4.2.4. Dynamic Phase Configuration Implementation
To perform one dynamic phase shift step, follow these steps:
- Set PHASEUPDOWN and PHASECOUNTERSELECT as required.
- Assert PHASESTEP for at least two SCANCLK cycles. Each PHASESTEP pulse allows one phase shift.
- Deassert PHASESTEP after PHASEDONE goes low.
- Wait for PHASEDONE to go high.
- Repeat steps 1 through 4 as many times as required to perform multiple phase shifts.
PHASEUPDOWN and PHASECOUNTERSELECT signals are synchronous to SCANCLK and must meet the set up time (tsu) and hold time (th) requirements with respect to the SCANCLK edges.
You can repeat dynamic phase-shifting indefinitely. For example, in a design where the VCO frequency is set to 1,000 MHz and the output clock frequency is set to 100 MHz, performing 40 dynamic phase shifts (each one yields 125 ps phase shift) results in shifting the output clock by 180º, which is a phase shift of 5 ns.
The PHASESTEP signal is latched on the negative edge of SCANCLK (a,c) and must remain asserted for at least two SCANCLK cycles. Deassert PHASESTEP after PHASEDONE goes low.
On the second SCANCLK rising edge (b,d) after PHASESTEP is latched, the values of PHASEUPDOWN and PHASECOUNTERSELECT are latched. The PLL starts dynamic phase-shifting for the specified counters and in the indicated direction.
The PHASEDONE signal is deasserted synchronous to SCANCLK at the second rising edge (b,d) and remains low until the PLL finishes dynamic phase-shifting. Depending on the VCO and SCANCLK frequencies, PHASEDONE low time may be greater than or less than one SCANCLK cycle.
You can perform another dynamic phase-shift after the PHASEDONE signal goes from low to high. Each PHASESTEP pulse enables one phase shift. The PHASESTEP pulses must be at least one SCANCLK cycle apart.