NUC970 Technical Reference Manual
Publication Release Date: Dec. 15, 2015
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Revision V1.30
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Synchronizations occur on edges from recessive to dominant, their purpose is to control the distance
between edges and Sample Points.
Edges are detected by sampling the actual bus level in each time quantum and comparing it with the
bus level at the previous Sample Point. A synchronization may be done only if a recessive bit was
sampled at the previous Sample Point and if the bus level at the actual time quantum is dominant.
An edge is synchronous if it occurs inside of Sync_Seg, otherwise the distance between edge and the
end of Sync_Seg is the edge phase error, measured in time quanta. If the edge occurs before
Sync_Seg, the phase error is negative, else it is positive.
Two types of synchronization exist, Hard Synchronization and Re-synchronization.
A Hard Synchronization is done once at the start of a frame and inside a frame only when Re-
synchronizations occur.
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Hard Synchronization
After a hard synchronization, the bit time is restarted with the end of Sync_Seg, regardless of
the edge phase error. Thus hard synchronization forces the edge, which has caused the hard
synchronization to lie within the synchronization segment of the restarted bit time.
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Bit Re-synchronization
Re-synchronization leads to a shortening or lengthening of the bit time such that the position of the
sample point is shifted with regard to the edge.
When the phase error of the edge which causes Re-synchronization is positive, Phase_Seg1 is
lengthened. If the magnitude of the phase error is less than SJW, Phase_Seg1 is lengthened by the
magnitude of the phase error, else it is lengthened by SJW.
When the phase error of the edge, which causes Re-synchronization is negative, Phase_Seg2 is
shortened. If the magnitude of the phase error is less than SJW, Phase_Seg2 is shortened by the
magnitude of the phase error, else it is shortened by SJW.
When the magnitude of the phase error of the edge is less than or equal to the programmed value of
SJW, the results of Hard Synchronization and Re-synchronization are the same. If the magnitude of
the phase error is larger than SJW, the Re-synchronization cannot compensate the phase error
completely, an error (phase error - SJW) remains.
Only one synchronization may be done between two Sample Points. The Synchronizations maintain a
minimum distance between edges and Sample Points, giving the bus level time to stabilize and
filtering out spikes that are shorter than (Pr Phase_Seg1).
Apart from noise spikes, most synchronizations are caused by arbitration. All nodes synchronize
“hard” on the edge transmitted by the “leading” transceiver that started transmitting first, but due to
propagation delay times, t
hey cannot become ideally synchronized. The “leading” transmitter does not
necessarily win the arbitration, therefore the receivers have to synchronize themselves to different
transmitters that subsequently “take the lead” and that are differently synchronized to the previously
“leading” transmitter. The same happens at the acknowledge field, where the transmitter and some of
the receivers will have to synchronize to that receiver that “takes the lead” in the transmission of the
dominant acknowledge bit.
Synchronizations after the end of the arbitration will be caused by oscillator tolerance, when the
differences in the oscillator’s clock periods of transmitter and receivers sum up during the time
between synchronizations (at most ten bits). These summarized differences may not be longer than
the SJW, limiting the oscillator’s tolerance range.
The examples in the following figure show how the Phase Buffer Segments are used to compensate
for phase errors. There are three drawings of each two consecutive bit timings. The upper drawing