C515C Datasheet, PDF (38/69 Pages) Siemens Semiconductor Group – 8-Bit CMOS Microcontroller

C515C
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 7.
Table 7
USART Operating Modes
Mode
0
1
2
3
SCON
SM0
SM1
0
0
0
1
1
0
1
1
Description
Shift register mode, fixed baud rate
Serial data enters and exits through R×D; T×D outputs the shift
clock; 8-bit are transmitted/received (LSB first)
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 15 to the serial interface which - there divided
by 16 - results in the actual "baud rate". Further, the abrevation 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 either from timer 1
or from a decdicated baud rate generator (see figure 15).
Semiconductor Group
38
1997-07-01