请问stm32下载bootloader时出现Failed to init device.怎么解决

D: arduinohardware Arduino_ STM32- -masterltools win> stm32flash. exe -k COM3
stm32flash 0.4

http://stm32flash. googlecode. com/

Interface serial w32:57600 8E1
Failed to init device.

STM32进入串口下载模式了吗