Intel 8xC251TB - Manual (Page 13)

7
8xC251Tx Hardware Description
3.0 THE SECOND SERIAL I/O PORT
The second serial I/O port is functionally the same as the standard serial I/O port shared by both the
8xC251Tx and the 8xC251Sx. This section provides information about the new special function registers
(SFRs) associated with the second serial port. Detailed operation and programming of the serial I/O ports can
be obtained from Chapter 10 of the 8xC251SA, 8xC251SB, 8xC251SP, 8xC251SQ Embedded Microcon-
troller User’s Manual (272795). All the SFRs and control bits for the standard serial I/O port in both the
8xC251Sx and 8xC251Tx have an equivalent in the second serial I/O port. This should be kept in mind when
referencing Chapter 10 of the 8xC251SA, 8xC251SB, 8xC251SP, 8xC251SQ Embedded Microcontroller
User’s Manual (272795).
3.1 Overview
The second serial I/O port provides synchronous and asynchronous communications modes. It operates as a
universal asynchronous receiver and transmitter (UART) in three full-duplex modes (modes 1, 2 and 3).
Asynchronous transmission and reception can occur simultaneously and at different baud rates. The second
UART provides framing-bit error detection, multiprocessor communications and automatic address recog-
nition. The second serial port also operates in a single synchronous mode (mode 0).
The synchronous mode (mode 0) operates at a single baud rate. Mode 2 operates at two baud rates. Modes
1 and 3 operate over a wide range of frequencies, which are generated by Timer 1 and Timer 2.
The second serial I/O port signals are defined in Table 4 and the special function registers are described in
Table 5 .
For the three asynchronous modes, the second serial I/O port transmits on the TXD1 pin and receives on the
RXD1 pin. For the synchronous mode (mode 0), the second serial I/O port outputs a clock signal on the TXD1
pin and sends and receives messages on the RXD1 pin. The SBUF1 register holds received bytes and bytes
to be transmitted. To send, software writes a byte to SBUF1; to receive, software reads SBUF1. The receive
shift register allows reception of a second byte before the first byte has been read from SBUF1. However, if
software has not read the first byte by the time the second byte is received, the second byte will overwrite the
first. The second serial I/O port sets interrupts bits TI1 and RI1 on transmission and reception, respectively.
These two share a single interrupt request and interrupt vector.
The serial port control 1 (SCON1) and the secondary serial port control (BGCON) registers configures and
controls the second serial I/O port.
Table 4. Second Serial I/O Port Signals
Function
Name
Type Description
Multiplexed
With
TXD1 O Transmit Serial Data. TXD1 outputs the shift clock in serial I/O mode
0 and transmits serial data in serial I/O modes 1, 2 and 3 for the sec-
ond serial I/O port P1.3/CEX0
RXD1 I/O Receive Serial Data 1. RXD1 send and receives data in serial I/O
mode 0 and receives data in serial I/O modes 1, 2 and 3 for the sec-
ond serial I/O port P1.2/ECI