
MC68HC916X1
MOTOROLA
MC68HC916X1TS/D
115
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:
or
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 61 lists the SCBR[12:0] settings for standard and maximum baud rates using a 16.78 MHz sys-
 tem clock.
SCCR0 — SCI Control Register 0
$YFFC08
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
SCBR[12:0]
RESET:
0
1
0
Table 61 SCI Baud Rates
Nominal
Baud Rate
Actual Rate with
16.78 MHz Clock
SCBR[12:0] Value
64
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
32
SCBR[12:0]
×
--------------------------------------------
=
SCBR[12:0]
System Clock
32
SCI Baud Rate Desired
×
---------------------------------------------------------------------------
=
F
re
e
sc
a
le
S
e
m
ic
o
n
d
u
c
to
r,
I
Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
n
c
..
.