An image sensor system, including: an image sensor, a readout circuit, and a timing control circuit. The image sensor includes a plurality of diodes, and one of the plurality of diodes outputs a sensing current when a photon is detected. The readout circuit is coupled to the image sensor and arranged to selectively operate in at least a first mode and a second mode. The timing control circuit is coupled to the readout circuit and is arranged to determine if a coding condition is fit according to an input signal and generate a control signal when the coding condition is fit, wherein the input signal input signal includes a plurality of bits serially input to the timing control circuit, and each bit of the plurality of bits corresponds to each pulse of a clock signal respectively.