Communications Processor (CP)
MOTOROLA
MC68302 USER’S MANUAL
4-27
crystal, an externally supplied UART clock on the TCLK or RCLK pins can be as high as
6.67 MHz, giving a maximum baud rate of 417 kbaud.
The baud rate using the baud rate generator is (System Clock or TIN1 clock)/(1 or 4)/(Clock
Divider + 1)/16. The baud rate using the baud rate generator with an externally supplied
clock to the TCLK or RCLK pins is always (TCLK or RCLK)/16.
Table 4-4 shows examples of typical bit rates of asynchronous communication and how to
obtain them with the baud rate generator using an internally supplied clock.
4.5.2.2 Synchronous Baud Rate Generator Examples
For synchronous communication (HDLC/SDLC, BISYNC, DDCMP, Transparent, and
V.110), the internal clock is identical to the baud rate output. To obtain the desired rate, the
user selects the appropriate system clock according to the following equation:
Baud rate = (System Clock or TIN1 Clock)/(Clock Divider + 1)/(1 or 4) according
to the DIV4 bit
For example, to get the data rate of 64 kbps, the system clock can be 15.36 MHz, DIV4 = 0,
and the Clock Divider = 239. Of course, a 64 kbps rate provided externally on the TCLK or
RCLK pins could also be used.
4.5.3 SCC Mode Register (SCM)
Each SCC has a mode register. The functions of bits 5–0 are common to each protocol. The
function of the specific mode bits varies according to the protocol selected by the MODE1–
MODE0 bits. They are described in the relevant sections for each protocol type. Each SCM
is a 16-bit, memory-mapped, read-write register. The SCMs are cleared by reset.
DIAG1–DIAG0—Diagnostic Mode
00 = Normal operation (CTS, CD lines under automatic control)
In this mode, the CTS and CD lines are monitored by the SCC controller. The
SCC controller uses these lines to automatically enable/disable reception and
transmission.
Table 4-4. Typical Bit Rates of Asynchronous Communication
Baud Rates
15.36
16.0
16.667
DIV4
DIV
Actual
Frequency
DIV4
DIV
Actual
Frequency
DIV4
DIV
Actual
Frequency
150
300
600
1200
2400
4800
9600
19200
38400
1
1
0
0
0
0
0
0
0
1599
799
1599
799
399
199
99
49
24
150
300
600
1200
2400
4800
9600
19200
38400
1
1
0
0
0
0
0
0
0
1666
832
1666
832
416
207
103
51
25
149.97
300.12
599.88
1200.48
2398.08
4807.69
9615.34
19230.8
38461.53
1
1
0
0
0
0
0
0
0
1735
867
1735
867
433
216
108
53
26
150.01
300.02
600.05
1200.1
2400.2
4800.4
9556.76
19290.5
38581.0
15
6
5
4
3
2
1
0
SPECIFIC MODE BITS
DIAG1
DIAG0
ENR
ENT
MODE1
MODE0