Civil Engineering Reference
In-Depth Information
transmission of this message must be controlled. In order to activate this function
any number greater than 0 must be inserted. After a successful transmission and
independently of any filter conditions, the controller writes this message into the
receive buffer together with this number and a 32-bit-long time stamp, if desired.
Writing into address 0 automatically enables transmission of the message.
Removing a message from a CAN cell can also be done without inserting another
message (Remove Pending Message). In case the node is only alone at the bus,
because the communication lines were disconnected, the node would continuously
retry to transmit the message. After reconnecting the node, the node would retrans-
mit the message again. A removal of the actual message together with a reset of the
transmit FIFO pointer enables once again the setup of the messages to be transmit-
ted. The number of message within the FIFO can also be read back.
3.2.2
Masks and Filters
In order to ease the evaluation of received CAN messages, this controller provides
256 pairs of masks and filters. There is an object number assigned to each of the
filter pairs, which is written into the receive buffer after a successful check. Because
programming is done in two steps, each of the masks and each of the filters pro-
vide an additional bit. When applied, both of the bits must be set, in order to avoid
the controller to use non-valid combinations for comparison. This makes sure that
masks and filters can be reprogrammed even though the system is running. As only
those messages are written into the receive buffer which have passed the filter con-
dition, a filter can be applied as such to pass all messages. Setting a 1 in bit position
x of the mask defines that the value of the filter bit and the received Identifier (ID)
bit in position x must match. A 0 indicates that the comparison is switched off and
the comparator always would indicate a match.
3.2.3
Receive Buffer
The receive buffer is FIFO organized. It can be parameterized to be 32-256 mes-
sages long. Each message is stored as a sequence of four messages.
• The address 0 contains the 8-bit-long frame number for messages which had
been transmitted by this controller itself, the object number as an identification
which of the filters had passed this message, the remote frame bit, as well as the
data length code.
• The address 1 contains the standard and extended ID.
• The addresses 2 and 3 contain the received data bytes.
• An additional address allows the receive time stamp of the message to be read.
