Abstract: A testable circuit includes a first function logic, an input output cell including an input/output unit and a first control multiplexer; and a first testing block is provided, wherein the input/output unit has at least a connection terminal. The first control multiplexer has an output port coupled to the connection terminal, a first input port coupled to the first functional logic, and a second input port. The first testing block is coupled between the first functional logic and the second input port, wherein when the testable circuit is under a testing mode, the first control multiplexer couples the second input port to the output port; and when the testable circuit is under a normal mode, the first control multiplexer couples the first input port to the output port.