AT89C51CC02CA-RATUM datasheet(75/156 Pages) ATMEL

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AT/T89C51CC02

4126J–CAN–05/06

Figure 34. CAN Bit Construction

Synchronization Segment

The first segment is used to synchronize the various bus nodes.

On transmission, at the start of this segment, the current bit level is output. If there is a

bit state change between the previous bit and the current bit, then the bus state change

is expected to occur within this segment by the receiving nodes.

Propagation Time Segment

This segment is used to compensate for signal delays across the network.

This is necessary to compensate for signal propagation delays on the bus line and

through the transceivers of the bus nodes.

Phase Segment 1

Phase Segment 1 is used to compensate for edge phase errors.

This segment may be lengthened during resynchronization.

Sample Point

The sample point is the point of time at which the bus level is read and interpreted as the

value of the respective bit. Its location is at the end of Phase Segment 1 (between the

two Phase Segments).

Phase Segment 2

This segment is also used to compensate for edge phase errors.

This segment may be shortened during resynchronization, but the length has to be at

least as long as the information processing time and may not be more than the length of

Phase Segment 1.

Information Processing Time

It is the time required for the logic to determine the bit level of a sampled bit.

The Information processing Time begins at the sample point, is measured in TQ and is

fixed at 2 TQ for the Atmel CAN. Since Phase Segment 2 also begins at the sample

point and is the last segment in the bit time, Phase Segment 2 minimum shall not be

less than the Information processing Time.

Bit Lengthening

As a result of resynchronization, Phase Segment 1 may be lengthened or Phase Seg-

ment 2 may be shortened to compensate for oscillator tolerances. If, for example, the

transmitter oscillator is slower than the receiver oscillator, the next falling edge used for

resynchronization may be delayed. So Phase Segment 1 is lengthened in order to

adjust the sample point and the end of the bit time.

Time Quantum

(producer)

Nominal CAN Bit Time

Segments

(producer)

SYNC_SEG

PROP_SEG

PHASE_SEG_1

PHASE_SEG_2

propagation

delay

Segments

(consumer)

SYNC_SEG

PROP_SEG

PHASE_SEG_1

PHASE_SEG_2

Sample Point

Transmission Point

(producer)

CAN Frame

(producer)

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