Hardware manual

RX600 Series CAN Application Programming Interface

R01AN0339EU0203 Rev. 2.03 Page 20 of 29

Mar 23, 2013

Fast filtering of messages with Acceptance Filter Support

If you have used a mask to receive a broad range of message IDs, you must filter for the actual desired messages

with firmware. To increase the speed of this search one may use the Acceptance Filter Support instead.

The Acceptance Filter Support Unit (ASU) provides a faster search compared to software filtering of messages

using a mask (with the R_CAN_RxSetMask API). Software filtering can be time consuming as the Standard

ID bits are rearranged and not stored as a normal word in memory. Another problem could be that the

acceptance mask may not be able to be set to receive the particular combination of messages you want. If you

set the mask to accept all messages you may have to ‘waste’ time by checking a long list of the messages using

software for each incoming ID. This manual filtering’ would also involve having all the IDs in a readable

format. An efficient solution in such cases is to use the Acceptance Filter Support Unit.

To use it, one writes the CAN-ID as it is stored in the message box into the ASU. When read back from the ASU

Bit 0-7 = Table Address Search Info, ‘ASI’

Bit 8-15 = Bit Search Information, ‘BSI’. SID0-3 has now been converted to a bit position to enable faster

table searches. Use the output to search through a table.

Figure 3. The Acceptance Filter Support Unit (ASU). When read, the representation of the ID

is formatted to enable a fast search through a table. This provides a faster response than a

search through a ‘normal’ array of CAN IDs.

The search table. A table must be prepared by the user to check whether an ID is of interest to the application. The

firmware must search the table at each byte address ASI and bit position BSI. If a bit BSI-value is set in the

user’s table, the bit pattern matches the BSI pattern of the register which means the address is of interest to the

node, and the frame should be processed by the application.

See REJ05B0276 “CAN Application Note” for more information on how to use the ASU. (Download from