xPC Target

Acceptance Filters

As mentioned earlier, the CAN controller's acceptance filters can be used to ensure that certain received messages referenced by their identifiers get written into the receive FIFO. Therefore, fewer read attempts are necessary to get at the messages which are of importance for the target application.

The behavior of the acceptance filter is described for standard and extended identifier ranges individually (one for standard identifiers and one for extended identifiers). Each acceptance filter is defined by a mask parameter and a code parameter.

The mask parameter defines for each bit of the identifier, if the filtering process cares about this bit or not. A 0 means "don't care" and a 1 means "do care."

The code parameter then defines for each bit of the identifier the filtering process cares about (defined by the mask parameter), what the bit value has to be (0 or 1).

For standard identifiers the mask parameter and code parameter have to be both in the range 0 to 2047. For extended identifiers the mask parameter and code parameter have to be both in the range 0 to 2²⁹-1.

The filtering process evaluates the following binary expression

• and( xor( mask, identifier ), code )
  

If all bits of the resulting value are 0, the message with this identifier will be accepted, if one single bit is 1 the message will be voided.

According to this description, acceptance filters work using binary evaluation while most applications use the mental model of differentiating messages (identifiers) in a decimal or hexadecimal manner. As a consequence, it is possible to filter messages, which identifiers are above a certain decimal number, while the opposite (identifiers below a certain decimal number) can not be achieved in a general way.

Examples

The default values in the FIFO setup driver block are both 0 for the mask parameter and the code parameter. These parameter values assure (the above expression always evaluates to 0) that all incoming messages will reach the receive FIFO (no filtering takes place). All parameter values have to be defined using decimal numbers. You can use the MATLAB function hex2dec to also define hexadecimal numbers in the dialog box entry.

Lets assume a CAN application where messages with the following identifiers (standard) are crossing the CAN network:

• 2-30, 48-122 (decimal)
  

Additionally, only incoming messages 4-29 have to be processed by the target application. Ideally, all messages not having identifier 4-29 would be filtered out, but the mask and code parameters don't allow this exact scheme. The closest we can achieve here, is filtering out all messages with identifiers above 31 by using value 2047-31=2016 for the mask parameter and value 0 for the code parameter. The messages with identifier 0,1,2, and 3 cannot be filtered out and will be returned by the FIFO read driver block, even if they have not to be processed by the target application.

FIFO Reset RCV Driver Block

Examples