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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

INITIATE TRANSMIT REGISTERS

These registers tell the CS8900A-CQ how to transmit the next packet. The first four definitions in Listing 9 are values that can be used to instruct the CS8900A-CQ to start transmitting after a certain number of bytes have been transferred to the CS8900A-CQ transmit buffer area. For instance, TXCMD_AFTER_5 says start transmitting after five bytes are in the CS8900A-CQ buffer. In the code snippet that follows the defines, you see that I used TXCMD_AFTER_ALL, which translates to wait for every byte sent to be loaded into the CS8900A-CQ transmit buffer before transmitting.

Letýs talk about the high-order byte of the TXCMD register. If the TXCMD_NO_CRC bit is included in the mask, the CS8900A-CQ generated CRC value is not appended to the transmission. Because you are appending a CRC and the TXCMD_NO_PAD bit is clear, any packet less than 60 bytes will be padded to 60 bytes and the CRC will be appended to the transmission.

The reason for control is to allow illegal packets to be transmitted. This illegal activity could be used in the development or test phase of a design to test other Ethernet devices or Ethernet data capture software like the Sniffer. The TXCMD_FORCE bit flushes any frames waiting to be transmitted and aborts the transmission of any frame in the process of being transmitted.

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