ATMEGA328P_14 Datasheet, PDF (184/344 Pages) ATMEL Corporation – ATMEL 8-BIT MICROCONTROLLER WITH 4/8/16/32KBYTES

The following equations can be used to calculate the ratio of the incoming data rate and internal
receiver baud rate.
R_slow
=
-----------⁽--^D------+-----¹----⁾--^S------------
S – 1 + D ⋅ S + S_F
R_fast
=
-------⁽---^D------+----²----⁾---^S--------
(D + 1)S + S_M
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
S_M
R_slow
First sample number used for majority voting. S_F = 8 for normal speed and
S_F = 4 for Double Speed mode.
Middle sample number used for majority voting. S_M = 9 for normal speed and
S_M = 5 for Double Speed mode.
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 19-2 on page 184 and Table 19-3 on page 185 list the maximum receiver baud rate error
that can be tolerated. Note that Normal Speed mode has higher toleration of baud rate
variations.
Table 19-2. Recommended Maximum Receiver Baud Rate Error for Normal Speed Mode
(U2Xn = 0)
D
# (Data+Parity Bit)
5
R_slow (%)
93.20
R_fast (%)
106.67
Max Total Error (%)
+6.67/-6.8
Recommended Max
Receiver Error (%)
± 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
184 Atmel ATmega328P [Preliminary]
7810C–AVR–10/12