Technical Data
MC68HC11E Family
—
Rev. 4
146
Serial Communications Interface (SCI)
MOTOROLA
Serial Communications Interface (SCI)
transmitter and receiver are independent, but use the same data format
and bit rate.
All members of the E series contain the same SCI, with one exception.
The SCI system in the MC68HC11E20 and MC68HC711E20 MCUs
have an enhanced SCI baud rate generator. A divide-by-39 stage has
been added that is enabled by an extra bit in the BAUD register. This
increases the available SCI baud rate selections. Refer to
Figure 7-8
and
7.8.5 Baud Rate Register
.
7.3 Data Format
The serial data format requires these conditions:
1.
An idle line in the high state before transmission or reception of a
message
2.
A start bit, logic 0, transmitted or received, that indicates the start
of each character
3.
Data that is transmitted and received least significant bit (LSB) first
4.
A stop bit, logic 1, used to indicate the end of a frame. A frame
consists of a start bit, a character of eight or nine data bits, and a
stop bit.
5.
A break, defined as the transmission or reception of a logic 0 for
some multiple number of frames
Selection of the word length is controlled by the M bit of SCI control
register (SCCR1).
7.4 Transmit Operation
The SCI transmitter includes a parallel transmit data register (SCDR)
and a serial shift register. The contents of the serial shift register can be
written only through the SCDR. This double buffered operation allows a
character to be shifted out serially while another character is waiting in
the SCDR to be transferred into the serial shift register. The output of the
serial shift register is applied to TxD as long as transmission is in