34.6.5.1 Acceptance Filtering

The CAN offers the possibility to configure two sets of acceptance filters, one for standard identifiers and one for extended identifiers. These filters can be assigned to an Rx Buffer or to Rx FIFO 0,1. For acceptance filtering each list of filters is executed from element #0 until the first matching element. Acceptance filtering stops at the first matching element. The following filter elements are not evaluated for this message.

The main features are:

  • Each filter element can be configured as
    • range filter (from - to)
    • filter for one or two dedicated IDs
    • classic bit mask filter
  • Each filter element is configurable for acceptance or rejection filtering
  • Each filter element can be enabled / disabled individually
  • Filters are checked sequentially, execution stops with the first matching filter element

Related configuration registers are:

  • Global Filter Configuration GFC
  • Standard ID Filter Configuration SIDFC
  • Extended ID Filter Configuration XIDFC
  • Extended ID AND Mask XIDAM

Depending on the configuration of the filter element (SFEC/EFEC) a match triggers one of the following actions:

  • Store received frame in FIFO 0 or FIFO 1
  • Store received frame in Rx Buffer
  • Store received frame in Rx Buffer and generate pulse at filter event pin
  • Reject received frame
  • Set High Priority Message interrupt flag IR.HPM
  • Set High Priority Message interrupt flag IR.HPM and store received frame in FIFO 0 or FIFO 1

Acceptance filtering is started after the complete identifier has been received. After acceptance filtering has completed, and if a matching Rx Buffer or Rx FIFO has been found, the Message Handler starts writing the received message data in portions of 32 bit to the matching Rx Buffer or Rx FIFO. If the CAN protocol controller has detected an error condition (e.g. CRC error), this message is discarded with the following impact on the affected Rx Buffer or Rx FIFO:

Rx Buffer
New Data flag of matching Rx Buffer is not set, but Rx Buffer (partly) overwritten with received data. For error type see PSR.LEC respectively PSR.FLEC.

Rx FIFO
Put index of matching Rx FIFO is not updated, but related Rx FIFO element (partly) overwritten with received data. For error type see PSR.LEC respectively PSR.FLEC. In case the matching Rx FIFO is operated in overwrite mode, the boundary conditions described in Rx FIFO Overwrite Mode have to be considered.

Note: When an accepted message is written to one of the two Rx FIFOs, or into an Rx Buffer, the unmodified received identifier is stored independent of the filter(s) used. The result of the acceptance filter process is strongly depending on the sequence of configured filter elements.

Range Filter

The filter matches for all received frames with Message IDs in the range defined by SF1ID/SF2ID for standard frames or EF1ID/EF2ID for extended frames.

There are two possibilities when range filtering is used together with extended frames:

EFT = “00”
The Message ID of received frames is AND’ed with the Extended ID AND Mask (XIDAM) before the
 range filter is applied
EFT = “11”
The Extended ID AND Mask (XIDAM) is not used for range filtering

Filter for specific IDs

A filter element can be configured to filter for one or two specific Message IDs. To filter for one specific Message ID, the filter element has to be configured with SF1ID = SF2ID resp. EF1ID = EF2ID.

Classic Bit Mask Filter

Classic bit mask filtering is intended to filter groups of Message IDs by masking single bits of a received Message ID. With classic bit mask filtering SF1ID/EF1ID is used as Message ID filter, while SF2ID/EF2ID is used as filter mask.

A zero bit at the filter mask will mask out the corresponding bit position of the configured ID filter, e.g. the value of the received Message ID at that bit position is not relevant for acceptance filtering. Only those bits of the received Message ID where the corresponding mask bits are one are relevant for acceptance filtering.

In case all mask bits are one, a match occurs only when the received Message ID and the Message ID filter are identical. If all mask bits are zero, all Message IDs match.

Standard Message ID Filtering

The figure below shows the flow for standard Message ID (11-bit Identifier) filtering. The Standard Message ID Filter element is described in 34.9.5 Standard Message ID Filter Element.

Controlled by the Global Filter Configuration GFC and the Standard ID Filter Configuration SIDFC Message ID, Remote Transmission Request bit (RTR), and the Identifier Extension bit (IDE) of received frames are compared against the list of configured filter elements.

Figure 34-5. Standard Message ID Filtering

Extended Message ID Filtering

The figure below shows the flow for extended Message ID (29-bit Identifier) filtering. The Extended Message ID Filter element is described in 34.9.6 Extended Message ID Filter Element.

Controlled by the Global Filter Configuration GFC and the Extended ID Filter Configuration XIDFC Message ID, Remote Transmission Request bit (RTR), and the Identifier Extension bit (IDE) of received frames are compared against the list of configured filter elements.

The Extended ID AND Mask XIDAM is AND’ed with the received identifier before the filter list is executed.

Figure 34-6. Extended Message ID Filtering