Holdover occurs when the DPLL reference clocks become unavailable, as seen in Figure 3-5. During holdover, the APLL numerator is set by the DPLL tuning word history, which determines the output frequency accuracy upon entry into holdover—any error on entry is considered a short-term holdover error. The tuning word history can be configured as either the accumulated averaged reference phase history, a user-specified value, or the last APLL numerator value.

The quality of the external oscillator (provided to XO input) determines the long-term frequency stability and accuracy of the output frequency. Over time, temperature fluctuations impact the frequency accuracy of the output clocks. Hence, the decision to choose between an XO, TCXO, or OCXO lies primarily on the long-term holdover requirements of the system.

Figure 3-5 Holdover Functionality

For applications requiring output frequency correction during long-term holdover, the output frequency can be corrected by making APLL DCO adjustments through software to compensate for temperature variations, as shown in Figure 3-6.

Figure 3-6 APLL DCO Operation During Holdover