Datasheet

The following figure shows the sampling of the stop bit and the earliest possible beginning of the start bit
of the next frame.
Figure 21-7. Stop Bit Sampling and Next Start Bit Sampling
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
(U2X = 0)
Sample
(U2X = 1)
(A) (B) (C)
The same majority voting is done to the stop bit as done for the other bits in the frame. If the stop bit is
registered to have a logic '0' value, the Frame Error (UCSRnA.FE) Flag will be set.
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 taken at point marked
(A) in the figure above. For Double Speed mode, the first low level must be delayed to (B). (C) marks a
stop bit of full length. The early start bit detection influences the operational range of the Receiver.
21.9.3. 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 the Transmitter is sending frames at too fast or too slow bit rates, or
the internally generated baud rate of the Receiver does not have a similar base frequency (see
recommendations below), 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.

slow
=
 + 1 
  1 +    + 


fast
=
 + 2 
 + 1  + 

• D: Sum of character size and parity size (D = 5 to 10 bit)
• S: Samples per bit. S = 16 for Normal Speed mode and S = 8 for Double Speed mode.
• S
F
: First sample number used for majority voting. S
F
= 8 for normal speed and S
F
= 4
for Double Speed mode.
• S
M
: Middle sample number used for majority voting. S
M
= 9 for normal speed and
S
M
= 5 for Double Speed mode.
• R
slow
: is the ratio of the slowest incoming data rate that can be accepted in relation to the
receiver baud rate. R
fast
is the ratio of the fastest incoming data rate that can be
accepted in relation to the receiver baud rate.
The following tables list the maximum receiver baud rate error that can be tolerated. Note that Normal
Speed mode has higher toleration of baud rate variations.
Table 21-2. Recommended Maximum Receiver Baud Rate Error for Normal Speed Mode (U2X = 0)
D
# (Data+Parity Bit)
R
slow
[%] R
fast
[%] Max. Total Error [%] Recommended Max. Receiver Error [%]
5 93.20 106.67 +6.67/-6.8 ±3.0
6 94.12 105.79 +5.79/-5.88 ±2.5
Atmel ATmega644A [DATASHEET]
Atmel-42716C-ATmega644A_Datasheet_Complete-10/2016
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