A host circuit includes a first clock generator, a first input output interface, a first communication interface, and a first processor. The first clock generator generates a first clock signal. The first processor outputs a trigger signal through the first input output interface, records a first clock count of the first clock generator at the same time, and outputs the first clock count through the first communication interface. A slave circuit includes a second clock generator, a second input output interface, a second communication interface, and a second processor. The second clock generator generates a second clock signal. When receiving the trigger signal, the second processor records a second clock count of the second clock generator, and calculates a time difference between the first clock signal and the second clock signal according to the first clock count and the second clock count.