LM3S9792 Datasheet, PDF (776/1279 Pages) Texas Instruments – Stellaris® LM3S9792 Microcontroller

Controller Area Network (CAN) Module
The last two functions, combined with the readable CAN receive pin CANnRX, can be used to check
the physical layer of the CAN bus.
The Transmit Control function is enabled by programming the TX[1:0] field in the CANTST register.
The three test functions for the CANnTX signal interfere with all CAN protocol functions. TX[1:0]
must be cleared when CAN message transfer or Loopback Mode, Silent Mode, or Basic Mode are
selected.
18.3.14
Bit Timing Configuration Error Considerations
Even if minor errors in the configuration of the CAN bit timing do not result in immediate failure, the
performance of a CAN network can be reduced significantly. In many cases, the CAN bit
synchronization amends a faulty configuration of the CAN bit timing to such a degree that only
occasionally an error frame is generated. In the case of arbitration, however, when two or more
CAN nodes simultaneously try to transmit a frame, a misplaced sample point may cause one of the
transmitters to become error passive. The analysis of such sporadic errors requires a detailed
knowledge of the CAN bit synchronization inside a CAN node and of the CAN nodes' interaction on
the CAN bus.
18.3.15
Bit Time and Bit Rate
The CAN system supports bit rates in the range of lower than 1 Kbps up to 1000 Kbps. Each member
of the CAN network has its own clock generator. The timing parameter of the bit time can be
configured individually for each CAN node, creating a common bit rate even though the CAN nodes'
oscillator periods may be different.
Because of small variations in frequency caused by changes in temperature or voltage and by
deteriorating components, these oscillators are not absolutely stable. As long as the variations
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
18-4 on page 777): 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 18-4 on page 777). The length of the time quantum (t_q), which is
the basic time unit of the bit time, is defined by the CAN controller's input clock (fsys) and the Baud
Rate Prescaler (BRP):
t_q = BRP / fsys
The fsys input clock is the system clock frequency as configured by the RCC or RCC2 registers
(see page 129 or page 137).
The Synchronization Segment Sync 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 and the Sync is
called the phase error of that edge.
The Propagation Time Segment Prop is intended to compensate for the physical delay times within
the CAN network.
The Phase Buffer Segments Phase1 and Phase2 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.
776
June 14, 2010
Texas Instruments-Advance Information