AS8218 datasheet(110/123 Pages) AMSCO

110 / 123 page

Data Sheet AS8218 / AS8228

Revision 3.0, 31-May-06

Page 110 of 123

The UART1 asserts an interrupt whenever flag SCON.RI is ‘1’ or SCON.TI is ‘1’. These flags are set if a

successful receive or transmit operation has taken place. The flags to SCON.RI and SCON.TI must be cleared

by software. The MCU program branches to the interrupt routine if the serial interrupt is enabled in the IE

interrupt service routine for both transmitting and receiving. The interrupt service routine has to decide which

event triggered the interrupt request (by querying the flags RI and TI). It is important to clear the flags before

leaving the interrupt service routine.

Multiprocessor Communications

Mode 2 has a special provision for multiprocessor communications. In this mode, a 9

th data bit is received and

goes into RB8. Then a stop bit follows.

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

only if RB8 = 1. This feature is enabled by setting bit SM2 in SCON. A way to use this feature in multiprocessor

systems is as follows:

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

address byte which identifies the target slave. An address byte differs from a data byte in that the ninth bit is 1

in an address byte and 0 in a data byte. With SM2 = 1, no slave will be interrupted by a data byte. An address

byte, however, will interrupt all slaves, so that each slave can examine the received byte and see if it is being

addressed. The addressed slave will clear its SM2 bit and prepare to receive the data bytes that will be

coming. The slaves that were not being addressed leave their SM2s set and go on about their business,

ignoring the coming data bytes.

Modes

The UART1 can be used in two different modes: mode 0 (= mode 1) and mode 2 (= mode 3). The mode

selection is due the bits SM0, SM1 in the SCON register.

Mode 0 and 1

8 bit UART1 with variable baudrate controlled by the baudrate generator.

WrStrobeSFR

TransmitEnable

TransmitSBUF

Transmitting

EndOfTransmission

TxD

Start Bit

Stop Bit

001000010010

000100001001 000010000100 000001000010

000000100001 000000010000 000000001000 000000000100

000000000010

Figure 29: Transmitting in mode 1: here ‘09h’ is sent. The resulting bit stream on the TxD line is:

start bit (=‘0’) + ‘10010000’, for LSB is sent first.

The process of transmitting is initiated by writing to SBUF. The byte written to SBUF is held in register

TransmitSBUF. The transmission starts with the next 1-pulse on the internal signal TransmitEnable. Output TxD is

driven with a start bit (‘0’), eight data bits with the LSB first shifted out from

TransmitSBUF, and a stop bit (‘1’). At

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