Configuration of the CAN bit timing

AN2352

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Figure 1.

Bit timing

Table 1 describes the minimum programmable ranges required by the CAN protocol.

A given bit rate may be met by different bit time configurations but for the proper functioning

of the CAN network, the physical delay times and the oscillator’s tolerance range must be

taken into consideration.

1.2

Propagation time segment

This part of the bit time compensates physical delay times within the network. These delay

times consist of the signal propagation time on the bus and the internal delay time of the

CAN nodes.

Any CAN node synchronized to the bit stream on the CAN bus will be out of phase with the

transmitter of that bit stream due to the signal propagation time between the two nodes. The

CAN protocol’s nondestructive bit-wise arbitration and the dominant acknowledge bit

provided by the receivers of CAN messages require a CAN node transmitting a bit stream to

also receive dominant bits transmitted by other CAN nodes that are synchronized to that bit

stream. The example in

Figure 2 shows the phase shift and propagation times between two

CAN nodes.

Table 1.

Parameters of the CAN bit time

Parameter

Range

Remark

BRP

[1 .. 32]

Sync_Seg

Fixed length, synchronization of bus input to system clock

Prop_Seg

Compensates the physical delay times

Phase_Seg1

May be lengthened temporarily by synchronization

Phase_Seg2

May be shortened temporarily by synchronization

SJW

May not be longer than either phase buffer segment

1 Time quantum

(t

q

)

Sync_ Prop_Seg

Phase_Seg1

Phase_Seg2

Sample point

Nominal CAN bit time

Seg