
MC68336/376
QUEUED SERIAL MODULE
MOTOROLA
USER’S MANUAL
Rev. 15 Oct 2000
9-27
Start Bit — One bit-time of logic zero that indicates the beginning of a data frame.
A start bit must begin with a one-to-zero transition and be preceded by at least
three receive time samples of logic one.
Stop Bit— One bit-time of logic one that indicates the end of a data frame.
Frame — A complete unit of serial information. The SCI can use 10-bit or 11-bit
frames.
Data Frame — A start bit, a specified number of data or information bits, and at
least one stop bit.
Idle Frame — A frame that consists of consecutive ones. An idle frame has no
start bit.
Break Frame — A frame that consists of consecutive zeros. A break frame has
no stop bits.
9.4.3.2 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 SCCR1 specifies the number of
bits per frame.
The most common data frame format for NRZ serial interfaces is one start bit, eight
data bits (LSB first), and one stop bit; a total of ten bits. The most common 11-bit data
frame contains one start bit, eight data bits, a parity or control bit, and one stop bit.
Ten-bit and eleven-bit frames are shown in Table 9-5.
9.4.3.3 Baud Clock
The SCI baud rate is programmed by writing a 13-bit value to the SCBR field in SCI
control register zero (SCCR0). The baud rate is derived from the MCU system clock
by a modulus counter. Writing a value of zero to SCBR[12:0] disables the baud rate
generator. Baud rate is calculated as follows:
or
Table 9-5 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
System Clock
32
SCBR[12:0]
×
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