Although the Realtek 8019AS can be used, I consider it to be relatively crappy. It is doubtful that you will be able to get any useful information on the chip from Realtek, since it was originally designed for the ISA bus on standard PCs. However, because there aren't that many Ethernet controllers out there for embedded use, this non-PCI chip is relatively popular. The Microchip PICDEM.NET demo board's Users Guide has schematics showing it in use. Microchip App Note AN833 "The Microchip TCP/IP Stack" may be useful if you're wondering how to interface with the Realtek's ISA interface. I would provide links to the afformentioned documents, but unfortunately Microchip's website is down for maintenance right now.
The cons of this chip are:
- High pin count wastes board space and increases soldering work
- ISA bus requires bit banging, diminishing performance
- Chip is old and costs a lot
- Data sheet is gobbledygook and incomplete (for the purposes of doing non-ISA-PC oriented designs)
The pros of this chip:
- It exists
- It works at 5V
I'd recommend the Microchip ENC28J60 stand alone Ethernet controller with SPI. It's awesome compared to the 8019AS. The only significant problem is that the chip is new and Microchip's standard distributors don't appear to have any chips to sell you at this point. Unless you can wait, you may have to contact Microchip to obtain chips to play with.
I've written a great deal of code for the ENC28J60 including my own TCP/IP stack. I like it.