Datasheet

203
ATmega16U4/32U4 [DATASHEET]
Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016
18.7.3 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 (see Table 18-6 on page 208) 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.
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.
The recommendations of the maximum receiver baud rate error was made under the assumption that the
Receiver and Transmitter equally divides the maximum total error.
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
Table 18-2. Recommended Maximum Receiver Baud Rate Error for Normal Speed Mode (U2Xn = 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
7 94.81 105.11 +5.11/-5.19 ±2.0
8 95.36 104.58 +4.58/-4.54 ±2.0
9 95.81 104.14 +4.14/-4.19 ±1.5
10 96.17 103.78 +3.78/-3.83 ±1.5
Table 18-3. Recommended Maximum Receiver Baud Rate Error for Double Speed Mode (U2Xn = 1)
D
# (Data+Parity Bit)
R
slow
[%] R
fast
[%] Max. total error [%]
Recommended max. receiver
error [%]
5 94.12 105.66 +5.66/-5.88 ±2.5
6 94.92 104.92 +4.92/-5.08 ±2.0
7 95.52 104,35 +4.35/-4.48 ±1.5
8 96.00 103.90 +3.90/-4.00 ±1.5
9 96.39 103.53 +3.53/-3.61 ±1.5
10 96.70 103.23 +3.23/-3.30 ±1.0
R
slow
D 1+S
S
1– DS S
F
++
-------------------------------------------=
R
fast
D 2+S
D
1+SS
M
+
-----------------------------------=