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Functional Description
797
SLAU723A – October 2017 – Revised October 2018
Copyright © 2017–2018, Texas Instruments Incorporated
Controller Area Network (CAN) Module
11.3.13.4 Basic Mode
Basic Mode allows the CAN Controller to be operated without the Message RAM. In Basic Mode, The
CANIF1 registers are used as the transmit buffer. The transmission of the contents of the IF1 registers is
requested by setting the BUSY bit of the CANIF1CRQ register. The CANIF1 registers are locked while the
BUSY bit is set. The BUSY bit indicates that a transmission is pending. As soon the CAN bus is idle, the
CANIF1 registers are loaded into the shift register of the CAN Controller and transmission is started.
When the transmission has completed, the BUSY bit is cleared and the locked CANIF1 registers are
released. A pending transmission can be aborted at any time by clearing the BUSY bit in the CANIF1CRQ
register while the CANIF1 registers are locked. If the CPU has cleared the BUSY bit, a possible
retransmission in case of lost arbitration or an error is disabled.
The CANIF2 Registers are used as a receive buffer. After the reception of a message, the contents of the
shift register are stored in the CANIF2 registers, without any acceptance filtering. Additionally, the actual
contents of the shift register can be monitored during the message transfer. Each time a read message
object is initiated by setting the BUSY bit of the CANIF2CRQ register, the contents of the shift register are
stored into the CANIF2 registers.
In Basic Mode, all message-object-related control and status bits and of the control bits of the
CANIFnCMSK registers are not evaluated. The message number of the CANIFnCRQ registers is also not
evaluated. In the CANIF2MCTL register, the NEWDAT and MSGLST bits retain their function, the
DLC[3:0] field shows the received DLC, the other control bits are cleared.
Basic Mode is enabled by setting the BASIC bit in the CANTST register.
11.3.13.5 Transmit Control
Software can directly override control of the CANnTX signal in four different ways, as follows:
•
CANnTX is controlled by the CAN Controller.
•
The sample point is driven on the CANnTX signal to monitor the bit timing.
•
CANnTX drives a low value.
•
CANnTX drives a high value.
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.
11.3.14 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.
11.3.15 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.