An asynchronous FIFO interface having a readout clock asynchronous with a write clock is provided. The asynchronous FIFO interface includes a FIFO buffer, a clock controller, a reference source and a signal source. The FIFO buffer receives a digital signal from an ADC according to the write clock and outputs a digital signal to a processor according to the readout clock. The clock controller outputs a clock control signal according to the amount of data stored in the FIFO buffer. The reference source provides an oscillation frequency. The signal source divides the oscillation frequency by a first integer divisor to generate a reference frequency, divides the readout clock by a second integer divisor to generate an input frequency, and outputs a control signal by comparing the reference frequency with the input frequency.