AT91 datasheet(101/153 Pages) ATMEL

101 / 153 page

101

AT91X40 Series

1354D–ATARM–08/02

Receiver

Asynchronous Receiver

The USART is configured for asynchronous operation when SYNC = 0 (bit 7 of

US_MR). In Asynchronous Mode, the USART detects the start of a received character

by sampling the RXD signal until it detects a valid start bit. A low level (space) on RXD is

interpreted as a valid start bit if it is detected for more than 7 cycles of the sampling

clock, which is 16 times the baud rate. Hence a space which is longer than 7/16 of the

bit period is detected as a valid start bit. A space which is 7/16 of a bit period or shorter

is ignored and the receiver continues to wait for a valid start bit.

When a valid start bit has been detected, the receiver samples the RXD at the theoreti-

cal mid-point of each bit. It is assumed that each bit lasts 16 cycles of the sampling clock

(one bit period) so the sampling point is 8 cycles (0.5 bit periods) after the start of the bit.

The first sampling point is therefore 24 cycles (1.5 bit periods) after the falling edge of

the start bit was detected. Each subsequent bit is sampled 16 cycles (1 bit period) after

the previous one.

Figure 38. Asynchronous Mode: Start Bit Detection

Figure 39. Asynchronous Mode: Character Reception

16 x Baud

Rate Clock

RXD

True Start

Detection

Sampling

RXD

True Start Detection

Sampling

Parity Bit

Stop Bit

Example: 8-bit, parity enabled 1 stop

1 bit

period

0.5 bit

periods

AT91 Datasheet(PDF) 101 Page - ATMEL Corporation