STR720 datasheet(254/401 Pages) STMICROELECTRONICS

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STR720 - CONTROLLER AREA NETWORK (CAN)

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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 example, the magnitude of phase error of this “

early” edge’s 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 these examples, the bit timing is seen from the point of view of the CAN state machine,

where the bit time starts and ends at the Sample Points. The state machine omits Sync_Seg

when synchronizing on an “early” edge, because it cannot subsequently redefine that time

quantum of Phase_Seg2 where the edge occurs to be the Sync_Seg.

Figure 37. Synchronization on “late” and “early” Edges

recessive

dominant

recessive

dominant

Sync_Seg

Prop_Seg

Phase_Seg1

Phase_Seg2

“late” Edge

“early” Edge

Rx-Input

Sample-Point

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