CANwireless CR3133
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8.3.3 Filtering CAN messages
You can configure in the device which CAN messages are forwarded from the CAN interface to the
radio interface. The bus load of the CAN and radio interfaces is reduced if no more data than
necessary is allowed.
A filter and a mask for the CAN ID decide whether a message is forwarded to the radio interface. Up
to eight filters can be configured. If there are less than eight messages, a separate filter can be used
for each message. Use mask 0x7FF for an 11-bit CAN ID and mask 0x1FFFFFFF for a 29-bit CAN ID.
The device uses the following variables to calculate whether to allow or block a CAN message:
Variable name Description
RxCANid CAN ID of the received CAN message
FilterValue Filter with which a CAN ID is identified as authorised
MaskValue Mask with which a CAN ID is identified as authorised
The verification mechanism works as follows:
RxCANid XOR FilterValue = checkedCANid
checkedCANid AND MaskValue = RESULT
If RESULT = 0, the message is authorised.
Example 1:
RxCANid: 0x123
FilterValue: 0x421
MaskValue: 0xF8
Description Hex Binary
Received CAN message 0x123 0b001.0010.0011
FilterValue 0x421 0b100.0010.0001
XOR
checkedCANid 0x502 0b101.0000.0010
MaskValue 0x0F8 0b000.1111.1000
AND
RESULT 0x000 0b000.0000.0000
→ With these settings, the CAN ID 0x123 is authorised; messages with this ID are forwarded.
Example 2:
RxCANid: 0x123
FilterValue: 0x431
MaskValue: 0xF8
Description Hex Binary
Received CAN message 0x123 0b001.0010.0011
FilterValue 0x431 0b100.0011.0001
XOR
checkedCANid 0x512 0b101.0001.0010
MaskValue 0x0F8 0b000.1111.1000
AND
RESULT 0x010 0b000.0001.0000
→ With these settings, the CAN ID 0x123 is rejected; messages with this ID will not be forwarded.
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