The device can also support APLL frequency and
phase control through writing the 40-bit register DPLLx_FREE_RUN[39:0] while the DPLL is in
holdover or not used. If the reference clock in a free-run mode or disabled, the DPLL
disconnects with the APLL, but users can still adjust frequency and phase accuracy.
To enable APLL DCO control, set DPLLx_LOOP_EN = 1,
and PLLx_MODE = 1 for 40-bit fractional denominator. DPLLx_EN can be
set = 0.
There are two alternative methods in adjusting the APLL DCO.
- Absolute frequency adjustment
- Set DPLLx_HIST_EN = 0
- Effective APLLx_NUM (APLLx_NUM_STAT) = APLLx_NUM + DPLLx_FREE_RUN
- The APLLx_NUM_STAT is a read-only register and can be read back.
- The DPLL loop filter block modifies the
APLLx_NUM_STAT based on DPLLx_FREE_RUN value.
- DPLLx_FREE_RUN is a 40-bit 2's complement
number
- Relative frequency adjustment
- Set DPLLx_HIST_EN = 1
- DPLLx_FREE_RUN value is fed into the APLLx_NUM at a controlled rate defined by a step size register and step period register.
- If another DPLLx_FREE_RUN write occurs before the LMK is complete in making the last adjustment, any remaining steps are lost and the new value begins to feed the APLL numerator.
- A flag is set when the DPLLx_FREE_RUN word is fully fed into the effective APLLx_NUM (APLL_NUM_STAT).