A flash memory controller includes a read/write unit, a state machine, a processing unit, and an auxiliary unit. The read/write unit is connected to a flash memory and performs a writing command or a reading command. The state machine is configured to determine a state of the flash memory controller. The processing unit is connected to the read/write unit and the state machine and configured to control the read/write unit. The auxiliary unit is connected to a first data line and a second data line and the processing unit and configured to receive and store a string output from the processing unit. The auxiliary unit outputs the string through the first and second data lines when the flash memory controller completes a writing data transmission.