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THE ETHERNET DEVELOPMENT BOARD

Part 2: The Software and Firmware Exposed
by Fred Eady

Start ý The Basics ý Registers ý Bus Interface Registers ý Status and Control Registers ý InitChip ý Initiate Transmit Registers ý Address Filter Registers ý Receive and Transmit Frame Locations ý CS8900A-CQ Transmit and Receive Operations ý The Next Read ý Broadcast ý ARP ý No Cheating ý Tiger Woodsý Putter ý Whatýs the Point? ý And It Programs, Too! ý Sources and PDF

NO CHEATING

If youýre reading ahead, and I expect you are, you already know that the fourth event is unmasked in Photo 8. You also know it is a UDP transaction between the PC called SPEADY and the Ethernet development board. Iýve already presented lots of words on IP protocol in other Circuit Cellar articles. (Is that a low-down way to get you to read my stuff, or what?) So, Iým not going to go into detail about what the IP fields mean and do here. I do want to point out that the Sniffer fields match the PIC IP field layout in the Ethernet development board code you see in Listing 15.

Photo 9 highlights the DLC-type field in the hex dump area containing 0800, which signifies an IP frame. Following the logic in Listing 16, you see that 0806 caused an ARP replay frame to be built and transmitted, and 0800 sends the code down the path to process an IP frame. After youýre headed in the IP frame direction, there are still decisions to be made as to where the code will branch to next. The protocol field within the IP header determines that. In Photo 9, the protocol value is 0x11, or 17 decimal. That means that UDP header information and data is contained within the IP data area.

Photo 9ýAll this work for one byte of data! (enlarge)

The Ethernet development board firmware examines the UDP destination port to determine how to handle the UDP data. In this case, the destination is Port 7. When UDP Port 7 is addressed in the Ethernet development board firmware, an echo operation is performed. That is, the data, in this case an "a" (0x61), is echoed back to the source hostýs port 5002. You can see from the fifth event at the top of Photo 9 that this entails swapping the source and destination ports, IP addresses, and physical addresses. This swapped frame is returned to the senderýs UDP port with the UDP data untouched. Compare Photo 9 and Photo 10 to realize the concept.

Photo 10ýAnd, one byte of data back to you. (enlarge)

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