Microchip Technology MCP2515DM-PTPLS Data Sheet
2003-2012 Microchip Technology Inc.
DS21801G-page 41
MCP2515
5.2
Synchronization
To compensate for phase shifts between the oscillator
frequencies of each of the nodes on the bus, each CAN
controller must be able to synchronize to the relevant
signal edge of the incoming signal. Synchronization is
the process by which the DPLL function is
implemented.
frequencies of each of the nodes on the bus, each CAN
controller must be able to synchronize to the relevant
signal edge of the incoming signal. Synchronization is
the process by which the DPLL function is
implemented.
When an edge in the transmitted data is detected, the
logic will compare the location of the edge to the
expected time (SyncSeg). The circuit will then adjust
the values of PS1 and PS2 as necessary.
logic will compare the location of the edge to the
expected time (SyncSeg). The circuit will then adjust
the values of PS1 and PS2 as necessary.
There are two mechanisms used for synchronization:
1.
Hard synchronization
2.
Resynchronization
5.2.1
HARD SYNCHRONIZATION
Hard synchronization is only performed when there is a
recessive-to-dominant edge during a BUS IDLE
condition, indicating the start of a message. After hard
synchronization, the bit time counters are restarted with
SyncSeg.
recessive-to-dominant edge during a BUS IDLE
condition, indicating the start of a message. After hard
synchronization, the bit time counters are restarted with
SyncSeg.
Hard synchronization forces the edge that has
occurred to lie within the synchronization segment of
the restarted bit time. Due to the rules of
synchronization, if a hard synchronization occurs, there
will not be a resynchronization within that bit time.
occurred to lie within the synchronization segment of
the restarted bit time. Due to the rules of
synchronization, if a hard synchronization occurs, there
will not be a resynchronization within that bit time.
5.2.2
RESYNCHRONIZATION
As a result of resynchronization, PS1 may be
lengthened or PS2 may be shortened. The amount of
lengthening or shortening of the phase buffer segments
has an upper-bound, given by the Synchronization
Jump Width (SJW).
lengthened or PS2 may be shortened. The amount of
lengthening or shortening of the phase buffer segments
has an upper-bound, given by the Synchronization
Jump Width (SJW).
The value of the SJW will be added to PS1 or
subtracted from PS2 (see
subtracted from PS2 (see
). The SJW
represents the loop filtering of the DPLL. The SJW is
programmable between 1 TQ and 4 TQ.
programmable between 1 TQ and 4 TQ.
5.2.2.1
Phase Errors
The NRZ bit coding method does not encode a clock
into the message. Clocking information will only be
derived from recessive-to-dominant transitions. The
property which states that only a fixed maximum
number of successive bits have the same value (bit-
stuffing) ensures resynchronization to the bit stream
during a frame.
into the message. Clocking information will only be
derived from recessive-to-dominant transitions. The
property which states that only a fixed maximum
number of successive bits have the same value (bit-
stuffing) ensures resynchronization to the bit stream
during a frame.
The phase error of an edge is given by the position of
the edge relative to SyncSeg, measured in TQ. The
phase error is defined in magnitude of TQ as follows:
the edge relative to SyncSeg, measured in TQ. The
phase error is defined in magnitude of TQ as follows:
• e = 0 if the edge lies within SYNCSEG
• e > 0 if the edge lies before the SAMPLE POINT
(TQ is added to PS1)
• e < 0 if the edge lies after the SAMPLE POINT of
the previous bit (TQ is subtracted from PS2)
5.2.2.2
No Phase Error (e =
0
)
If the magnitude of the phase error is less than or equal
to the programmed value of the SJW, the effect of a
resynchronization is the same as that of a hard
synchronization.
to the programmed value of the SJW, the effect of a
resynchronization is the same as that of a hard
synchronization.
5.2.2.3
Positive Phase Error (e >
0
)
If the magnitude of the phase error is larger than the
SJW and, if the phase error is positive, PS1 is
lengthened by an amount equal to the SJW.
SJW and, if the phase error is positive, PS1 is
lengthened by an amount equal to the SJW.
5.2.2.4
Negative Phase Error (e <
0
)
If the magnitude of the phase error is larger than the
resynchronization jump width and the phase error is
negative, PS2 is shortened by an amount equal to the
SJW.
resynchronization jump width and the phase error is
negative, PS2 is shortened by an amount equal to the
SJW.
5.2.3
SYNCHRONIZATION RULES
1.
Only recessive-to-dominant edges will be used
for synchronization.
for synchronization.
2.
Only one synchronization within one bit time is
allowed.
allowed.
3.
An edge will be used for synchronization only if
the value detected at the previous sample point
(previously read bus value) differs from the bus
value immediately after the edge.
the value detected at the previous sample point
(previously read bus value) differs from the bus
value immediately after the edge.
4.
A transmitting node will not resynchronize on a
positive phase error (e > 0).
positive phase error (e > 0).
5.
If the absolute magnitude of the phase error is
greater than the SJW, the appropriate phase
segment will adjust by an amount equal to the
SJW.
greater than the SJW, the appropriate phase
segment will adjust by an amount equal to the
SJW.