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Controller Area Network (CAN)
If a transmission fails due to a loss of arbitration or an error, the message transmission will be
reattempted. Before reattempting the transmission, the CAN module checks if other transmissions are
requested. If the TRS bit of a higher-priority (determined either by the MBX number or by the associated
TPL value) mailbox had been set before the message in the transmit-buffer has lost arbitration, the
transmit-buffer contents will be replaced with that of the higher-priority mailbox and the higher priority
mailbox will be transmitted after the arbitration loss. However, if that TRS bit was set after the message in
the transmit-buffer has lost arbitration, the higher priority MBX will be transmitted only after the current
message in the transmit-buffer has been transmitted.
13.6.2 Receive Mailbox
The identifier of each incoming message is compared to the identifiers held in the receive mailboxes using
the appropriate mask. When equality is detected, the received identifier, the control bits, and the data
bytes are written into the matching RAM location. At the same time, the corresponding receive-message-
pending bit, RMP[n] (RMP.31-0), is set and a receive interrupt is generated if enabled. If no match is
detected, the message is not stored.
When a message is received, the message controller starts looking for a matching identifier at the mailbox
with the highest mailbox number. Mailbox 15 of the eCAN in SCC compatible mode has the highest
receive priority; mailbox 31 has the highest receive priority of the eCAN in eCAN mode.
RMP[n] (RMP.31-0) has to be reset by the CPU after reading the data. If a second message has been
received for this mailbox and the receive-message-pending bit is already set, the corresponding message-
lost bit (RML[n] (RML.31-0)) is set. In this case, the stored message is overwritten with the new data if the
overwrite-protection bit OPC[n] (OPC.31-0) is cleared; otherwise, the next mailboxes are checked.
If a mailbox is configured as a receive mailbox and the RTR bit is set for it, the mailbox can send a remote
frame. Once the remote frame is sent, the TRS bit of the mailbox is cleared by the CAN module.
13.6.3 CAN Module Operation in Normal Configuration
If the CAN module is being used in normal configuration (that is, not in self-test mode), there should be at
least one more CAN module on the network, configured for the same bit rate. The other CAN module
need NOT be configured to actually receive messages into the mailbox RAM from the transmitting node.
But, it should be configured for the same bit rate. This is because a transmitting CAN module expects at
least one node in the CAN network to acknowledge the proper reception of a transmitted message. Per
CAN protocol specification, any CAN node that received a message will acknowledge (unless the
acknowledge mechanism has been explicitly turned off), irrespective of whether it has been configured to
store the received message or not. It is not possible to turn off the acknowledge mechanism in the eCAN
module.
The requirement of another node does not exist for the self-test mode (STM). In this mode, a transmitting
node generates its own acknowledge signal. The only requirement is that the node be configured for any
valid bit-rate. That is, the bit timing registers should not contain a value that is not permitted by the CAN
protocol.
It is not possible to achieve a direct digital loopback externally by connecting the CANTX and CANRX pins
together (as is possible with SCI/SPI/McBSP modules). An internal loopback is possible in the self-test
mode (STM).
13.7 eCAN Configuration
This section explains the process of initialization and describes the procedures to configure the eCAN
module.
13.7.1 CAN Module Initialization
The CAN module must be initialized before the utilization. Initialization is only possible if the module is in
initialization mode. Figure 13-6 is a flow chart showing the process.
Programming CCR (CANMC.12) = 1 sets the initialization mode. The initialization can be performed only
when CCE (CANES.4) = 1. Afterwards, the configuration registers can be written.
SCC mode only:
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