9.3.4.2 Manual Input Selection
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 (see Table 4) or hardware pin control (see Table 5).
- 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.
Table 4. Manual Input Selection by Register Bits
DPLL_REF_MAN_REG_SEL BIT |
DPLLx_REF_MAN_SEL BIT |
SELECTED INPUT |
0 |
0 |
PRIREF |
1 |
0 |
SECREF |
Table 5. Manual Input Selection by Hardware Pins
REFSEL PIN |
DPLL_REF_MAN_SEL BIT |
SELECTED INPUT |
0 |
1 |
PRIREF |
Float (VIM) |
1 |
Auto Select |
1 |
1 |
SECREF |
The reference input selection flowchart is shown in Figure 36.