CAN filter configuration to use along libcanard V1

noxuz · December 11, 2020, 8:27pm

Perfect!

The default filter config I was looking for will end up with the ID field as (0x1FFF << 8), and in the case of the mask, same as line #195 from the PyUAVCAN function you shared, adding 3 bits to the mask aside from the (0x1D55 << 8).

github.com/OpenCyphal/pycyphal

pyuavcan/transport/can/_identifier.py

f43afb54e


      
                  # If the local node-ID is not set, we may need to receive loopback frames for sent anonymous transfers.
                  # This essentially means that we need to allow ALL anonymous transfers. Those may be only messages, as there
                  # is no such thing as anonymous service transfer.
                  full.append(ext(idn=_BIT_MSG_ANON,
                                  msk=_BIT_SRV_NOT_MSG | _BIT_MSG_ANON | _BIT_R23 | _BIT_MSG_R7))
          
              # One filter per unique subject-ID. Sorted for testability.
              for sid in sorted(set(subject_id_list)):
                  s_mask = pyuavcan.transport.MessageDataSpecifier.SUBJECT_ID_MASK
                  assert sid <= s_mask
                  full.append(ext(idn=int(sid) << 8,
                                  msk=_BIT_SRV_NOT_MSG | _BIT_R23 | (s_mask << 8) | _BIT_MSG_R7))
          
              return full
          
          
          def _unittest_can_filter_configuration() -> None:
              from .media import FilterConfiguration, optimize_filter_configurations, FrameFormat
          
              def ext(idn: int, msk: int) -> FilterConfiguration:
                  assert idn <= _CANID_EXT_MASK and msk <= _CANID_EXT_MASK

I also found the migration guide below that clarifies why the bits 22 and 21 are being ignored in the mask. This completes the explanation of all the bits particular when dealing solely with messages, as in the simplest case mentioned earlier.

I think this step of configuring the filter is the only one that is skipped from the libcanard example provided in the repo’s readme; and adding this step to the section that demonstrates how to setup reception could be of support for the documentation.

Quick idea is a single line just below the previous section:

configureHWfilter(7509, 2**13 - 1); // ID-Mask pair for receiving Hearbeat messages

Thank you Pavel for the explanation.