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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

ARP

If you look at the top of any of the Sniffer screen shots, youýll notice that the second event is an ARP broadcast, and the third event is the ARP reply from physical address 00EDTP. Photo 8 looks similar to the ARP request screen shot with the exception of an additional physical address supplied by the Ethernet development board (000045445450) and an Opcode 2 in the ARP frame defining the frame as an ARP reply. The ARP process is somewhat like an algebra problem. You use the knowns (IP addresses) to solve for the unknowns (physical addresses).

Photo 8ýThis shot is a steak dinner perfectly cooked. (enlarge)

If you start at the DA and count the total number of bytes in the ARP reply, youýll come up with 42. Thatýs 60 bytes total frame length minus 18 bytes of padding. In Listing 14, Port C is commanded to output 0x2A and 42 bytes of ARP response buffer area are requested from the CS8900A-CQ. This is called a bid in the CS8900A-CQ datasheet. After the CS8900A-CQ allocates the space and sets the RDY4TXNOW_BIT, the bits of the ARP reply flow out of the PIC16F877 RAM in the order of the subareas you see in Figure 1 and Photo 8, starting with the DA. After all of the bytes are collected in the CS8900A-CQ transmit buffer, the CS8900A-CQ generates a preamble that is immediately followed by an SFD. And assuming no collision occurs, the ARP reply hits the ether followed by a CS8900A-CQ-generated CRC.

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