C515A_9708 Datasheet, PDF (99/182 Pages) Siemens Semiconductor Group – 8-Bit CMOS Microcontroller

On-Chip Peripheral Components
C515A
6.3.3.3.2 Using Timer 1 to Generate Baud Rates
In mode 1 and 3 of the serial port also timer 1 can be used for generating baud rates. Then the baud
rate is determined by the timer 1 overflow rate and the value of SMOD as follows:
2^SMOD
Mode 1, 3 baud rate =
x (timer 1 overflow rate)
32
The timer 1 interrupt is usually disabled in this application. Timer 1 itself can be configured for either
"timer" or "counter" operation, and in any of its operating modes. In most typical applications, it is
configured for "timer" operation in the auto-reload mode (high nibble of TMOD = 0010B). In this
case the baud rate is given by the formula:
2^SMOD x oscillator frequency
Mode 1, 3 baud rate =
32 x 12 x (256 – (TH1))
Very low baud rates can be achieved with timer 1 if leaving the timer 1 interrupt enabled,
configuring the timer to run as 16-bit timer (high nibble of TMOD = 0001B), and using the timer 1
interrupt for a 16-bit software reload.
Table 6-6 lists various commonly used baud rates and shows how these baud rates can be
obtained from timer 1 or from the baud rate generator.
Semiconductor Group
6-48
1997-08-01