302
8331B–AVR–03/12
Atmel AVR XMEGA AU
23.8.2
Asynchronous Data Recovery
The data recovery unit uses sixteen samples in normal mode and eight samples in double speed
mode for each bit.
shows the sampling process of data and parity bits.
Figure 23-7.
Sampling of data and parity bits.
As for start bit detection, an identical majority voting technique is used on the three center sam-
ples for deciding of the logic level of the received bit. The process is repeated for each bit until a
complete frame is received. It includes the first stop bit, but excludes additional ones. If the sam-
pled stop bit is a 0 value, the frame error (FERR) flag will be set.
shows the sampling of the stop bit in relation to the earliest possible
beginning of the next frame's start bit.
Figure 23-8.
Stop bit and next start bit sampling.
A new high-to-low transition indicating the start bit of a new frame can come right after the last of
the bits used for majority voting. For normal speed mode, the first low level sample can be at the
point marked (A) in Stop Bit Sampling and Next Start Bit Sampling. For double speed mode, the
first low level must be delayed to point (B). Point (C) marks a stop bit of full length at nominal
baud rate. The early start bit detection influences the operational range of the receiver.
23.8.3
Asynchronous Operational Range
The operational range of the receiver is dependent on the mismatch between the received bit
rate and the internally generated baud rate. If an external transmitter is sending using bit rates
that are too fast or too slow, or if the internally generated baud rate of the receiver does not
match the external source’s base frequency, the receiver will not be able to synchronize the
frames to the start bit.
The following equations can be used to calculate the ratio of the incoming data rate and internal
receiver baud rate.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
1
BIT n
1
2
3
4
5
6
7
8
1
RxD
Sample
(CLK2X = 0)
Sample
(CLK2X = 1)
1
2
3
4
5
6
7
8
9
10
0/1
0/1
0/1
STOP 1
1
2
3
4
5
6
0/1
RxD
Sample
(CLK2X = 0)
Sample
(CLK2X = 1)
(A)
(B)
(C)
