ATMEGA640 datasheet(215/436 Pages) MICROCHIP

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ATmega640/V-1280/V-1281/V-2560/V-2561/V [DATASHEET]

DS40002211A

The same majority voting is done to the stop bit as done for the other bits in the frame. If the stop bit is registered to

have a logic 0 value, the Frame Error (FEn) Flag will be set.

A new high to low transition indicating the start bit of a new frame can come right after the last of the bits used for

majority voting. For Normal Speed mode, the first low level sample can be at point marked (A) in Figure 22-7 on

page 214. For Double Speed mode the first low level must be delayed to (B). (C) marks a stop bit of full length. The

early start bit detection influences the operational range of the Receiver.

22.8.3

Asynchronous Operational Range

The operational range of the Receiver is dependent on the mismatch between the received bit rate and the inter-

nally 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 22-2) 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.

Sum of character size and parity size (D = 5 to 10 bit).

Samples per bit. S = 16 for Normal Speed mode and S = 8 for Double Speed mode.

First sample number used for majority voting. SF = 8 for normal speed and SF = 4 for Double Speed

mode.

Middle sample number used for majority voting. SM = 9 for normal speed and SM = 5 for Double

Speed mode.

Rslow

is the ratio of the slowest incoming data rate that can be accepted in relation to the receiver baud

rate. Rfast is the ratio of the fastest incoming data rate that can be accepted in relation to the receiver baud rate.

Table 22-2 and Table 22-3 on page 216 list the maximum receiver baud rate error that can be tolerated. Note that

Normal Speed mode has higher toleration of baud rate variations.

Table 22-2.

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

Rslow

D 1

S

S 1

–



-------------------------------------------

Rfast

D 2

S

D 1

SS

-----------------------------------

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