AT91M40400-25C ATMEL Corporation, AT91M40400-25C Datasheet - Page 66

• Manufacturer Part Number: AT91M40400-25C
• Description: 16/32-bit Microcontroller, 2.7V to 3.6V Operating Range
• Manufacturer: ATMEL Corporation

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.
Figure 36. Asynchronous Mode: Start Bit Detection
Figure 37. Asynchronous Mode: Character Reception
66
Rate Clock
16 x Baud
Sampling
Example: 8-bit, parity enabled 1 stop
Sampling
RXD
AT91M40400
RXD
periods
0.5 bit
True Start Detection
period
1 bit
D0
D1
True Start
Detection
D2
D3
When a valid start bit has been detected, the receiver sam-
ples the RXD at the theoretical 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.
D4
D5
D6
D7
Parity Bit
Stop Bit
D0