Hello folks.
I have designed a DM641 DSP-based board with a National Semiconducto DP83848C Ethernet PHY chip, and am facing a rather strange (or not problem.
I have connected a PC to the board and am running a TCP/IP stack on th DSP. At 10Mbps, everything works great - I am able to ping the board (0 packet loss), telnet into it, etc. However, at 100Mbps, no packets ge through to the DSP. Autonegotiation works fine, though.
However, when I run a diagnostic on the PC-side Ethernet card, I get message that says the board is unable to support 100Mbps operation Further, when I place a breakpoint in the TCP/IP stack code whenever th link state changes and force a link speed and duplex on the PC side, th board autonegotiates to the right speed, but not always to the correc duplex.
I have scoped the cable side Tx outputs of the transformer at 100Mbps an all I see is something like noise. The peak voltage is about 1.5 (relative to ground). I am, however, using a 100MHz scope.
AFAIK, I have followed all PCB design guidelines as shown in the dat sheet. Since I don't know what the output waveforms look like, I am no sure how to go about debugging this problem.
Could any of you please throw some light on this problem? Your help woul be greatly appreciated.
Thanks!
--Vishnu