MEGA128CAN Datasheet, PDF(186/413 Page) ATMEL Corporation – Microcontroller WITH 128K BYTES OF ISP FLASH AND CAN CONTROLLER

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MEGA128CAN Datasheet, PDF (186/413 Pages) ATMEL Corporation – Microcontroller WITH 128K BYTES OF ISP FLASH AND CAN CONTROLLER

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Figure 89 shows the sampling of the stop bit and the earliest possible beginning of the

start bit of the next frame.

Figure 89. Stop Bit Sampling and Next Start Bit Sampling

RxDn

STOP 1

(A)

(B)

(C)

Sample

(U2Xn = 0)

Sample

(U2Xn = 1)

1 2 3 4 5 6 7 8 9 10 0/1 0/1 0/1

1

2

3

4

5

6

0/1

Asynchronous Operational

Range

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 89. 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.

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 77) 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.

Rslow = -S----â----1--(--D-+-----+D-----1-â---)S--S---+----S----F-

Rfast = -(--D----(--+-D----1--+-)---S2----+)---S--S---M---

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

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

mode.

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

for Double Speed mode.

SM 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 77 and Table 78 list the maximum receiver baud rate error that can be tolerated.

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

186 AT90CAN128

4250EâCANâ12/04

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