C501_1 Datasheet, PDF (67/121 Pages) Siemens Semiconductor Group – 8-Bit Single-Chip Microcontroller

On-Chip Peripheral Components
C501
6.3.3.1 Using Timer 1 to Generate Baud Rates
When timer 1 is used as the baud rate generator, the baud rates in modes 1 and 3 are determined
by the timer 1 overflow rate and the value of SMOD as follows:
Modes 1,3 baud rate = 2^SMOD/32×(timer 1 overflow rate)
The timer 1 interrupt should be disabled in this application. The timer itself can be configured for
either “timer” or “counter” operation, and in any of its 3 running modes. In the most typical
applications, it is configured for “timer” operation, in the auto-reload mode (high nibble of
TMOD=0010B). In that case, the baud rate is given by the formula
Modes 1,3 baud rate = 2^SMOD/32×f_OSC/ [12×(256–TH1)]
One can achieve very low baud rates with timer 1 by leaving the timer 1 interrupt enabled, and
configuring the timer to run as a 16-bit timer (high nibble of TMOD = 0001B), and using the timer 1
interrupt to do a 16-bit software reload.
Table 6-7 lists commonly used baud rates and how they can be obtained from timer 1.
Table 6-7
Timer 1 Generated Commonly Used Baud Rates
Baud Rate
f_OSC
SMOD
C/T
Mode 0 max: 1 MHz
Mode 2 max: 375 K
Modes 1, 3: 62.5 K
19.2 K
9.6 K
4.8 K
2.4 K
1.2 K
110
110
12 MHz
X
X
12 MHz
1
X
12 MHz
1
0
11.059 MHz 1
0
11.059 MHz 0
0
11.059 MHz 0
0
11.059 MHz 0
0
11.059 MHz 0
0
6 MHz
0
0
12 MHz
0
0
Timer 1
Mode
Reload
Value
X
X
X
X
2
FFH
2
FDH
2
FDH
2
FAH
2
F4H
2
E8H
2
72H
1
FEEBH
Semiconductor Group
6-33