
Serial Communications Interface Module (SCI)
MC68HC812A4 Data Sheet, Rev. 7
156
Freescale Semiconductor
14.5.2 Baud Rate Generation
A 13-bit modulus counter in the baud rate generator derives the baud rate for both the receiver and the
transmitter. The value from 0 to 8191 written to the SBR12–SBR0 bits determines the module clock
divisor. The SBR bits are in the SCI baud rate registers (SCBDH and SCBDL). The baud rate clock is
synchronized with the bus clock and drives the receiver. The baud rate clock divided by 16 drives the
transmitter. The receiver has an acquisition rate of 16 samples per bit time.
Baud rate generation is subject to two sources of error:
Integer division of the module clock may not give the exact target frequency.
Synchonization with the bus clock can cause phase shift.
Table 14-3
lists some examples of achieving target baud rates with a module clock frequency of 10.2
MHz.
14.5.3 Transmitter
A block diagram of the SCI transmitter is shown in
Figure 14-4
.
14.5.3.1 Character Length
The SCI transmitter can accommodate either 8-bit or 9-bit data characters. The state of the M bit in SCI
control register 1 (SCCR1) determines the length of data characters. When transmitting 9-bit data, bit T8
in SCI data register high (SCDRH) is the ninth bit (bit 8).
14.5.3.2 Character Transmission
During an SCI transmission, the transmit shift register shifts a frame out to the TXD pin. The SCI data
registers (SCDRH and SCDRL) are the write-only buffers between the internal data bus and the transmit
shift register.
Table 14-3. Baud Rates (Module Clock = 10.2 MHz)
Baud Rate
Divisor
(1)
1. The baud rate divisor is the value written to the SBR12–SBR0 bits.
2. The receiver clock frequency is the MCLK frequency divided by the baud rate divisor.
3. The transmitter clock frequency is the receiver clock frequency divided by 16.
Receiver
Clock Rate (Hz)
(2)
Transmitter
Clock Rate (Hz)
(3)
Target
Baud Rate
Error (%)
17
600,000.0
37,500.0
38,400
2.3
33
309,090.9
19,318.2
19,200
0.62
66
154,545.5
9659.1
9600
0.62
133
76,691.7
4793.2
4800
0.14
266
38,345.9
2396.6
2400
0.14
531
19,209.0
1200.6
1200
0.11
1062
9604.5
600.3
600
0.05
2125
4800.0
300.0
300
0.00
4250
2400.0
150.0
150
0.00
5795
1760.1
110.0
110
0.00