LPC1800 Ethernet Tx underflow

Discussion created by lpcware Employee on Jun 15, 2016
Latest reply on Jun 15, 2016 by lpcware
Content originally posted in LPCWare by Grant.Edwards on Thu May 16 14:19:57 MST 2013

I'm having problems with tx underflow errors on an LPC1857 eval board
from Keil.

Any time I attempt to send a packet longer than about 300 bytes, I
get a tx underflow error, and the tx side of the the Ethernet
controller locks up until it is reset.  I've tried DMA burst lengths
of 1,8,16, and 64: they all behave the same.  I don't seem to have any
problem receiving long packets.

I've also noted that when I send packets shorter than 300 bytes, I
have about 10-15% packet loss: all of the the packets appear to be
sent (link LED flashes), but 10-15% of them are invalid and get
dropped by the receiving device(s).  I've tried a variety of devices
on the "other end" and I get the same 10-15% packet loss with all of

My M3 clock is 120MHz, and the Ethernet link is running at 100Mb
full duplex.  The Ethernet descriptors and buffers are all in SDRAM
that's also running at 120Mhz.

Any advice on how to get Ethernet tx to work?