You will be amazed at the performance of the C8051F120 if you read the data sheet and see just how fast it can go. 489 nanoseconds would be a huge disappointment.

Please read the data sheet for the part. It is available as a PDF file from here:
http://www.silabs.com/pages/Dow...DocLibrary

Look in that document at Table 11.1 beginning on page 129. The number of clock cycles for each instruction are specified in detail.

Also take a close look at the Section 14 called Oscillators starting on Page 185. This will explain the many options available for generating the SYSCLK that drives the internal CPU and other things. Note that the internal calibrated 24.5 MHz oscillator is just one of the available choices.

Michael Karas