LM3S5C36 Datasheet, PDF (756/1068 Pages) Texas Instruments – Stellaris® LM3S5C36 Microcontroller

Controller Area Network (CAN) Module
16.3.7
16.3.8
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 on 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, causing
the CANINT register to point to the message object that just received a message. The TXRQST bit
of this message object should be cleared to prevent the transmission of a remote frame.
Receiving a Remote Frame
A remote frame contains no data, but instead specifies which object should be transmitted. When
a remote frame is received, three different configurations of the matching message object have to
be considered:
Table 16-2. Message Object Configurations
Configuration in CANIFnMCTL
Description
■ DIR = 1 (direction = transmit); programmed in the At the reception of a matching remote frame, the TXRQST bit of this
CANIFnARB2 register
message object is set. The rest of the message object remains
unchanged, and the controller automatically transfers the data in
■ RMTEN = 1 (set the TXRQST bit of the
the message object as soon as possible.
CANIFnMCTL register at reception of the frame
to enable transmission)
■ UMASK = 1 or 0
■ DIR = 1 (direction = transmit); programmed in the At the reception of a matching remote frame, the TXRQST bit of this
CANIFnARB2 register
message object remains unchanged, and the remote frame is
ignored. This remote frame is disabled, the data is not transferred
■ RMTEN = 0 (do not change the TXRQST bit of the _{and nothing indicates that the remote frame ever happened.}
CANIFnMCTL register at reception of the frame)
■ UMASK = 0 (ignore mask in the CANIFnMSKn
register)
■ DIR = 1 (direction = transmit); programmed in the At the reception of a matching remote frame, the TXRQST bit of this
CANIFnARB2 register
message object is cleared. The arbitration and control field (ID +
XTD + RMTEN + DLC) from the shift register is stored into the message
■ RMTEN = 0 (do not change the TXRQST bit of the _{object in the message RAM, and the NEWDAT bit of this message}
CANIFnMCTL register at reception of the frame) _{object is set. The data field of the message object remains}
unchanged; the remote frame is treated similar to a received data
■ UMASK = 1 (use mask (MSK, MXTD, and MDIR in _{frame. This mode is useful for a remote data request from another}
the CANIFnMSKn register) for acceptance filtering) _{CAN device for which the Stellaris controller does not have readily}
available data. The software must fill the data and answer the frame
manually.
756
January 23, 2012
Texas Instruments-Production Data