SAH-C515 SIEMENS [Siemens Semiconductor Group], SAH-C515 Datasheet - Page 89

• Manufacturer Part Number: SAH-C515
• Description: 8-Bit CMOS Microcontroller
• Manufacturer: SIEMENS [Siemens Semiconductor Group]

6.3.3.3
In these modes the baud rate is variable and can be generated alternatively by a baud rate
generator with a fixed prescaler or by timer 1.
6.3.3.3.1 Using the Baud Rate Generator
In modes 1 and 3, the C515 can use the internal baud rate generator for the serial port. To enable
this feature, bit BD (bit 7 of special function register ADCON) must be set. This baud rate generator
divides the oscillator frequency by 2 x 39=78. Bit SMOD (PCON.7) also can be used to enable a
divide by 2 prescaler. For baud rate calculation the baud rate clock must be further divided by 16.
Therefore, at 12 MHz oscillator frequency, the commonly used baud rates 4800 baud (SMOD=0)
and 9600 (SMOD=1) are available (with 0.16 % deviation). The baud rate is determined by SMOD
and the oscillator frequency as follows :
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. To enable this
feature, bit BD (bit 7 of special function register ADCON) must be reset.Then the baud rate is
determined by the timer 1 overflow rate and the value of SMOD as follows :
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 = 0010 B ). In this
case the baud rate is given by the formula:
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 = 0001 B ), and using the timer 1
interrupt for a 16-bit software reload.
Table 6-5 lists various commonly used baud rates of the different modes. It also shows how these
baud rates can be obtained using timer 1 in mode 1 or 3.
Semiconductor Group
Baud Rate in Mode 1 and 3
Mode 1, 3 baud rate =
Mode 1,3 baud rate =
Mode 1, 3 baud rate =
2
2
2496
2
SMOD
32
SMOD
SMOD
32 x 12 x (256 – (TH1))
6-46
x oscillator frequency
x
x
(timer 1 overflow rate)
oscillator frequency
On-Chip Peripheral Components
C515