Dynamic Switching of the FX2 CPU Clock.

Question: Is it possible to change clock frequency while the chip is running?

 

Answer:

Response:Yes. Bits 3 (CLKSPD0) and 4 (CLKSPD1) of the CPUCS register are used to set the CPU clock speed. At power-on-reset, these bits default to 00 (12 MHz). Firmware may modify these bits at any time.

CLKSPD1/CLKSPD0 -> CPU Clock
---------------------------------------
00 -> 12 MHz (Default)
01 -> 24 MHz
10 -> 48 MHz
11 -> Reserved

Section 15.5.1 of the EZ-USB 
Technical Reference Manual explains about the CPUCS register.