6.4.3.1 Definition of Terms
Bit-Time — The time required to transmit or receive one bit of data; one cycle of the
baud frequency.
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 re-
ceive time (RT) 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.
6
6.4.3.2 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 sup-
port for both ten-bit and eleven-bit frames. The serial mode (M) bit in SCI control
register one (SCCR1) specifies the number of bits per frame.
The most common ten-bit data frame format for NRZ serial interface consists of one
start bit, eight data bits (LSB first), and one stop bit. The most common eleven-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 6-5.
Table 6-5 Serial Frame Formats
10-Bit Frames
Start
Data
Parity/Control
Stop
1
1
1
7
7
8
—
1
2
1
1
—
11-Bit Frames
Start
Data
Parity/Control
Stop
1
1
7
8
1
1
2
1
6.4.3.3 Baud Clock
The SCI baud clock is programmed by writing a 13-bit value to the baud rate (SCBR)
field in SCI control register zero (SCCR0). Baud clock is derived from the MCU system
clock by a modulus counter. Writing a value of zero to SCBR disables the baud rate
generator. Baud clock rate is calculated as follows:
MOTOROLA
6-26
QUEUED SERIAL MODULE
MC68331
USER’S MANUAL
MOTOROLA [ MOTOROLA ]
