1.1.4 Locking from INT and FRAC PRESET Settings
(Ask a Question)The JAPLL supports locking from preset INT and FRAC values to speed up lock time. The feature is activated by providing values to INT PRESET and FRAC PRESET and setting PRESET EN = 1'b1. When the loop detects a PRESET EN transition from 1'b0 to 1'b1, it assigns the values of INT PRESET and FRAC PRESET to the frequency detector and reenter frequency tracking mode and then proceed to lock as normal.
PRESET EN must maintain a value of 1'b1 for at least one FFB clock cycle to guarantee that the preset mode takes effect, however, PRESET EN may be left at a value of 1'b1 due to the fact that the preset logic only activates on a positive edge transition of PRESET EN.
In addition, PRESET EN must be set to 1'b1 only after the values of INT PRESET and FRAC PRESET have been written to ensure no glitch occurs. Specifying a new INT PRESET and FRAC PRESET requires that PRESET EN transition to 1'b0 for at least one FFB clock cycle and then to transition to 1'b1 again.