AT89LP428 datasheet(77/149 Pages) ATMEL

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3654A–MICRO–8/09

AT89LP428/828

16. Serial Interface (UART)

The serial interface on the AT89LP428/828 implements a Universal Asynchronous

Receiver/Transmitter (UART). The UART has the following features:

•Full-duplex Operation

• 8 or 9 Data Bits

•Framing Error Detection

•Multiprocessor Communication Mode with Automatic Address Recognition

•Baud Rate Generator Using Timer 1 or Timer 2

• Interrupt on Receive Buffer Full or Transmission Complete

The serial interface is full-duplex, which means it can transmit and receive simultaneously. It is

also receive-buffered, which means it can begin receiving a second byte before a previously

received byte has been read from the receive register. (However, if the first byte still has not

been read when reception of the second byte is complete, one of the bytes will be lost.) The

serial port receive and transmit registers are both accessed at the Special Function Register

SBUF. Writing to SBUF loads the transmit register, and reading SBUF accesses a physically

separate receive register. The serial port can operate in the following four modes.

• Mode 0: Serial data enters and exits through RXD. TXD outputs the shift clock. Eight data

bits are transmitted/received, with the LSB first. The baud rate is programmable to 1/2 or

1/4 the oscillator frequency, or variable based on Time 1.

• Mode 1: 10 bits are transmitted (through TXD) or received (through RXD): a start bit (0),

8data bits (LSB first), and a stop bit (1). On receive, the stop bit goes into RB8 in the Special

Function Register SCON. The baud rate is variable based on Timer 1 or Timer 2.

• Mode 2: 11 bits are transmitted (through TXD) or received (through RXD): a start bit (0),

8data bits (LSB first), a programmable 9th data bit, and a stop bit (1). On transmit, the 9th

data bit (TB8 in SCON) can be assigned the value of “0” or “1”. For example, the parity bit

(P, in the PSW) can be moved into TB8. On receive, the 9th data bit goes into RB8 in the

Special Function Register SCON, while the stop bit is ignored. The baud rate is

programmable to either 1/16 or 1/32 the oscillator frequency.

• Mode 3: 11 bits are transmitted (through TXD) or received (through RXD): a start bit (0),

8data bits (LSB first), a programmable 9th data bit, and a stop bit (1). In fact, Mode 3 is the

same as Mode 2 in all respects except the baud rate, which is variable based on Timer 1 or

Timer 3 in Mode 3.

In all four modes, transmission is initiated by any instruction that uses SBUF as a destination

ated in the other modes by the incoming start bit if REN = 1.

16.1

Multiprocessor Communications

Modes 2 and 3 have a special provision for multiprocessor communications. In these modes,

9data bits are received, followed by a stop bit. The 9th bit goes into RB8. Then comes a stop bit.

The port can be programmed such that when the stop bit is received, the serial port interrupt is

activated only if RB8 = 1. This feature is enabled by setting bit SM2 in SCON.

The following example shows how to use the serial interrupt for multiprocessor communications.

When the master processor must transmit a block of data to one of several slaves, it first sends

out an address byte that identifies the target slave. An address byte differs from a data byte in

that the 9th bit is “1” in an address byte and “0” in a data byte. With SM2 = 1, no slave is

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