LM3S8971 Datasheet, PDF (379/577 Pages) List of Unclassifed Manufacturers – Microcontroller

LM3S8971 Microcontroller
remain inside a specific oscillator's tolerance range, the CAN nodes are able to compensate for the
different bit rates by periodically resynchronizing to the bit stream.
According to the CAN specification, the bit time is divided into four segments (see Figure
15-2 on page 379): the Synchronization Segment, the Propagation Time Segment, the Phase Buffer
Segment 1, and the Phase Buffer Segment 2. Each segment consists of a specific, programmable
number of time quanta (see Table 15-3 on page 379). The length of the time quantum (tq), which
is the basic time unit of the bit time, is defined by the CAN controller's system clock (fsys) and the
Baud Rate Prescaler (BRP):
tq = BRP / fsys
The CAN module's system clock fsys is the frequency of its CAN module clock (CAN_CLK) input.
The Synchronization Segment Sync_Seg is that part of the bit time where edges of the CAN bus
level are expected to occur; the distance between an edge that occurs outside of Sync_Seg and
the Sync_Seg is called the phase error of that edge.
The Propagation Time Segment Prop_Seg is intended to compensate for the physical delay times
within the CAN network.
The Phase Buffer Segments Phase_Seg1 and Phase_Seg2 surround the Sample Point.
The (Re-)Synchronization Jump Width (SJW) defines how far a resynchronization may move the
Sample Point inside the limits defined by the Phase Buffer Segments to compensate for edge phase
errors.
A given bit rate may be met by different bit-time configurations, but for the proper function of the
CAN network, the physical delay times and the oscillator's tolerance range have to be considered.
Figure 15-2. CAN Bit Time
Table 15-3. CAN Protocol Ranges^a
Parameter Range Remark
BRP
[1 .. 32] Defines the length of the time quantum t_q
Sync_Seg 1 t_q
Fixed length, synchronization of bus input to system clock
Prop_Seg [1 .. 8] t_q Compensates for the physical delay times
Phase_Seg1 [1 .. 8] t_q May be lengthened temporarily by synchronization
Phase_Seg2 [1 .. 8] t_q May be shortened temporarily by synchronization
October 01, 2007
379
Preliminary