NUC970 Technical Reference Manual
Publication Release Date: Dec. 15, 2015
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Revision V1.30
NUC97
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shows the synchronization on a “late” edge, the lower drawing shows the synchronization on an “early”
edge, and the middle drawing is the reference without synchronization.
Rx-Input
recessive
dominant
Sync_Seg
Prop_Seg
Phase_Seg
Prop_Seg
Sample-Point
Sample-Point
Sample-Point
Sample-Point
Sample-Point
Sample-Point
recessive
dominant
Rx-Input
“
early Edge
Figure 5.24-9
Synchronization on “late” and “early” Edges
In the first example an edge from recessive to dominant occurs at the end of Prop_Seg. The edge is
“late” since it occurs after the Sync_Seg. Reacting to the “late” edge, Phase_Seg1 is lengthened so
that the distance from the edge to the Sample Point is the same as it would have been from the
Sync_Seg to the Sample Point if no edge had occurred. The phase error of this “late” edge is less than
SJW, so it is fully compensated and the edge from dominant to recessive at the end of the bit, which is
one nominal bit time long, occurs in the Sync_Seg.
In the second example an edge from recessive to dominant occurs during Phase_Seg2. The edge is
“early” since it occurs before a Sync_Seg. Reacting to the “early” edge, Phase_Seg2 is shortened and
Sync_Seg is omitted, so that the distance from the edge to the Sample Point is the same as it would
have been from an Sync_Seg to the Sample Point if no edge had occurred. As in the previous
exam
ple, the magnitude of this “early” edge’s phase error is less than SJW, so it is fully compensated.
The Phase Buffer Segments are lengthened or shortened temporarily only; at the next bit time, the
segments return to their nominal programmed values.
In the
se examples, the bit timing is seen from the point of view of the CAN implementation’s state
machine, where the bit time starts and ends at the Sample Points. The state machine omits Sync_Seg
when synchronis
ing on an “early” edge because it cannot subsequently redefine that time quantum of
Phase_Seg2 where the edge occurs to be the Sync_Seg.
The examples in the following figure show how short dominant noise spikes are filtered by
synchronisations. In both examples the spike starts at the end of Prop_Seg and has the length of
(Pr Phase_Seg1).
In the first example, the Synchronization Jump Width is greater than or equal to the phase error of the
spike’s edge from recessive to dominant. Therefore the Sample Point is shifted after the end of the
spike; a recessive bus level is sampled.
In the second example, SJW is shorter than the phase error, so the Sample Point cannot be shifted far