Designers Corner with Prof. Mike Smith  Part 1

Dividing the work

At this point we had a plan. We would take a divide and conquer approach. We would use a PHY chip for the Ethernet interface. This meant we had to design and build our own MAC. To do that we would first learn how the PHY chip worked by using a Level One evaluation board.

The Level One evaluation Network Interface Card (NIC). This card contains the same Level One PHY chip we used in the final design. By soldering a few wires to the back of the NIC we were able to access the signals from the PHY and MAC chips. This allowed us to figure out how to build our own MAC core. We attached a logic analyzer to the PHY chip, captured a few Ethernet packets, then converted the trace to a spreadsheet and from there converted the signals to VHDL for simulation purposes.

We had a choice of Level One evaluation boards. One board contained just the LXT970A PHY chip, which meant we would have to build a MAC that connected to the PHY chip via a standard media-independent interface (MII). The other board was a complete Network Interface Card (NIC). There were advantages of both the NIC and the MII board. I tried to find a commercial NIC with a LXT970 part but could not (most of the Level One chips were used in embedded applications). In fact, we had a hard time finding any commercial NICs with separate PHY and MAC chips. While we waited for the evaluation boards from Wyle, the Level One distributor, we ended up buying five different NICs from Fry's and using a $400 in-circuit attach probe from Pomona to probe the 100-pin QFP PHY chip. The MAC on the Level One card is a DEC chip (the Tulip chip). The Level One board had an advantage over the other NICs that we bought, because the Tulip chip is very well understood and documented (largely because of the Linux crowd) as far as drivers are concerned. This made the Level One NIC card particularly useful for development purposes.

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