A low pin count bus serial interrupt system including a host and an external device sending interrupt signals to each other and exchanging signals with each other, the low pin count bus serial interrupt system comprising: an interrupt direction signal generator configured to determine a current interrupt direction signal (SerIRQ_dir) according to whether the host currently sends a first interrupt signal to an external device; and a level-shifter configured to receive the interrupt direction signal (SerIRQ_dir) in a static mode and to convert a voltage level of one of the signals exchanged between the host and the external device according to the current interrupt direction signal (SerIRQ_dir), wherein the first interrupt signal includes an end frame, wherein determining the current interrupt direction signal (SerIRQ_dir) by the interrupt direction signal generator according to whether the host currently sends the first interrupt signal to the external device includes: when the host currently sends the end frame to the external device, determining that the current interrupt direction is from the host to the external device, and generating a first signal indicating a direction from the host to the external device; and when the host currently does not send the end frame to the external device, determining that the current interrupt direction is from the external device to the host, and generating a second signal indicating a direction from the external device to the host. an interrupt direction signal generator configured to determine a current interrupt direction signal (SerIRQ_dir) according to whether the host currently sends a first interrupt signal to an external device; and a level-shifter configured to receive the interrupt direction signal (SerIRQ_dir) in a static mode and to convert a voltage level of one of the signals exchanged between the host and the external device according to the current interrupt direction signal (SerIRQ_dir), wherein the first interrupt signal includes an end frame, wherein determining the current interrupt direction signal (SerIRQ_dir) by the interrupt direction signal generator according to whether the host currently sends the first interrupt signal to the external device includes: when the host currently sends the end frame to the external device, determining that the current interrupt direction is from the host to the external device, and generating a first signal indicating a direction from the host to the external device; and when the host currently does not send the end frame to the external device, determining that the current interrupt direction is from the external device to the host, and generating a second signal indicating a direction from the external device to the host. when the host currently sends the end frame to the external device, determining that the current interrupt direction is from the host to the external device, and generating a first signal indicating a direction from the host to the external device; and when the host currently does not send the end frame to the external device, determining that the current interrupt direction is from the external device to the host, and generating a second signal indicating a direction from the external device to the host.