AcroPack Series AP560
CAN Bus Interface Module
- 38 -
BIT TIMING
The CAN protocol supports a broad range of bit rates, from a few kHz up to
1MHz (
Note:
the minimum bit rate of the HI-3111 is limited to 40kHz by the
permanent dominant timeout protection of the transceiver). Every node on
the network has its own clock generator (typically a quartz oscillator),
however the bit rate must obviously be the same for every node on the bus.
Therefore, each CAN node must be configurable to generate the nominal bit
rate as a function of its own oscillator frequency, f
OSC
. This is done by
generating a time quanta (TQ) clock, whose period t
TQ
is related to the
oscillator frequency by a Baud Rate Prescaler value, BRP as follows:
t
TQ
=
2 • BRP / f
OSC
(1)
The TQ clock is used to construct the bit time in terms of time quanta, such
that one time quantum, Tq, equals one TQ clock period, t
TQ
, as shown in
Figure 8 below.
The CAN system nominal bit rate (BR) is defined in terms of the nominal bit
time, t
b
, as
BR = 1 / t
b
(2)
Therefore, the nominal bit rate is related to the TQ clock period by the
following relationship
BR = 1 / (t
TQ
x (number of time quanta per bit))
(3)
The CAN standard divides the bit time into four segments, namely,
synchronization segment (Sync Seg), propagation time segment (Prop Seg),
phase buffer segment 1 (Phase Seg1) and phase buffer segment 2 (Phase
Seg2). This is illustrated in Figure 8. The HI-3111 fixes the Sync Seg at 1Tq.
Prop Seg and Phase Seg1 are treated as one-time segment, TSeg1, which is
programmable from 2Tq to 16Tq. Phase Seg2 is a second time segment,
TSeg2, which is programmable from 2Tq to 8Tq (
Note:
Not all combinations
are valid, see below for examples).
Synchronization Segment (Sync Seg)
The Sync Seg is the first segment of the bit time and is used to synchronize
the various nodes on the bus. A bit edge is expected to occur within the Sync
Seg.
Propagation Time Segment (Prog Seg)
The Prog Seg is used to compensate for physical delays on the bus, which
include signal propagation delay time on the bus and internal node delay
times. For two nodes A and B communicating on the bus, Prog Seg must be
greater than or equal to the sum of both nodes internal delays plus twice the
bus line propagation delay between the two nodes.
Содержание AcroPack AP560
Страница 28: ...AcroPack Series AP560 CAN Bus Interface Module 28 Figure 2 Standard Data Frame...
Страница 30: ...AcroPack Series AP560 CAN Bus Interface Module 30 Figure 3 Extended Data Frame...
Страница 32: ...AcroPack Series AP560 CAN Bus Interface Module 32 Figure 4 Remote Frame...
Страница 34: ...AcroPack Series AP560 CAN Bus Interface Module 34 Figure 5 Error Frame...
