80C186EC/80C188EC Microprocessor User's Manual

SERIAL COMMUNICATIONS UNIT
Due to internal synchronization requirements, the maximum input frequency to BCLK is one-half
the CPU operating frequency. See “BCLK Pin Timings” on page 11-18 for more information. Table
11-1 shows the correct BxCMP values for common baud rates.
Table 11-1. BxCMP Values for Typical Baud Rates and CPU Frequencies
CPU Frequency
Baud
Rate
BxCMP
Value
25 MHz 20 MHz 16 MHz 8 MHz
%
Error
BxCMP
Value
%
Error
BxCMP
Value
%
Error
BxCMP
Value
%
Error
19,200 80A2H –0.14 8081H 0.16 8067H 0.16 8033H 0.16
9,600 8145H –0.14 8103H 0.16 80CFH 0.16 8067H 0.16
4,800 828AH 0.00 8208H –0.03 81A0H –0.08 80CFH 0.16
2,400 8515H 0.00 8411H –0.03 8340H 0.04 81A0H –0.08
1,200 8A2BH 0.00 8822H 0.01 8682H –0.02 8340H 0.04
NOTE
A zero or one value for BxCMP is illegal and results in unpredictable
operation. Programming BxCMP during a transmission or reception causes
indeterminate operation.
11.2.2 Asynchronous Mode Programming
The serial port operation is controlled by two registers. The Serial Port Control (SxCON) Register
controls the mode of operation of the serial port (see Figure 11-13). The Serial Port Status
(SxSTS) Register acts as the flags register, reporting on errors and the state of the RX and TX
machines (see Figure 11-14). Depending on the serial port mode, these registers can have different
functionality. This section outlines how to use SxCON and SxSTS to obtain the desired operation
from the serial port.
11.2.2.1 Modes 1, 3 and 4 for Stand-alone Serial Communications
When using these modes for their respective seven, eight or nine bit data modes, operation is fairly
straightforward. The serial port must be initialized correctly (through SxCON), then SxSTS
needs to be interpreted.
To configure the serial port, first program the baud rate through the BxCMP register, then program
SxCON (Figure 11-13 on page 11-15) as follows.
1. Determine the values for M2:0 for the desired serial port mode.
2. If parity is used, enable it with the PEN bit. Set the sense of parity (even or odd) with the
EVN bit. Note that parity is not available in Mode 4 (seven bit data).
11-13