ATMEGA649 datasheet(176/375 Pages) ATMEL

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176

ATmega329/3290/649/6490

2552H–AVR–11/06

Figure 76. Stop Bit Sampling and Next Start Bit Sampling

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 76. 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 detec-

tion influences the operational range of the Receiver.

Asynchronous Operational

Range

The operational range of the Receiver is dependent on the mismatch between the

received bit rate and the internally 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 78) 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. S

F = 8 for normal speed and SF = 4

for Double Speed mode.

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 78 and Table 79 list the maximum receiver baud rate error that can be tolerated.

Note that Normal Speed mode has higher toleration of baud rate variations.

0/1

STOP 1

123

0/1

RxD

Sample

(U2X = 0)

Sample

(U2X = 1)

(A)

(B)

(C)

slow

()S

–

⋅

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

fast

()S

()SS

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

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