CAN Basic Architectures - CAN System Engineering - page 23

Fig. 1.11 Phase shift between the nodes in the CAN network

and

SJW
Bitperiod

(1.2)

df = .

20•

Both inequalities must hold, with df indicating how much a CAN node's oscillator
frequency may deviate from its reference value without causing disturbances on the
CAN bus.
The oscillator tolerance df (given in per cent) thus depends on the length of the
Phase Buffer Segments as well as the length of the Resynchronization Jump Width ,
compared to the entire bit time. It must be noted that the Resynchronization Jump
Width may not be longer than the shorter of the Phase Buffer Segments and that the
partition of the bit time available for the Phase Buffer Segments is limited by the
Propagation Time Segment .

1.2.6.4 Setting the Bit Timing Registers

Two cases have to be distinguished when programming a CAN node's bit time
configuration. When the CAN node is integrated into an existing network, it needs
to adopt the bit time configuration that was specified for the network. Here, the
only variable is the BRP . Depending on the node's fsys frequency, BRP defines the
length of the tq, with the requirement that an integer number of tq needs to be placed
in one bit time, before and after the Sample Point . In CANopen-based networks, for
example, the Sample Point should be set to 85 % of the bit time. The standardized
configuration allows the easy integration of modules into a common CAN network,
even when the modules are supplied by different vendors.
If no default configuration is available, the parameters of the bit time are largely
determined by the desired bit rate and by the signal propagation delay. The compo-
nents of the bit time are calculated as follows: First, the delay times in the network