ST72561 datasheet(185/262 Pages) STMICROELECTRONICS

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ST72561

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beCAN CONTROLLER (Cont’d)

10.9.4.6 Bit Timing

The bit timing logic monitors the serial bus-line and

performs sampling and adjustment of the sample

point by synchronizing on the start-bit edge and re-

synchronizing on the following edges.

Its operation may be explained simply by splitting

nominal bit time into three segments as follows:

– Synchronization segment (SYNC_SEG): a bit

change is expected to occur within this time seg-

ment. It has a fixed length of one time quantum

(1 x tCAN).

– Bit segment 1 (BS1): defines the location of the

sample point. It includes the PROP_SEG and

PHASE_SEG1 of the CAN standard. Its duration

is programmable between 1 and 16 time quanta

but may be automatically lengthened to compen-

sate for positive phase drifts due to differences in

the frequency of the various nodes of the net-

work.

– Bit segment 2 (BS2): defines the location of the

transmit point. It represents the PHASE_SEG2

of the CAN standard. Its duration is programma-

ble between 1 and 8 time quanta but may also be

automatically shortened to compensate for neg-

ative phase drifts.

– Resynchronization Jump Width (RJW): de-

fines an upper bound to the amount of lengthen-

ing or shortening of the bit segments. It is

programmable between 1 and 4 time quanta.

To guarantee the correct behaviour of the CAN

controller, SYNC_SEG + BS1 + BS2 must be

greater than or equal to 5 time quanta.

For a detailed description of the CAN resynchroni-

zation mechanism and other bit timing configura-

tion constraints, please refer to the Bosch CAN

standard 2.0.

As a safeguard against programming errors, the

configuration of the Bit Timing Registers CBTR1

and CBTR0 is only possible while the device is in

Initialization mode.

Figure 105. Bit Timing

Figure 106. CAN Frames (Part 1of 2)

SYNC_SEG

BIT SEGMENT 1 (BS1)

BIT SEGMENT 2 (BS2)

NOMINAL BIT TIME

1 x tCAN

tBS1

tBS2

SAMPLE POINT

TRANSMIT POINT

Data Field

8 * N

Control Field

Arbitration Field

CRC Field

Ack Field

DLC

CRC

Data Frame (Standard identifier)

44 + 8 * N

Inter-Frame Space

or Overload Frame

Inter-Frame Space

EOF

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