C511_1 Datasheet, PDF (63/128 Pages) Siemens Semiconductor Group – 8-Bit CMOS Microcontroller

On-Chip Peripheral Units
6.3.3 Baud Rates
There are several possibilities to generate the baud rate clock for the serial interface depending on
the mode in which it is operated.
To clearify the terminology, something should be mentioned about the differences between "baud
rate clock" and "baud rate".
The serial interface requires a clock rate which is 16 times the baud rate for the 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 chapter already includes the factor and calculate the
final baud rate.
Mode 0
The baud rate in mode 0 is fixed:
Mode 0 baud rate = oscillator frequency/12 = f_OSC/12
Mode 2
The baud rate in mode 2 depends on the value of bit SMOD in special function register PCON
(87H). If SMOD = 0 (which is the value on reset), the baud rate is f_OSC/64. If SMOD = 1, the baud
rate is f_OSC/32.
Mode 2 baud rate = 2^SMOD/64×(f_OSC)
Modes 1 and 3
The baud rates in mode 1 and 3 are determined by the timer overflow rate. These baud rates can
be determined by timer 1 or by timer 2 or by both (one for transmit and the other for receive).
Semiconductor Group
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