HCS08 datasheet(174/290 Pages) MOTOROLA

174 / 290 page

Serial Communications Interface (SCI) Module

MC9S08GB/GT

174

MOTOROLA

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 MC9S08GB/GT 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 Motorola 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.

11.5 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.

11.5.1 Transmitter Block Diagram

Figure 11-3 shows the transmitter portion of the SCI.

HCS08 데이터시트(PDF) 174 Page - Motorola, Inc