C515 Datasheet, PDF (30/55 Pages) Siemens Semiconductor Group – 8-Bit CMOS Microcontroller

C515
Serial Interface (USART)
The serial port is full duplex and can operate in four modes (one synchronous mode, three
asynchronous modes) as illustrated in table 5. The possible baudrates can be calculated using the
formulas given in table 5.
Table 5
USART Operating Modes
Mode
0
1
2
3
SCON
SM0
SM1
0
0
0
1
1
0
1
1
Description
Shift register mode
Serial data enters and exits through R×D/ T×D outputs the shift
clock; 8-bit are transmitted/received (LSB first); fixed baud rate
8-bit UART, variable baud rate
10 bits are transmitted (through T×D) or received (at R×D)
9-bit UART, fixed baud rate
11 bits are transmitted (through T×D) or received (at R×D)
9-bit UART, variable baud rate
Like mode 2
For clarification some terms regarding the difference between "baud rate clock" and "baud rate"
should be mentioned. In the asynchronous modes the serial interfaces require a clock rate which is
16 times the baud rate for internal synchronization. Therefore, the baud rate generators/timers have
to provide a "baud rate clock" (output signal in figure 13 to the serial interface which - there divided
by 16 - results in the actual "baud rate". Further, the abbrevation f_OSC refers to the oscillator
frequency (crystal or external clock operation).
The variable baud rates for modes 1 and 3 of the serial interface can be derived from either timer 1
or from the system clock (see figure 13).
Semiconductor Group
30
1997-08-01