Datasheet

753
SAM3X / SAM3A [DATASHEET]
Atmel-11057C-ATARM-SAM3X-SAM3A-Datasheet_23-Mar-15
34.5.2.2 Start Detection and Data Sampling
The UART only supports asynchronous operations, and this affects only its receiver. The UART receiver detects
the start of a received character by sampling the URXD signal until it detects a valid start bit. A low level (space) on
URXD 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 that 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 URXD at the theoretical midpoint of each bit. It
is assumed that each bit lasts 16 cycles of the sampling clock (1-bit period) so the bit sampling point is eight cycles
(0.5-bit period) 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 34-3. Start Bit Detection
Figure 34-4. Character Reception
34.5.2.3 Receiver Ready
When a complete character is received, it is transferred to the UART_RHR and the RXRDY status bit in UART_SR
(Status Register) is set. The bit RXRDY is automatically cleared when the receive holding register UART_RHR is
read.
Figure 34-5. Receiver Ready
Sampling Clock
URXD
Tr ue Start
Detection
D0
Baud Rate
Clock
D0 D1 D2 D3 D4 D5 D6 D7
URXD
True Start Detection
Sampling
Parity Bit
Stop Bit
Example: 8-bit, parity enabled 1 stop
1 bit
period
0.5 bit
period
D0 D1 D2 D3 D4 D5 D6 D7 PS
S
D0 D1 D2 D3 D4 D5 D6 D7 P
URXD
Read UART_RHR
RXRDY