How to convert Normal Mode to DFU Mode

[premanandaembed]

with usb1.USBContext() as context:
    for device in context.getDeviceList():
        if int(0xXXXX) == device.getVendorID() and int(0x0001) == device.getProductID():
            dfu_device = dfu.DFU(dev.open())
            dfu_device.startDFU() 

how to convert normal mode to dfu mode.