ATmega325P datasheet(179/364 Pages) ATMEL

179 / 364 page

179

8023F–AVR–07/09

ATmega325P/3250P

Figure 18-8. 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 18-8. 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.

18.8.3

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 18-1) 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 18-1 and Table 18-2 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)

Rslow

D 1

()S

S 1

–

⋅

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

Rfast

D 2

()S

D 1

()SS

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

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