RC32012A datasheet(39/66 Pages) RENESAS

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R31DS0078EU0103 Rev.1.03

Page 39

Sep 15, 2022

RC32012A Datasheet

4.13.1 Write-Frequency Mode

For the DPLL channel in this mode a Frequency Control Word (FCW) is used to adjust the frequency output of the

DCO (by steering the FOD) and the phase detector and loop filter are essentially bypassed. All the filtering is done

by an external device and the frequency offset written into the Write Frequency Configuration register is passed

on directly to the output clocks. This is shown in Figure 12. When applied, the FCW will not cause any missing

pulses or glitches in the output clock, although a large frequency jump may cause issues with devices receiving

this clock. The output will remain at this frequency until a new FCW is written.

Figure 12. External DCO Control via Frequency Control Word

The FCW is a 42-bit 2’s-complement value. The FCW has a granularity of 1.11x10-10ppm and a full range of

+244.20ppm to -244.08 ppm of the nominal DCO frequency. A positive value will increase the output frequency

and a negative one will decrease the output frequency. The formula for calculation of the FCW from the fractional

frequency offset (FFO) is:

Where,

FFO = Fractional Frequency Offset, in ppm

FCW = Frequency Control Word (Positive or Negative Integer)

The value resulting from the above calculation must be converted to a 42-bit 2’s complement value and then sign-

extended to 48-bits to be written into the register.

Write frequency mode may be used to make phase changes on the output. For fine resolution, phase changes are

done by controlling the DCO’s frequency. Coarse phase adjustments should be done by snap-alignment method

by using the Phase Offset registers. Using the Phase Offset registers is referred to as the snap-alignment method

since the output will snap directly to that new phase rather than moving smoothly to it over time. The snap-

alignment method provides fast coarse phase alignments and therefore it should be used to bring the phase close

to the desired value and then use the DCO in write frequency mode to fine tune it. Since write frequency mode is

changing the frequency, the phase will move smoothly over time without any jumps.

4.13.2 Increment / Decrement Registers and Pins

The DCO frequency update can also be done by applying a preset frequency offset value to be added or to be

subtracted from a cumulative FCW value. The cumulative FCW value behaves as described in the previous

section.

Steerable

Fractional

Divider

Output Stage

With

Integer Divider(s)

Low-Noise

Analog PLL

Frequency

Control Word

From

System DPLL

Digital PLL

FCW

1 FFO

106

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