MCU Serial Communications Interface (SCI)
MC1321x Reference Manual, Rev. 1.6
18-2
Freescale Semiconductor
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 HCS08 re-synchronizes 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.
18.4
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.
18.4.1
Transmitter Block Diagram
shows the transmitter portion of the SCI.
Figure 18-2. SCI Transmitter Block Diagram
H 8 7 6 5 4 3 2 1 0 L
SCID – Tx BUFFER
(WRITE-ONLY)
INTERNAL BUS
STOP11-BIT TRANSMIT SHIFT REGISTER START
SHIFT DIRECTION
LSB
1 ¥ BAUD
ATE CLOCK
PARITY
GENERATION
TRANSMIT CONTROL
SHIFT ENA
B
LE
PREAMBLE (ALL 1
s
)
BREAK (ALL 0
s
)
ENABLE
SCI CONTROLS TxD1
TxD1 DIRECTION
TO TxD1
PIN LOGIC
LOOP
CONTROL
TO RECEIVE
DATA IN
TO TxD1 PIN
Tx INTERRUPT
REQUEST
LO
AD
F
R
O
M
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