
M68HC11E Family — Rev. 5
Data Sheet
MOTOROLA
Serial Communications Interface (SCI)
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117
Data Sheet — M68HC11E Family
Section 7. Serial Communications Interface (SCI)
7.1 Introduction
The serial communications interface (SCI) is a universal asynchronous receiver
transmitter (UART), one of two independent serial input/output (I/O) subsystems in
the M68HC11 E series of microcontrollers. It has a standard non-return-to-zero
(NRZ) format (one start bit , eight or nine data bits, and one stop bit). Several baud
rates are available. The SCI transmitter and receiver are independent, but use the
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
7.7.5 Baud Rate Register
.
7.2 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).
7.3 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
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n
.