Datasheet

ATmega48A/PA/88A/PA/168A/PA/328/P
 2018 Microchip Technology Inc. Data Sheet Complete DS40002061A-page 194
20.8.3 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 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 20-2 on page 194) 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.
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 20-2 on page 194 and Table 20-3 on page 194 list the maximum receiver baud rate error that can be
tolerated. Note that Normal Speed mode has higher toleration of baud rate variations.
Equation 1
Table 20-2. 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 20-3. 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
R
slow
D 1+S
S 1– DS S
F
++
-------------------------------------------=
R
fast
D 2+S
D 1+SS
M
+
-----------------------------------=