RM0033 Rev 9 803/1381
RM0033 Controller area network (bxCAN)
837
signals the overrun condition by setting the FOVR bit in the CAN_RFR register. Which
message is lost depends on the configuration of the FIFO:
• If the FIFO lock function is disabled (RFLM bit in the CAN_MCR register cleared) the
last message stored in the FIFO will be overwritten by the new incoming message. In
this case the latest messages will be always available to the application.
• If the FIFO lock function is enabled (RFLM bit in the CAN_MCR register set) the most
recent message will be discarded and the software will have the three oldest messages
in the FIFO available.
Reception related interrupts
Once a message has been stored in the FIFO, the FMP[1:0] bits are updated and an
interrupt request is generated if the FMPIE bit in the CAN_IER register is set.
When the FIFO becomes full (i.e. a third message is stored) the FULL bit in the CAN_RFR
register is set and an interrupt is generated if the FFIE bit in the CAN_IER register is set.
On overrun condition, the FOVR bit is set and an interrupt is generated if the FOVIE bit in
the CAN_IER register is set.
27.7.4 Identifier filtering
In the CAN protocol the identifier of a message is not associated with the address of a node
but related to the content of the message. Consequently a transmitter broadcasts its
message to all receivers. On message reception a receiver node decides - depending on
the identifier value - whether the software needs the message or not. If the message is
needed, it is copied into the SRAM. If not, the message must be discarded without
intervention by the software.
To fulfill this requirement, the bxCAN Controller provides 28 configurable and scalable filter
banks (27-0) to the application. In other devices the bxCAN Controller provides 14
configurable and scalable filter banks (13-0) to the application in order to receive only the
messages the software needs. This hardware filtering saves CPU resources which would be
otherwise needed to perform filtering by software. Each filter bank x consists of two 32-bit
registers, CAN_FxR0 and CAN_FxR1.
Scalable width
To optimize and adapt the filters to the application needs, each filter bank can be scaled
independently. Depending on the filter scale a filter bank provides:
• One 32-bit filter for the STDID[10:0], EXTID[17:0], IDE and RTR bits.
• Two 16-bit filters for the STDID[10:0], RTR, IDE and EXTID[17:15] bits.
Refer to Figure 305.
Furthermore, the filters can be configured in mask mode or in identifier list mode.
Mask mode
In mask mode the identifier registers are associated with mask registers specifying which
bits of the identifier are handled as “must match” or as “don’t care”.
Identifier list mode
In identifier list mode, the mask registers are used as identifier registers. Thus instead of
defining an identifier and a mask, two identifiers are specified, doubling the number of single
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