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Controller Area Network
22.2.2.2 Remote Frame
The Remote frame is a request for information from a particular node. It is a message
frame with the RTR bit set, followed by the control field that indicates the number of infor-
mation bytes (0 to 8) it is requesting. When the node with the information receives the
remote frame, it responds by sending the information onto the network.
NOTE: The identifier determines the priority of the node. The lower the identifier, the higher the node priority.
22.2.2.3 Bit Errors
Bit errors occur when either a bit-stuffing violation occurs or when the transmitting node
detects that its transmission of data is different than the data it had intended to send.
Transmitting nodes normally listen to the outgoing data to ensure that it matches the
internal data.
22.2.2.4 Message Errors
Message errors occur if:
• The checksum does not match the data field.
• A predefined bit field is not at its predefined position.
22.2.2.5 Acknowledgement Errors
Acknowledgement errors occur if the transmitter does not receive an acknowledgement
after sending a message frame.
Since errors can occur when data is being transmitted or received, the CAN Controller can
track these errors and generate an interrupt based on a predetermined value. This type of
constant checking for potential errors makes the CAN bus extremely desirable in any elec-
trically hostile environment.
22.2.3 Transmitted and Received Data
Data transmitted to the CAN Controller can follow either the standard (11-bit) or extended
(29-bit) frame format. The Transmit Buffer has 13 bytes between addresses 0xFFFC5180
and 0xFFFC51B0 for saving messages consisting of up to eight bits of data. Before writing
the data to the buffer, the Transmit Buffer Status (bit 2) should be checked to ensure that
the buffer is not busy. If the buffer is busy, the data written is lost and there is no indication
that the data has been lost.
Data transmission is initiated by setting the Transmit Request bit or the Self-Reception
Request bit in the Command Register. If the Self-Reception bit is used, the Transmit Sta-
tus bit in the Status Register is set to 1 and the Transmission Request bit is cleared. The
output appears on the TX output signal. If bus arbitration is lost or if transmission errors
occur while the message is being sent, the CAN Controller tries to send the message again
automatically. Data transmission can be aborted by writing to the Command Register, pro-
vided the transmission has not yet started. To verify the status of the data transmission,
the Status Register can be checked.
22-4
Communication onto the bus is non-destructive; this means that if two nodes are requesting the bus
at the same time, the node with the higher priority gets access to the bus.
LH75400/01/10/11 (Preliminary) User's Guide
6/17/03
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