Identifier Filtering - ST STM32F412 Reference Manual

Controller area network (bxCAN)
Overrun
Once the FIFO is in pending_3 state (i.e. the three mailboxes are full) the next valid
message reception leads to an overrun and a message is lost. The hardware 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 is overwritten by the new incoming message. In this
case the latest messages are always available to the application.
•
If the FIFO lock function is enabled (RFLM bit in the CAN_MCR register set) the most
recent message is discarded and the software has 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.
28.7.4

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 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
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".
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Figure 326.
RM0402 Rev 6
RM0402