LM3S2730 Datasheet, PDF (442/562 Pages) Texas Instruments – Stellaris® LM3S2730 Microcontroller

Controller Area Network (CAN) Module
CANIFnMSKn register to filter which frames are received. The MXTD bit in the CANIFnMSK2 register
should be set if only 29-bit extended identifiers are expected by this message object.
12.3.11
Handling of Received Message Objects
The CPU may read a received message any time via the CAN Interface registers because the data
consistency is guaranteed by the message handler state machine.
Typically, the CPU first writes 0x007F to the CANIFnCMSK register and then writes the number of
the message object to the CANIFnCRQ register. That combination transfers the whole received
message from the message RAM into the Message Buffer registers (CANIFnMSKn, CANIFnARBn,
and CANIFnMCTL). Additionally, the NEWDAT and INTPND bits are cleared in the message RAM,
acknowledging that the message has been read and clearing the pending interrupt generated by
this message object.
If the message object uses masks for acceptance filtering, the CANIFnARBn registers show the
full, unmasked ID for the received message.
The NEWDAT bit in the CANIFnMCTL register shows whether a new message has been received
since the last time this message object was read. The MSGLST bit in the CANIFnMCTL register
shows whether more than one message has been received since the last time this message object
was read. MSGLST is not automatically cleared, and should be cleared by software after reading its
status.
Using a remote frame, the CPU may request new data from another CAN node on the CAN bus.
Setting the TXRQST bit of a receive object causes the transmission of a remote frame with the receive
object's identifier. This remote frame triggers the other CAN node to start the transmission of the
matching data frame. If the matching data frame is received before the remote frame could be
transmitted, the TXRQST bit is automatically reset. This prevents the possible loss of data when the
other device on the CAN bus has already transmitted the data slightly earlier than expected.
12.3.11.1 Configuration of a FIFO Buffer
With the exception of the EOB bit in the CANIFnMCTL register, the configuration of receive message
objects belonging to a FIFO buffer is the same as the configuration of a single receive message
object (see “Configuring a Receive Message Object” on page 441). To concatenate two or more
message objects into a FIFO buffer, the identifiers and masks (if used) of these message objects
have to be programmed to matching values. Due to the implicit priority of the message objects, the
message object with the lowest message object number is the first message object in a FIFO buffer.
The EOB bit of all message objects of a FIFO buffer except the last one must be cleared. The EOB
bit of the last message object of a FIFO buffer is set, indicating it is the last entry in the buffer.
12.3.11.2 Reception of Messages with FIFO Buffers
Received messages with identifiers matching to a FIFO buffer are stored starting with the message
object with the lowest message number. When a message is stored into a message object of a
FIFO buffer, the NEWDAT of the CANIFnMCTL register bit of this message object is set. By setting
NEWDAT while EOB is clear, the message object is locked and cannot be written to by the message
handler until the CPU has cleared the NEWDAT bit. Messages are stored into a FIFO buffer until the
last message object of this FIFO buffer is reached. If none of the preceding message objects has
been released by clearing the NEWDAT bit, all further messages for this FIFO buffer will be written
into the last message object of the FIFO buffer and therefore overwrite previous messages.
442
June 18, 2012
Texas Instruments-Production Data