Datasheet

C515C
Data Sheet 39 2003-02
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 8.
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 abbreviation
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 dedicated baud rate generator (see Figure 15).
Table 8 USART Operating Modes
Mode
SCON Description
SM0 SM1
000Shift 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)
1018-bit UART, variable baud rate
10 bits are transmitted (through T
×D) or received (at
R
×D)
2109-bit UART, fixed baud rate
11 bits are transmitted (through T
×D) or received (at
R
×D)
3119-bit UART, variable baud rate
Like mode 2