HI-3110
The CAN standard divides the bit time into four segments,
BIT TIMING
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-3110 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).
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-
3110 is limited to 40kHz by the permanent dominant timeout
protection of the transceiver). Every node on the network
has it’s 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
it’s own oscillator frequency, fOSC. This is done by generating
a time quanta (TQ) clock, whose period tTQ is related to the
oscillator frequency by a Baud Rate Prescaler value, BRP as
follows:
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.
tTQ = 2•BRP/fOSC
(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, tTQ, as shown in figure 8 below.
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.
The CAN system nominal bit rate (BR) is defined in terms of
the nominal bit time, tb, as
BR = 1/tb
(2)
Therefore, the nominal bit rate is related to the TQ clock
period by the following relationship
BR = 1/(tTQ x (number of time quanta per bit))
(3)
tOSC
OSC
Baud Rate Prescaler
tTQ
TQ
Clock
Nominal bit time, tb
Tq
Sync
Seg
Sync
Seg
TSeg1 =
Prop Seg + Phase Seg 1
TSeg2 =
Phase Seg2
TSeg1
TSeg2
Sample
Point
Figure 8. CAN Bit Time
HOLT INTEGRATED CIRCUITS
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HOLTIC [ HOLT INTEGRATED CIRCUITS ]
