4.155.2 clock_prescale_set
clock_prescale_set(clock_div_t __x)
Set the clock prescaler register select bits, selecting a system clock division setting. This function is inlined, even if compiler optimizations are disabled.
The type of x is clock_div_t.
Note: 
For device with XTAL Divide Control Register (XDIV), x can actually range from 1 to 129. Thus, one does not need to use clock_div_t type as argument. 
