When I try to send a unicast packet, the packet is never sent but I can see a lots of ARP requests sent by the w5100 but these requests look odds:
The IP address to resolve in the ARP requests are always 0.0.0.0, regardless the destination IP I set in the S0_DIPR register.
If I set the MAC address of the host in the S0_DHAR register in addition to the previous register, then the packet is sent correclty and received by the remote host.
On the other hand, the w5100 responds correctly to ARP requests from other hosts and also responds to ping requests.
The S0_CR register is well cleared when the packet is (or should be) sent and the SEND_OK bit of the S0_IR register is 1.
Here is a screenshot of a Wireshark capture of the strange ARP requests:
the w5100 has the 192.168.188.181 address after the DHCP process. As you can see, UDP broadcast works fine (do not take care of duplicated packets, it is because of the switch port mirroring), the w5100 event reply to an ARP request but then send 8 strange ARP requests with an interval of 200 ms:
Someone has already had this kind of issue ?