There is a register (CLKPR ) that can be accessed by software that can divide the system clock but it requires a special write special bit -write value sequence that must be done in 4 cycles for it to work ( don't know if it can be done other than using inline assembly to do it). There is a factor of 2 available so the UNO can be made to actually run at 8MHz ( have not done this but it is a feature of the mega328 chip).