ZHCSMJ6B November 2020 – March 2021 LMK5C33216
PRODUCTION DATA
In APLL-only mode, the external XO input source determines the free-run frequency stability and accuracy of the output clocks. The DPLL blocks are not used and do not affect the APLLs. APLLs still can operate in cascaded mode or non-cascaded mode and also have DCO option through control register writes.
The principle of operation for APLL-only mode after power-on reset and initialization is as follows. If APLL1 or APLL2 is in cascaded mode as shown in Figure 9-6 (DPLL3 also is not used), VCO1 or VCO2 will track the VCO3 domain. APLLs lock in APLL priority order using bits: APLLx_STRT_PRTY. Cascading APLL1 or APLL2 from VCO3 provides a high-frequency, ultra-low-jitter reference clock to minimize the APLL2 or APLL3 in-band phase noise/jitter degradation could otherwise occur from a lower performance XO/TCXO/OCXO.
If APLL1 or APLL2 is not cascaded as shown in Figure 9-7, VCO1 or VCO2 will lock to the XO input in APLLx_STRT_PRTY order after initialization and operate independent of the APLL3 domain.
When operating in APLL-Only mode, it is recommended for frequency accuracy to use a 24-bit numerator and a programmable 24-bit denominator (PLLx_MODE = 0) instead of a fixed 40-bit denominator (PLLx_MODE = 1).