DSPIC30F4012 Microchip Technology Inc., DSPIC30F4012 Datasheet - Page 131

• Manufacturer Part Number: DSPIC30F4012
• Description: Dspic30f4011/4012 Enhanced Flash 16-bit Digital Signal Controller
• Manufacturer: Microchip Technology Inc.

19.6.2
There is a programmable prescaler with integral values
ranging from 1 to 64 in addition to a fixed divide-by-2 for
clock generation. The Time Quantum (T
unit of time derived from the oscillator period, shown in
Equation 19-1, where F
is set) or 4 F
EQUATION 19-1:
19.6.3
This part of the bit time is used to compensate physical
delay times within the network. These delay times con-
sist of the signal propagation time on the bus line and
the internal delay time of the nodes. The propagation
segment can be programmed from 1 T
setting the PRSEG<2:0> bits (C1CFG2<2:0>).
19.6.4
The phase segments are used to optimally locate the
sampling of the received bit within the transmitted bit
time. The sampling point is between Phase1 Seg and
Phase2 Seg. These segments are lengthened or short-
ened by resynchronization. The end of the Phase1 Seg
determines the sampling point within a bit period. The
segment is programmable from 1 T
Seg provides delay to the next transmitted data transi-
tion. The segment is programmable from 1 T
or it may be defined to be equal to the greater of
Phase1 Seg or the information processing time (2 T
The Phase1 Seg is initialized by setting bits
SEG1PH<2:0> (C1CFG2<5:3>), and Phase2 Seg is
initialized by setting SEG2PH<2:0> (C1CFG2<10:8>).
The following requirement must be fulfilled while setting
the lengths of the phase segments:
© 2007 Microchip Technology Inc.
Propagation Segment + Phase1 Seg > = Phase2 Seg
Note:
PRESCALER SETTING
F
CANCKS = 0, then F
7.5 MHz.
PROPAGATION SEGMENT
PHASE SEGMENTS
CY
CAN
T
Q
(if CANCKS is cleared).
= 2 (BRP<5:0> + 1)/F
must not exceed 30 MHz. If
TIME QUANTUM FOR
CLOCK GENERATION
CAN
is F
CY
CY
(if the CANCKS bit
Q
must not exceed
CAN
to 8 T
Q
Q
) is a fixed
to 8 T
Q
Q
. Phase2
to 8 T
Q
Q
by
Q
).
,
19.6.5
The sample point is the point of time at which the bus
level is read and interpreted as the value of that respec-
tive bit. The location is at the end of Phase1 Seg. If the bit
timing is slow and contains many T
specify multiple sampling of the bus line at the sample
point. The level determined by the CAN bus then cor-
responds to the result from the majority decision of three
values. The majority samples are taken at the sample
point and twice before with a distance of T
module allows the user to choose between sampling
three times at the same point, or once at the same point,
by setting or clearing the SAM bit (C1CFG2<6>).
Typically, the sampling of the bit should take place at
about 60-70% through the bit time depending on the
system parameters.
19.6.6
To compensate for phase shifts between the oscillator
frequencies of the different bus stations, each CAN
controller must be able to synchronize to the relevant
signal edge of the incoming signal. When an edge in
the transmitted data is detected, the logic will compare
the location of the edge to the expected time
(synchronous segment). The circuit will then adjust the
values of Phase1 Seg and Phase2 Seg. There are
2 mechanisms used to synchronize.
19.6.6.1
Hard synchronization is only done whenever there is a
‘recessive’ to ‘dominant’ edge during bus Idle, indicating
the start of a message. After hard synchronization, the
bit time counters are restarted with the synchronous
segment. Hard synchronization forces the edge which
has caused the hard synchronization to lie within the
synchronization segment of the restarted bit time. If a
hard synchronization is done, there will not be a
resynchronization within that bit time.
19.6.6.2
As a result of resynchronization, Phase1 Seg may be
lengthened or Phase2 Seg may be shortened. The
amount of lengthening or shortening of the phase buffer
segment has an upper bound known as the
synchronization jump width, and is specified by the
SJW<1:0> bits (C1CFG1<7:6>). The value of the
synchronization jump width will be added to Phase1 Seg
or subtracted from Phase2 Seg. The resynchronization
jump width is programmable between 1 T
The following requirement must be fulfilled while setting
the SJW<1:0> bits:
dsPIC30F4011/4012
Phase2 Seg > Synchronization Jump Width
SAMPLE POINT
SYNCHRONIZATION
Hard Synchronization
Resynchronization
DS70135E-page 129
Q
, it is possible to
Q
Q
/2. The CAN
and 4 T
Q
.