
MC68F375
QUEUED SERIAL MULTI-CHANNEL MODULE
MOTOROLA
REFERENCE MANUAL
Rev. 25 June 03
6-51
bit.
Break frame — A frame that consists of consecutive zeros. A break frame has no
stop bits.
6.8.7.2 Serial Formats
All data frames must have a start bit and at least one stop bit. Receiving and transmit-
ting devices must use the same data frame format. The SCI provides hardware
support for both 10-bit and 11-bit frames. The M bit in SCCxR1 specifies the number
of bits per frame.
The most common data frame format for NRZ (non-return to zero) serial interfaces is
one start bit, eight data bits (LSB first), and one stop bit (ten bits total). The most com-
mon 11-bit data frame contains one start bit, eight data bits, a parity or control bit, and
one stop bit. Ten-bit and 11-bit frames are shown in Table 6-28.
6.8.7.3 Baud Clock
The SCI baud rate is programmed by writing a 13-bit value to the SCxBR field in SCI
control register zero (SCCxR0). The baud rate is derived from the MCU system clock
by a modulus counter. Writing a value of zero to SCxBR[12:0] disables the baud rate
generator. The baud rate is calculated as follows:
or
where SCxBR is in the range {1, 2, 3, ..., 8191}.
The SCI receiver operates asynchronously. An internal clock is necessary to synchro-
nize with an incoming data stream. The SCI baud rate generator produces a receive
time sampling clock with a frequency 16 times that of the SCI baud rate. The SCI deter-
Table 6-28 Serial Frame Formats
10-bit Frames
Start
Data
Parity/Control
Stop
17
—
2
1711
18
—
1
11-Bit Frames
Start
Data
Parity/Control
Stop
1712
1811
SCI Baud Rate
f
SYS
32
SCxBR
×
--------------------------------
=
SCxBR
f
SYS
32
SCI Baud Rate Desired
×
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=
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Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
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