LM3S2276 Datasheet, PDF (615/785 Pages) Texas Instruments – Stellaris® LM3S2276 Microcontroller

Stellaris® LM3S2276 Microcontroller
16.3.5
16.3.6
16.3.7
8. Program the number of the message object to be transmitted in the MNUM field in the CAN IFn
Command Request (CANIFnCRQ) register.
9. When everything is properly configured, set the TXRQST bit in the CANIFnMCTL register. Once
this bit is set, the message object is available to be transmitted, depending on priority and bus
availability. Note that setting the RMTEN bit in the CANIFnMCTL register can also start message
transmission if a matching remote frame has been received.
Updating a Transmit Message Object
The CPU may update the data bytes of a Transmit Message Object any time via the CAN Interface
Registers and neither the MSGVAL bit in the CANIFnARB2 register nor the TXRQST bits in the
CANIFnMCTL register have to be cleared before the update.
Even if only some of the data bytes are to be updated, all four bytes of the corresponding
CANIFnDAn/CANIFnDBn register have to be valid before the content of that register is transferred
to the message object. Either the CPU must write all four bytes into the CANIFnDAn/CANIFnDBn
register or the message object is transferred to the CANIFnDAn/CANIFnDBn register before the
CPU writes the new data bytes.
In order to only update the data in a message object, the WRNRD, DATAA and DATAB bits in the
CANIFnMSKn register are set, followed by writing the updated data into CANIFnDA1, CANIFnDA2,
CANIFnDB1, and CANIFnDB2 registers, and then the number of the message object is written to
the MNUM field in the CAN IFn Command Request (CANIFnCRQ) register. To begin transmission
of the new data as soon as possible, set the TXRQST bit in the CANIFnMSKn register.
To prevent the clearing of the TXRQST bit in the CANIFnMCTL register at the end of a transmission
that may already be in progress while the data is updated, the NEWDAT and TXRQST bits have to be
set at the same time in the CANIFnMCTL register. When these bits are set at the same time, NEWDAT
is cleared as soon as the new transmission has started.
Accepting Received Message Objects
When the arbitration and control field (the ID and XTD bits in the CANIFnARB2 and the RMTEN and
DLC[3:0] bits of the CANIFnMCTL register) of an incoming message is completely shifted into
the CAN controller, the message handling capability of the controller starts scanning the message
RAM for a matching valid message object. To scan the message RAM for a matching message
object, the controller uses the acceptance filtering programmed through the mask bits in the
CANIFnMSKn register and enabled using the UMASK bit in the CANIFnMCTL register. Each valid
message object, starting with object 1, is compared with the incoming message to locate a matching
message object in the message RAM. If a match occurs, the scanning is stopped and the message
handler proceeds depending on whether it is a data frame or remote frame that was received.
Receiving a Data Frame
The message handler stores the message from the CAN controller receive shift register into the
matching message object in the message RAM. The data bytes, all arbitration bits, and the DLC bits
are all stored into the corresponding message object. In this manner, the data bytes are connected
with the identifier even if arbitration masks are used. The NEWDAT bit of the CANIFnMCTL register
is set to indicate that new data has been received. The CPU should clear this bit when it reads the
message object to indicate to the controller that the message has been received, and the buffer is
free to receive more messages. If the CAN controller receives a message and the NEWDAT bit is
already set, the MSGLST bit in the CANIFnMCTL register is set to indicate that the previous data
was lost. If the system requires an interrupt upon successful reception of a frame, the RXIE bit of
the CANIFnMCTL register should be set. In this case, the INTPND bit of the same register is set,
November 17, 2011
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