A USB device includes a main body, a PCB fixed in the main body and a USB connector connected to the main body. In addition, the device may determine a target packet drop rate based on the packet drop rate, the processor utilization rate, and the target processor utilization rate. The method and circuit also provides a protection mechanism for an overload of the circuit system overload, so that the integrated circuit can moderate surges and prevent damages. The trapping wall has a first trapping surface and a second trapping surface.