USB-UART (universal asynchronous receiver/transmitter) downloading adapter

The invention discloses a USB-UART (universal asynchronous receiver/transmitter) downloading adapter which comprises an external interface and a mainboard circuit connected with the external interface. The mainboard circuit comprises a voltage converter and a USB/UART selector, wherein the voltage converter is used to make the voltage input from the external interface stably output so as to provide a stable working voltage for the USB-UART downloading adapter; the USB/UART selector is connected with the voltage converter and is used to select to connect a USB downloading line or a UART downloading line according to an enable signal of the external interface. By using the USB-UART downloading adapter, a power supply does not need to be added, and the USB downloading and the UART downloading are supported simultaneously. Application programs and a BOOTROOT program used to guide a file can be rapidly downloaded to a mobile phone through the USB-UART downloading adapter. A plug-and-play function can be achieved.