Datasheet
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 24-2. 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
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 (U2X = 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 divides the maximum total error.
There are two possible sources for the Receivers Baud Rate error. The Receiver’s system clock (XTAL)
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 of the resonators tolerance. The second source for the error is more
AVR 8-Bit Microcontroller
USART - Universal Synchronous and Asynchrono...
© 2017 Microchip Technology Inc.
Datasheet Complete
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