MC68331TS datasheet(65/84 Pages) FREESCALE

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MC68331TS/D

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.

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 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)

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.

Pin Names

Mnemonics

Mode

Function

Receive Data

RXD

Receiver Disabled

Receiver Enabled

Not Used

Serial Data Input to SCI

Transmit Data

TXD

Transmitter Disabled

Transmitter Enabled

General-Purpose I/O

Serial Data Output from SCI

SCCR0 — SCI Control Register 0

$YFFC08

SCBR

RESET:

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