SFFS779 December 2024 TMS320F28P550SJ
PLL lock functionality is implemented by comparing the difference (error) between the feedback clock and reference clock through a phase frequency detector (PFD). When PLL is in lock and generating the correct frequency, the difference is <100pS≅300pS. Once there is any fault causing the PLL output frequency to drift, the difference goes outside of that range. In such a case, PLL lock signal goes from 1 to 0; indicating PLL is out of lock. DCC can be used to detect that drift has occurred.