LM3S2110 Datasheet, PDF (478/649 Pages) List of Unclassifed Manufacturers – Microcontroller

Controller Area Network (CAN) Module
13.3.1
13.3.2
The protocol controller transfers and receives the serial data from the CAN bus and passes the data
on to the message handler. The message handler then loads this information into the appropriate
message object based on the current filtering and identifiers in the message object memory. The
message handler is also responsible for generating interrupts based on events on the CAN bus.
The message object memory is a set of 32 identical memory blocks that hold the current configuration,
status, and actual data for each message object. These are accessed via either of the CAN message
object register interfaces.
The message memory is not directly accessible in the Stellaris memory map, so the Stellaris CAN
controller provides an interface to communicate with the message memory via two CAN interface
register sets for communicating with the message objects. As there is no direct access to the
message object memory, these two interfaces must be used to read or write to each message object.
The two message object interfaces allow parallel access to the CAN controller message objects
when multiple objects may have new information that must be processed. In general, one interface
is used for transmit data and one for receive data.
Initialization
Software initialization is started by setting the INIT bit in the CAN Control (CANCTL) register (with
software or by a hardware reset) or by going bus-off, which occurs when the transmitter's error
counter exceeds a count of 255. While INIT is set, all message transfers to and from the CAN bus
are stopped and the CANnTX signal is held High. Entering the initialization state does not change
the configuration of the CAN controller, the message objects, or the error counters. However, some
configuration registers are only accessible while in the initialization state.
To initialize the CAN controller, set the CAN Bit Timing (CANBIT) register and configure each
message object. If a message object is not needed, label it as not valid by clearing the MSGVAL bit
in the CAN IFn Arbitration 2 (CANIFnARB2) register. Otherwise, the whole message object must
be initialized, as the fields of the message object may not have valid information, causing unexpected
results. Both the INIT and CCE bits in the CANCTL register must be set in order to access the
CANBIT register and the CAN Baud Rate Prescaler Extension (CANBRPE) register to configure
the bit timing. To leave the initialization state, the INIT bit must be cleared. Afterwards, the internal
Bit Stream Processor (BSP) synchronizes itself to the data transfer on the CAN bus by waiting for
the occurrence of a sequence of 11 consecutive recessive bits (indicating a bus idle condition)
before it takes part in bus activities and starts message transfers. Message object initialization does
not require the CAN to be in the initialization state and can be done on the fly. However, message
objects should all be configured to particular identifiers or set to not valid before message transfer
starts. To change the configuration of a message object during normal operation, clear the MSGVAL
bit in the CANIFnARB2 register to indicate that the message object is not valid during the change.
When the configuration is completed, set the MSGVAL bit again to indicate that the message object
is once again valid.
Operation
There are two sets of CAN Interface Registers (CANIF1x and CANIF2x), which are used to access
the message objects in the Message RAM. The CAN controller coordinates transfers to and from
the Message RAM to and from the registers. The two sets are independent and identical and can
be used to queue transactions. Generally, one interface is used to transmit data and one is used to
receive data.
Once the CAN module is initialized and the INIT bit in the CANCTL register is cleared, the CAN
module synchronizes itself to the CAN bus and starts the message transfer. As each message is
received, it goes through the message handler's filtering process, and if it passes through the filter,
is stored in the message object specified by the MNUM bit in the CAN IFn Command Request
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July 15, 2014
Texas Instruments-Production Data