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, DPLL will disconnect with APLL, but we 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
may 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 will modify the APLLx_NUM_STAT
based on DPLLx_FREE_RUN value.
- DPLLx_FREE_RUN is a
40-bit 2s 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).