S08RC/RD/RE/RG resynchronizes to bit boundaries on every high-to-low transition, but 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.
11.4 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.
11.4.1 Transmitter Block Diagram
Figure 11-3 shows the transmitter portion of the SCI.
Freescale Semiconductor
MC9S08RC/RD/RE/RG
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