I have made a couple of improvements to the ethernet interface in the MEGA65:
1. The Ethernet PHY MIIM registers now work, so you can find out if the ethernet port is connected, and at what speed, for example. This was not too tricky, except that I had the bus running at 2x the correct speed, because I read the 400ns edge-to-edge time, i.e., half-clock time, as being the clock cycle time. After that, it was all peachy.
2. MAC address filtering, including recognising broadcast and multi-cast ethernet frames.
This is basically an extension of the "allow bad-CRC" flag, together with adding registers to store the MAC address of the machine, so that ethernet frames can be filtered if the CRC is bad, if the frame is not broadcast, not multicast, and not addressed to the MEGA65. The broadcast and multicast checks can be independently disabled, if you really want to cut down on processing load, by only listening for packets addressed to the machine itself.
After chasing down the bugs introduced in the process, it now all works. LGB wrote a nice little ethernet test utility, that responds to ARP requests and PING. This now works very nicely, and I can ping the MEGA65 from my linux laptop:
$ ping 192.168.1.65
PING 192.168.1.65 (192.168.1.65) 56(84) bytes of data.
64 bytes from 192.168.1.65: icmp_seq=1 ttl=64 time=0.316 ms
64 bytes from 192.168.1.65: icmp_seq=2 ttl=64 time=0.304 ms
64 bytes from 192.168.1.65: icmp_seq=3 ttl=64 time=0.304 ms
64 bytes from 192.168.1.65: icmp_seq=4 ttl=64 time=0.336 ms
64 bytes from 192.168.1.65: icmp_seq=5 ttl=64 time=0.306 ms
--- 192.168.1.65 ping statistics ---
5 packets transmitted, 5 received, 0% packet loss, time 4085ms
rtt min/avg/max/mdev = 0.304/0.313/0.336/0.016 ms
$ arp -na? (192.168.178.1) auf 38:10:d5:29:66:ef [ether] auf wlp1s0
? (192.168.1.65) auf 02:47:53:65:65:65 [ether] auf enp0s31f6
Here we can see the program running on the MEGA65, with counters on the top line for the number of packets received (RX), transmitted (TX), ARP packets replied to (ARP), PING packets replied to (PING) and UDP packets seen (UDP).
Below that is the MAC address of the MEGA65 displayed three times over: