SNAS835 September 2022 LMK5B33414
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 8-6 (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 in-band phase noise/jitter degradation that could otherwise occur from a lower performance XO/TCXO/OCXO.
If APLL1 or APLL2 is not cascaded as shown in Figure 8-7, 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 without DPLL control, TI recommends selecting the programmable 24-bit denominator (PLLx_MODE = 0) instead of a fixed 40-bit denominator (PLLx_MODE = 1) to synthesize exact frequency ratios and maintain 0 ppm frequency error.