TM4C1237H6PGE_15 Datasheet, PDF (1053/1324 Pages) Texas Instruments – Tiva Microcontroller

Tiva^™ TM4C1237H6PGE Microcontroller
17.3.1
17.3.2
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 memory blocks are accessed via either of
the CAN message object register interfaces.
The message memory is not directly accessible in the TM4C1237H6PGE memory map, so the
TM4C1237H6PGE CAN controller provides an interface to communicate with the message memory
via two CAN interface register sets for communicating with the message objects. 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
To use the CAN controller, the peripheral clock must be enabled using the RCGC0 register (see
page 443). In addition, the clock to the appropriate GPIO module must be enabled via the RCGC2
register (see page 449). To find out which GPIO port to enable, refer to Table 21-4 on page 1255. Set
the GPIO AFSEL bits for the appropriate pins (see page 664). Configure the PMCn fields in the
GPIOPCTL register to assign the CAN signals to the appropriate pins. See page 683 and Table
21-5 on page 1263.
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
Two sets of CAN Interface Registers (CANIF1x and CANIF2x) 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
June 12, 2014
Texas Instruments-Production Data
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