Tom,

The C8051F120 series parts from SiLabs have many more SFRs (special function registers) that what fit into the original 8051 SFR space of 128 bytes. So to get around this the SiLabs folks added the SFRPAGE register that is designed to be a bank selector register to select what bank of the SFRs to talk to at any given time. To access specific registers in the SFR space it is generally necessary to proceed the access with a load of the SFRPAGE register to the right page to access the desired register.

What Andy was referring to is a common pitfall of working with the 'F120 series parts. You merrily write your code and then start to debug it and then find that the I/O or peripheral functions are not working. Most often the mistake is that you've failed to setup the SFRPAGE register correctly !!!

Michael Karas