AS8218 datasheet(111/123 Pages) AMSCO

111 / 123 page

Data Sheet AS8218 / AS8228

Revision 3.0, 31-May-06

Page 111 of 123

begin of the stop bit the internal signal

EndOfTransmission is activated, causing flag SCON.TI going to high and

thus indicating the end of the transmission.

ReceiveShiftEnable

RxD

RB8

StartReception

Receiving

EndOfReception

ReceiveShiftRegister

ReceiveSBUF

Start

Bit

Stop Bit

011111111

001111111

100111111

110011111

011001111

101100111

110110011

111011001

111101100

Figure 30: Receiving in mode 1 and mode 2: Here the bit stream ‘0’+’01101111’ is received (see signal

ReceivedDataBit), that is:

start bit + F6h. The start bit is the first 0-pulse of signal

RxD, that is when signal ReceivingStartbit is active.

Receiving is only possible when SCON.REN = ‘1’. The process of receiving is started with a falling edge on

RxD

(internal signal

StartReceiption is activated) and controlled by a 4-bit counter, that means a bit time is divided into

16 time slices. The counter is reset when identifying a falling edge on

RxD and is consequently synchronized.

The value of

RxD is probed three times at the counter stage 6, 7, and 8 (counter range is from 0 to 15). The

final value (

ReceivedData-Bit) is determined by majority. The multiple probing ensures a more robust serial

connection.

At counter = 9 the received bit is transferred into the shift register (

ReceiveShiftRegister). If the first received bit

(stop bit) is not ‘0’, then the process is aborted and the UART1 waits for the next falling edge on

RxD. Due to

this procedure all data packets with an invalid start bit are automatically discarded.

When receiving the stop bit (

EndOfReceiption = ‘1’) the following condition is checked:

SCON.RI=’0’ and (SCON.SM2=’0’ or Received_Stop_Bit=’1’)

If this condition is true, then all eight data bits are transferred to

ReceiveSBUF, the stop bit is written to

SCON.RB8, and SCON.RI is set to ‘1’. Otherwise all the received data is discarded and the receiver waits for

the next falling edge on

RxD.

Mode 2 and 3

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

Mode 2 is very similar to mode 1 except that nine data bits are processed. The subsequent text deals only the

differences to mode 1. Mode 3 is the same as Mode 2.

The ninth data bit during transmission is taken from SCON.TB8 and is sent after the eight bits from SBUF.

When receiving the ninth data bit (

EndOfReceiption = ‘1’) the following condition is checked:

SCON.RI=’0’ and (SCON.SM2=’0’ or Ninth_Data_Bit=’1’)

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