National Semi DP83848C Ethernet issue.

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.



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