This is a simple driver for DAVICOM DM9000 A/B/BI Ethernet MACs running on 8-bit Atmel AVR microcontrollers I wrote some years ago. Tested and running for hours with continuous data flow on multiple ATmega128.

• Adjust dm9000.h to your needs:
  • Change DM9000_DATA_IN_PORT, DM9000_DATA_OUT_PORT, and DM9000_DDR_REG to the PIN, PORT, and DDR registers for the AVR I/O port that is connected to the data bus of the MAC. The data pins SD0 to SD7 must be connected to exactly one AVR I/O port and have their pin numbering match, i.e. connect SD3 to PA3 if you use PORTA. Using the DM9000 pins SD8 to SD15 for 16-bit I/O is not supported. If you use an AVR port with special functions for this, you need to make sure they are disabled.
  • Adjust NET_MAC_1 to NET_MAC_6 to the desired MAC address of your device.
  • Change DM9000_CS_PORT and DM9000_CS_PIN to the port and pin definition you connected the DM9000 CS# pin to.
  • Change DM9000_IOR_PORT and DM9000_IOR_PIN to the port and pin definition you connected the DM9000 IOR# pin to.
  • Change DM9000_IOW_PORT and DM9000_IOW_PIN to the port and pin definition you connected the DM9000 IOW# pin to.
  • Change DM9000_CMD_PORT and DM9000_CMD_PIN to the port and pin definition you connected the DM9000 CMD pin to.
• Make sure the DM9000 is configured to use a data bus width of 8 bit and a CS# polarity of active low as nothing else is supported. Check the data sheet for details on how to do this using the strap pins.
• Add dm9000.c into your firmware.
• Include dm9000.h somewhere in your program and use it:
  • Call ethernet_init_chip at some point in your global initialization routine. Also call ethernet_activate_rx if you want to receive packets.
  • If you want to transmit a packet: Call ethernet_buffer_tx_sync with a pointer to the data to send and the length of the buffer. You can call it multiple times to gradually fill the buffer, but all data has to stay in the same packet. Then call ethernet_tx_async which will actually start the transmission. It will return immediately after instructing the DM9000 to transmit and not wait for completion. It will, however, wait for the previous packet to finish transmission before sending a new one.
  • If you want to receive a packet: Call ethernet_is_packet_ready periodically to check whether a packet has been received (interrupts are not supported, but you can easily wire it up on your own if necessary). Then call ethernet_rx_sync with a pointer to a buffer that will receive the data and the length of the buffer, which is also the number of data bytes to read from the DM9000-internal RX ring buffer. You have to interpret the data yourself, which means that you will first want to read the DM9000_RX_HEADER_SIZE bytes that make up the DM9000 packet header. This tells you how many data bytes were received, which you can then again fetch via ethernet_rx_sync. If you don't want the packet, you still have to read it out in order to free space in the DM9000 RX buffer. You may use ethernet_dump_rx_sync if you have to skip some bytes without actually saving them anywhere at all.
  • If you need to do fancy things, you can use ethernet_read_register, ethernet_write_register, ethernet_read_phy_register and ethernet_write_phy_register to directly affect the internal workings of the DM9000. Be aware that this might interfere with the operation of this code, so be careful.
  • The MAC address is initialized with the data from NET_MAC_1 etc. in dm9000.h. If you want to change it later on, use ethernet_set_mac.

• All TX/RX data is entirely raw. Nothings is done to the data at all, which means you have to send/receive full Ethernet MAC-layer frames. For RX packets, the DM9000 4-byte packet header and the CRC32 trailer is also included.
• When receiving data, only trust the packet size advertised in the DM9000 packet header and not in any additional headers (IP etc.) for determining how much bytes you have to be read.
• Be sure to correctly set the MAC address. The DM9000 will be configured to drop all received packets that are not addressed either to the set address or the broadcast address by default.
• The throughput will obviously not reach 100 Mbps because of the bitbanging, but it should be enough for simple control applications.
• The code is very light on checking error conditions, so it's mainly useful for hobbyists.

Davicom Ethernet product page with datasheets