static const uint8_t SPIMaskFromSCKDuration[] PROGMEM =
{
#if (F_CPU == 8000000)
   SPI_SPEED_FCPU_DIV_2,    // AVRStudio =   8MHz SPI, Actual =   4MHz SPI
   SPI_SPEED_FCPU_DIV_2,    // AVRStudio =   4MHz SPI, Actual =   4MHz SPI
   SPI_SPEED_FCPU_DIV_4,    // AVRStudio =   2MHz SPI, Actual =   2MHz SPI
   SPI_SPEED_FCPU_DIV_8,    // AVRStudio =   1MHz SPI, Actual =   1MHz SPI
   SPI_SPEED_FCPU_DIV_16,   // AVRStudio = 500KHz SPI, Actual = 500KHz SPI
   SPI_SPEED_FCPU_DIV_32,   // AVRStudio = 250KHz SPI, Actual = 250KHz SPI
   SPI_SPEED_FCPU_DIV_64,   // AVRStudio = 125KHz SPI, Actual = 125KHz SPI
#elif (F_CPU == 16000000)
   SPI_SPEED_FCPU_DIV_2,    // AVRStudio =   8MHz SPI, Actual =   8MHz SPI
   SPI_SPEED_FCPU_DIV_4,    // AVRStudio =   4MHz SPI, Actual =   4MHz SPI
   SPI_SPEED_FCPU_DIV_8,    // AVRStudio =   2MHz SPI, Actual =   2MHz SPI
   SPI_SPEED_FCPU_DIV_16,   // AVRStudio =   1MHz SPI, Actual =   1MHz SPI
   SPI_SPEED_FCPU_DIV_32,   // AVRStudio = 500KHz SPI, Actual = 500KHz SPI
   SPI_SPEED_FCPU_DIV_64,   // AVRStudio = 250KHz SPI, Actual = 250KHz SPI
   SPI_SPEED_FCPU_DIV_128   // AVRStudio = 125KHz SPI, Actual = 125KHz SPI
#else
   #error No SPI prescaler masks for chosen F_CPU speed.
#endif
};

PLL Prescaler Clock – clkPllPresc
The PLL requires a 8 MHz input. A prescaler allows user to use either a 8MHz or a 16MHz
source (from a crystal or an external source), using a divider (by 2) if necessary. The output of
the prescaler goes into the PLL Input multiplexer, that allows the user to select either the prescaler output of the System Clock Multiplexer, or the Internal 8MHz Calibrated Oscillator.
USB Clock – clkUSB
The USB hardware module needs for a 48 MHz clock. This clock is generated from the on-chip
PLL. The output of the PLL passes through the PLL Postcaler where the frequency can be either
divided by 2 or directly connected to the clkUSB signal