Posted on May 17, 2011 at 09:56I'm trying to make ethernet mac to receive multicast frames using the hash registers as described in the str912 reference. The documentation is somewhat unspecific on the actual procedure. In my understanding the ether mac hardware calculates ethernet CRC32 over 6 bytes of destination address and then uses the most significant bit (bit 31) to determine which (hi/lo) hash register to use. The next 5 bits (bits 30-26) determine which bit in the hash register is checked to see if the frame is to be received.
This is pretty much how ethernet mac operates on some coldfire chips. If I use the same code (found in some linux distribution for coldfire) to calculate hash register values the bit I get is in correct. If I calculate the CRC32 using CRC32C-library I cannot get correct values. I can find the correct bit in hash register by trial and error (by setting bits in hash registers and trying to receive multicast frames until I succeed), but values I get by calculating CRC are not the same. The CRC functions I've used calculate correct CRC for test frames from IEEE ether standard example frames.
My asumption is that the CRC register is reset at the start of the frame, the 6 bytes of destination address are hashed into it and the current CRC value is used to determine if the frame is to be received by the hardware. If so, the CRC register will be updated until end of frame, then it is complemented and compared to FCS of the ether frame to determine if the frame was correctly received. I assume that the CRC value after the destination address is not to be complemented (I'm not getting correct values, with or without complementing the CRC).
Does anyone have code sample showing excatly how the correct hash register bit value is deduced from given ether multicast address?
Thank you in advance.
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niko
