參數(shù)資料
型號(hào): AT91M40807-33AI
廠商: ATMEL CORP
元件分類: 微控制器/微處理器
英文描述: Microcontroller
中文描述: 32-BIT, MROM, 33 MHz, RISC MICROCONTROLLER, PQFP100
封裝: 1.40 MM HEIGHT, TQFP-100
文件頁數(shù): 101/153頁
文件大小: 1837K
代理商: AT91M40807-33AI
101
AT91X40 Series
1354D
ATARM
05/02
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
D0
Sampling
D0
D1
D2
D3
D4
D5
D6
D7
RXD
True Start Detection
Sampling
Parity Bit
Stop Bit
Example: 8-bit, parity enabled 1 stop
1 bit
period
0.5 bit
periods
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