C515A_9708 Datasheet, PDF (95/182 Pages) Siemens Semiconductor Group – 8-Bit CMOS Microcontroller

On-Chip Peripheral Components
C515A
6.3.3
Baud Rate Generation
There are several possibilities to generate the baud rate clock for the serial port depending on the
mode in which it is operating.
For clarification some terms regarding the difference between "baud rate clock" and "baud rate"
should be mentioned. The serial interface requires a clock rate which is 16 times the baud rate for
internal synchronization. Therefore, the baud rate generators have to provide a "baud rate clock" to
the serial interface which - there divided by 16 - results in the actual "baud rate". However, all
formulas given in the following section already include the factor and calculate the final baud rate.
Further, the abrevation f_OSC refers to the external clock frequency (oscillator or external input clock
operation).
The baud rate of the serial port is controlled by two bits which are located in the special function
registers as shown below.
Special Function Register ADCON0 (Address D8H)
Special Function Register PCON (Address 87H)
Reset Value : 00H
Reset Value : 00H
Bit No.
D8H
MSB
DFH
BD
DEH DDH DCH DBH DAH D9H
CLK ADEX BSY ADM MX2 MX1
LSB
D8H
MX0
ADCON0
7
6
5
4
3
2
1
0
87H SMOD PDS IDLS SD GF1 GF0 PDE IDLE PCON
The shaded bits are not used in controlling the serial port baud rate.
Bit
BD
SMOD
Function
Baud rate generator enable
When set, the baud rate of the serial interface is derived from the dedicated
programmable baud rate generator. When cleared (default after reset), baud rate
is derived from the timer 1 overflow rate.
Double baud rate
When set, the baud rate of serial interface 0 in modes 1, 2, 3 is doubled. After
reset this bit is cleared.
Figure 6-23 shows the configuration for the baud rate generation of the serial port.
Semiconductor Group
6-44
1997-08-01