1. A signal-switching circuit for use in an electronic system, wherein the electronic system comprises a plurality of hardware circuits, the signal-switching circuit comprising:a control circuit, arranged to receive a trigger signal generated by a trigger circuit of the electronic system, and change a mode signal generated by the control circuit in response to the trigger signal; and
a switch circuit, arranged to electrically connect transmission signals from one of the hardware circuits to a test board external to the electronic system via a transmission interface of the electronic system in response to the mode signal,
wherein the trigger signal is a pulse signal,
wherein the signal-switching circuit comprises more than two operation modes, and the control circuit changes the mode signal according to the trigger signal to sequentially switch between the operation modes,
wherein the control circuit includes N serially-connected D flip-flops (DFFs), and each of the N DFFs comprises a data input pin, a clock input pin, a data output pin, and an inverse data output pin, and the inverse data output pin is connected to the data input pin in each of the N DFFs,
wherein the clock input pin of the first DFF of the N DFFs receives the trigger signal,
wherein the data output pin of each DFF is connected to the block input pin of the next DFF for each of the first DFF to the (N?1)-th DFF,
wherein the data output pin of each of the N DFFs is connected to the switch circuit.