AT91M40400 View Datasheet(PDF) - Atmel Corporation
Part Name
Description
MFG CO.
'AT91M40400' PDF : 116 Pages View PDF
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
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.
16 x Baud
Rate Clock
RXD
Sampling
True Start
D0
Detection
Figure 37. Asynchronous Mode: Character Reception
Example: 8-bit, parity enabled 1 stop
0.5 bit 1 bit
periods period
RXD
Sampling
D0 D1 D2 D3 D4 D5 D6 D7
Stop Bit
True Start Detection
Parity Bit
66
AT91M40400
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