ATmega644PA datasheet(188/468 Pages) ATMEL

188 / 468 page

188

8152G–AVR–11/09

ATmega164PA/324PA/644PA/1284P

Figure 18-7. 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 18-7 on page 188. 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.

18.9.3

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 18-2 on page 189) 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 18-2 on page 189 and Table 18-3 on page 189 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

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

ATmega644PA Datasheet(PDF) 188 Page - ATMEL Corporation

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