CAN Basic Architectures - CAN System Engineering - page 18

Table 1.2 Effective data rate
at 1 Mbit/s

Data field
in bytes

Effective data rate for frames with
11-bit identifier 29-bit identifier

0 ― ―

1 72.1 kbit/s 61.1 kbit/s

2 144.1 kbit/s 122.1 kbit/s

3 216.2 kbit/s 183.2 kbit/s

4 288.3 kbit/s 244.3 kbit/s

5 360.4 kbit/s 305.3 kbit/s

6 432.4 kbit/s 366.4 kbit/s

7 504.5 kbit/s 427.5 kbit/s

8 576.6 kbit/s 488.5 kbit/s

1.2.6 Bit-Timing and Synchronization

CAN supports data rates of less than 1 kbit/s and also up to 1,000 kbit/s. Each
CAN node in a CAN network is clocked by an individual clock generator (usually
a quartz oscillator). The parameters of the bit time (i.e. the inverse of the bit rate)
are individually adjusted in each CAN node, to achieve a uniform bit rate even from
different oscillator frequencies (fosc).
The parameters are written into the configuration registers of the bit timing logic
( BTL ). The Baud Rate Prescaler ( BRP ) determines the length of the time quantum
(tq), which is the basic unit of the bit time, while the timing segments determine the
number of time quanta in the bit time. The clock frequencies of these oscillators are
not absolutely stable, caused by temperature or voltage fluctuations and the aging of
components. As long as the deviations remain within a certain oscillator tolerance
range ( df ), the CAN nodes are able to compensate for the differences by synchroniz-
ing to the edges in CAN frames.
According to the CAN specification, a CAN bit time is divided into four seg-
ments (see Fig. 1.8 ): the Synchronization Segment , the Propagation Time Segment ,
the Phase Buffer Segment 1 and the Phase Buffer Segment 2 . Each segment consists
of a certain (programmable) number of time quanta . The duration of a time quantum
(t q ) is determined by the CAN controller's system clock f sys and the BRP : t q = BRP /
f sys . Common clocks are f sys = f osc or f sys = f osc /2.
The synchronization segment, Sync_Seg , is that part of the bit time where edg-
es of the CAN bus level are expected to occur. The Propagation Time Segment,
Prop_Seg, is intended to compensate for the physical delay times within the CAN
network. The Phase Buffer Segments, Phase_Seg1 and Phase_Seg2, surround the
Sample Point. The (re-) synchronization jump width ( SJW ) defines how far a re-
synchronization may move the Sample Point inside the limits, defined by the Phase
Buffer Segments.
The individual parameters are configurable in the following ranges:

• Sync_Seg : 1
• Prop_Seg : [1…8]