Following on Kai's recommendations. Analog Devices also even has application notes that can give you lots of good ideas.

Look at the following app note for example. This is for an D/A device which is a bit different than your AD7893 A/D but it does show a batch of 8051 code which will give a guide as to how to go about writing a serial interface to one of these types of analog parts.

http://www.analog.com/UploadedF...2AN325.pdf

Michael Karas