Very fast and accurate frequency tracking is essential for many photonic integrated circuit applications, such as sensing or in telecommunication systems. In particular, accurately locking a laser frequency to micro-ring wavelength resonance shifts is crucial for many applications like for example sensing in general and bio-sensing in particular. In this work, we present an innovative system capable of locking a tunable laser frequency to a silicon photonics microring resonator (MRR) and of tracking any changes in its wavelength over time with very high speed, accuracy and sensitivity. An electronic digital feedback loop is used to balance the power at the drop/through outputs of the micro-ring thus finely tuning the wavelength of the input laser. The silicon photonics chip is equipped with integrated photodiodes at each port of the MRR. Thanks to the low noise characteristics of integrated photodiodes and the high resolution of the laser tuning capability it was possible to lock shifts of less than 7 fm of residual noise at 1550 nm. The digital implementation of the feedback loop allowed to reach bandwidths of up to 1 kHz. Fully functional samples of the locking system including different micro-ring resonators, with Q-factor varying from 5000 to 60000 were manufactured showing excellent performance results.