the identifier stored in the message buffers (RXn_ARB0, RXn_ARB1). The result is
masker able (logic AND operation) with global mask (FILTER_AM_0, FILTER_AM_1).
Message Valid if
for i = 28 downto 18
(“MsgID(i)” XOR” RXn_arbitration (i) )AND “Accept. Mask(i)” = 0
If the incoming message is filtered out from the first message buffer, the arbitration field
of the incoming message is checked with the identifier of the second message buffer,
and if filtered out, with the identifier of the third message buffer. The global mask feature
(used to accept a set of incoming messages) permits that the incoming message could
match with all the three-messages buffers but the received message is stored on the
lowest numbered message buffer.
Interrupt Generation
The interrupt generation on the INT pin can be programmed by the external MCU writing
the internal configuration register. Interrupt can be generated for:
•
•
•
•
•
Either message correctly transmitted
Or message correctly received ( data frame, not remote frame reception).
Or overrun condition on the rx message buffer
Or bus-off condition
Or error passive condition
In the SETUP register, it is possible to configure which kind of interrupt will be generated
by the AT7908E device.
Fault Confinement and Error
Counters
The two internal error counters are readable from the external MCU to check the error
status of the CAN controller. The error counter operates according to the Fault Confine-
ment rules of the CAN Specification 2.0B, exception included. This means that if there is
only one node of the AT7908E connected on the BUS and this node starts to transmit
message, it meets the error passive condition but not the bus-off condition, because the
transmitter error counter will no longer be incremented for acknowledge error when it is
in error passive state.
Trigger Match Function
The trigger match function is a feature implemented to generate the HATRIG signal (a
pulse for a clock cycle) if the Identifier of the incoming message matches the
TRIGGER_MATCH register value. The incoming message identifier(msg_id28 down to
msg_id18) is checked with the TM28 down to the TM18 bits of the TRIGGER_MATCH
registers. This function is independent from the filtration of message and from the inter-
rupt generation. The HATRIG signal, together with the CAN node, regularly generating
the appropriate frame, could be used to force the bus switching in a system with two
physical bus used as nominal and redundant.
8
AT7908E
4268D–AERO–11/09
ATMEL [ ATMEL ]
