variable DAC_BITS for main PLL
Default DAC_BITS = 16 which computes to pi.y_max = 65530. Generalize pi.y_max for future DACs with higher granularity (more bits)
Default DAC_BITS = 16 which computes to pi.y_max = 65530. Generalize pi.y_max for future DACs with higher granularity (more bits)