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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

THE NEXT READ

The next required read brings in the length of the frame that will be transferred from the CS8900A-CQ to the PIC16F877. This is the total length of bytes starting with the DA and ending with the last byte before the 4-byte CRC value. The frame length value is stored in the first two locations of your PIC frame buffer area (0x30 and 0x31). The CS8900A-CQ receive buffer is then read low byte to high byte into the PIC16F877 RAM for the length of the buffered frame.

The PICýs frame buffer area extends from 0x32 to 0x7F, which works out to be 78 bytes. There is plenty of leftover RAM in the PIC16F877 in other banks that can be used to make a larger buffer, but for this application, 78 bytes are plenty. This means you must only accept frames of 78 bytes or less or only fill your PIC buffer area with 78 bytes maximum and ignore the rest.

Well, there are no bits to limit the incoming frame length, but you can throw away any bytes beyond 78. In Listing 12 under GetFrameData, you will find a flag bit called bitbucket. If incrementing the FSR (pointing to the next available PIC RAM register) puts the FSR over 0x7F, the rest of the remaining bytes are read from the CS8900A-CQ receive buffer and dumped into the bit bucket.

Instead of throwing away bytes beyond address 0x7F in the CS8900A-CQ receive buffer, you could read the RxEvent register after transferring byte 0x7F and trigger the CS8900A-CQ to move on to the next frame in its buffer. But because youýre reading the entire frame, thereýs no need to do that. My method also ensures that the entire frame is transferred or read whether or not all of the data is used.

Photo 1ýMy PICMASTER may be old, but it can still twiddle bits. (enlarge)

Letýs assume a host sent an ARP request to the Ethernet development board and all of the code in Listing 12 was executed successfully. Photo 1 shows how it looks inside the PIC16F877. Photo 2 shows what the Sniffer sees in the same situation and same frame.

Photo 2ýNot only is frame content broken out, itýs broken out in coordinating colors. (enlarge)

 The frame length is displayed at locations 0x30 and 0x31 in the PIC dump. The Sniffer shot simply tells you the frame size of the area I highlighted. Starting at location 0x32 in the PIC dump photo, you can follow along byte for byte in the Sniffer hex dump area at the bottom of Photo 2. Take another look at Figure 1 as I walk you through this frame. You can correlate the PIC and Sniffer data with the Figure 1 frame sub-areas. Because the Sniffer gives details not obvious in the PIC dump, use the Sniffer shots as your pointer to the relative (and identical) data in the PIC frame dump.

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