Datasheet
• 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 24-2. 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 24-3. 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
The recommendations of the maximum receiver baud rate error was made under the assumption that the
receiver and transmitter equally divide the maximum total error.
There are two possible sources for the receivers baud rate error. The receiver’s System Clock (EXTCLK)
will always have some minor instability over the supply voltage range and the temperature range. When
using a crystal to generate the system clock, this is rarely a problem, but for a resonator, the system clock
may differ more than 2% depending on the resonator's tolerance. The second source for the error is more
controllable. The baud rate generator cannot always do an exact division of the system frequency to get
the baud rate wanted. In this case, an UBRRn value that gives an acceptable low error can be used if
possible.
ATmega48PA/88PA/168PA
Universal Synchronous Asynchronous Receiver ...
© 2018 Microchip Technology Inc.
Datasheet Complete
DS40002011A-page 251