There are two manual input selection modes that
can be set by a register: Manual with Auto-Fallback and Manual with Auto-Holdover. In either
manual mode, the input selection can be done through register control (Register
DPLLx_MAN_REF_SEL) or hardware pin
control (GPIOs).
Manual with Auto-Fallback: In this mode, the manually selected reference is the active reference until it becomes invalid. If the reference becomes invalid, the DPLL will automatically fallback to the highest priority input that is valid or qualified. If no prioritized inputs are valid, the DPLL will enter holdover mode (if tuning word history is valid) or free-run mode. The DPLL will exit holdover mode when the selected input becomes valid.
Manual with Auto-Holdover: In this mode, the manually selected reference is the active reference until it becomes invalid. If the reference becomes invalid, the DPLL will automatically enter holdover mode (if tuning word history is valid) or free-run mode. The DPLL will exit holdover mode when the selected input becomes valid.
The reference input selection
flowchart is shown in Figure 8-14.