Serial i/o port, Framing error detection – Rainbow Electronics AT89C5131 User Manual

Page 80

AT89C5131

4136A–USB–03/03

Serial I/O Port

The serial I/O port in the AT89C5131 is compatible with the serial I/O port in the 80C52.
It provides both synchronous and asynchronous communication modes. It operates as
an Universal Asynchronous Receiver and Transmitter (UART) in three full-duplex
modes (modes 1, 2 and 3). Asynchronous transmission and reception can occur simul-
taneously and at different baud rates.

Serial I/O port includes the following enhancements:

•

Framing error detection

•

Automatic address recognition

Framing Error Detection

Framing bit error detection is provided for the three asynchronous modes (modes 1, 2
and 3). To enable the framing bit error detection feature, set SMOD0 bit in PCON regis-
ter (see Figure 34).

Figure 34. Framing Error Block Diagram

When this feature is enabled, the receiver checks each incoming data frame for a valid
stop bit. An invalid stop bit may result from noise on the serial lines or from simultaneous
transmission by two CPUs. If a valid stop bit is not found, the Framing Error bit (FE) in
SCON register (See Table 60) bit is set.
Software may examine FE bit after each reception to check for data errors. Once set,
only software or a reset can clear FE bit. Subsequently received frames with valid stop
bits cannot clear FE bit. When FE feature is enabled, RI rises on stop bit instead of the
last data bit (See Figure 35 and Figure 36).

Figure 35. UART Timings in Mode 1

RB8

TB8

REN

SM2

SM1

SM0/FE

IDL

GF0

GF1

POF

SMOD0

SMOD1

To UART Framing Error Control

SM0 to UART Mode Control (SMOD0 = 0)

Set FE Bit if Stop Bit is 0 (framing error) (SMOD0 = 1

SCON (98h)

PCON (87h)

Data Byte

SMOD0 = X

Stop

Bit

Start

Bit

RXD

SMOD0 = 1