MC68CK331CAG16 Datasheet, PDF (65/84 Pages) Freescale Semiconductor, Inc – 32-Bit Modular Microcontroller

Freescale Semiconductor, Inc.
5.5.1 SCI Pins
There are two unidirectional pins associated with the SCI. The SCI controls the transmit data (TXD) pin
when enabled, whereas the receive data (RXD) pin remains a dedicated input pin to the SCI. TXD is
available as a general-purpose I/O pin when the SCI transmitter is disabled. When used for I/O, TXD
can be configured either as input or output, as determined by QSM register DDRQS.
The following table shows SCI pins and their functions.
Pin Names
Receive Data
Transmit Data
Mnemonics
RXD
TXD
Mode
Function
Receiver Disabled Not Used
Receiver Enabled Serial Data Input to SCI
Transmitter Disabled General-Purpose I/O
Transmitter Enabled Serial Data Output from SCI
5.5.2 SCI Registers
The SCI programming model includes QSM global and pin control registers, and four SCI registers.
There are two SCI control registers, one status register, and one data register. All registers can be read
or written at any time by the CPU.
Changing the value of SCI control bits during a transfer operation may disrupt operation. Before chang-
ing register values, allow the transmitter to complete the current transfer, then disable the receiver and
transmitter. Status flags in the SCSR may be cleared at any time.
SCCR0 — SCI Control Register 0
$YFFC08
15
14
13
12
0
0
0
0
SCBR
RESET:
0
0
0
0
0
0
0
0
0
0
0
0
0
1
0
0
SCCR0 contains a baud rate selection parameter. Baud rate must be set before the SCI is enabled. The
CPU can read and write this register at any time.
Bits [15:13] — Not Implemented
SCBR — Baud Rate
SCI baud rate is programmed by writing a 13-bit value to BR. The baud rate is derived from the MCU
system clock by a modulus counter.
The SCI receiver operates asynchronously. An internal clock is necessary to synchronize with an in-
coming data stream. The SCI baud rate generator produces a receiver sampling clock with a frequency
16 times that of the expected baud rate of the incoming data. The SCI determines the position of bit
boundaries from transitions within the received waveform, and adjusts sampling points to the proper po-
sitions within the bit period. Receiver sampling rate is always 16 times the frequency of the SCI baud
rate, which is calculated as follows:
SCI Baud Rate = System Clock/(32SCBR)
or
SCBR = System Clock(32SCK)(Baud Rate Desired)
where SCBR is in the range {1, 2, 3, ..., 8191}
Writing a value of zero to SCBR disables the baud rate generator.
The following table lists the SCBR settings for standard and maximum baud rates using 16.78-MHz and
20.97-MHz system clocks.
MC68331TS/D
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