SPMC68CK338CPV14 datasheet(73/133 Pages) FREESCALE

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MC68CK338

MOTOROLA

MC68CK338TS/D

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[12:0] — 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:

where SCBR[12:0] is in the range {1, 2, 3, …, 8191}

Writing a value of zero to SCBR[12:0] disables the baud rate generator.

Table 43 lists the SCBR[12:0] settings for standard and maximum baud rates using a 14.44 MHz sys-

tem clock.

SCCR0 — SCI Control Register 0

$YFFC08

SCBR[12:0]

RESET:

Table 43 SCI Baud Rates

Nominal Baud Rate

Actual Rate with

14.44 MHz Clock

SCBR[12:0] Value

64.0

$1B8B

110

110.0

$1006

300

300.0

$05E0

600

600.1

$02F0

1200

1200.1

$0178

2400

2400.3

$00BC

4800

4800.5

$005E

9600

9601.1

$002F

19200

18802.1

$0018

38400

37604.2

$000C

76800

75208.3

$0006

Maximum rate

451250.0

$0001

SCI Baud Rate

System Clock

() SCBR[12:0]

()

------------------------------------------------

SCBR[12:0]

System Clock

SCI Baud Rate Desired

---------------------------------------------------------------------------

Freescale Semiconductor, Inc.

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