Please pardon the noob question, but I'm kinda stuck:

I have a Microblaze running on a Digilent Nexys 4 board (Artix 7 FPGA, SMC 8720 
RMII PHY) and I'm having some pretty serious difficulty getting any of the 
sample apps provided in their SDK to work properly. I'm using the 1.4.0 
distribution of lwIP that's included in the Xilinx 2014.2 SDK - latest stuff 
from them, although not 1.4.1 from lwIP.  I have done some poking around the 
Wiki and am getting my bearings with the lwIP implementation, but I'm not quite 
there yet.

In particular, I'm just trying to run the raw mode echo server app that listens 
on port 7 which is supposed to echo back anything that comes in on TCP to that 
port, however I've verified with netmon that the board is not responding to any 
ARP requests from the PC, so it's pretty much dead in the water.  However the 
application appears to start normally: On the console I can see the app 
assigning the static IP address/mask/gateway, and then the startup message from 
the echo server app appears.

When lwIP debugging is enabled, the startup messages from the app are followed 
by a long stream of pbuf frees of this sort:
"pbuf_free(0x6e01a8c0)
pbuf_free: 0x6e01a8c0 has ref 65535, ending here."

This is interspersed with this message:
"tcp_slowtmr: no active pcbs"

And eventually, the pbuf_free messages stop, leaving just the periodic "no 
active pcbs" message going as the timer fires.

I'm not really quite sure whether this behavior points the finger in any 
particular direction, so I'm also unsure where to go next in the debugging 
process. I'm also uncertain as to whether the problem is something in the 
underlying FPGA PHY->MAC implementation that I've put together, or whether the 
pbuf messages indicate that packets of some sort are being recognized and 
there's some purely software problem as to why the ARPs aren't getting handled 
and responded to.  Incidentally, I also tried turning on the ICMP debugging 
option, but this causes garbage to get spewed to the console, which doesn't 
occur with any of the other debug options.

Any pointers of what I might want to look at next would be greatly appreciated.

Thanks!
Andy
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