ATmega88 datasheet(182/340 Pages) ATMEL

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7530H–AVR–02/09

ATmega48/88/168 Automotive

Middle sample number used for majority voting. S

M = 9 for normal speed and

M = 5 for Double Speed mode.

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 17-2 and Table 17-3 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 assump-

tion 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 tempera-

ture 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 controllable. The baud rate generator can not 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.

Table 17-2.

Recommended Maximum Receiver Baud Rate Error for Normal Speed Mode

(U2Xn = 0)

# (Data+Parity Bit)

Rslow (%)

Rfast (%)

Max Total Error (%)

Recommended Max

Receiver Error (%)

93.20

106.67

+6.67/-6.8

± 3.0

94.12

105.79

+5.79/-5.88

± 2.5

94.81

105.11

+5.11/-5.19

± 2.0

95.36

104.58

+4.58/-4.54

± 2.0

95.81

104.14

+4.14/-4.19

± 1.5

96.17

103.78

+3.78/-3.83

± 1.5

Table 17-3.

Recommended Maximum Receiver Baud Rate Error for Double Speed Mode

(U2Xn = 1)

# (Data+Parity Bit)

Rslow (%)

Rfast (%)

Max Total Error (%)

Recommended Max

Receiver Error (%)

94.12

105.66

+5.66/-5.88

± 2.5

94.92

104.92

+4.92/-5.08

± 2.0

95.52

104,35

+4.35/-4.48

± 1.5

96.00

103.90

+3.90/-4.00

± 1.5

96.39

103.53

+3.53/-3.61

± 1.5

96.70

103.23

+3.23/-3.30

± 1.0

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