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TMS320F28388D: usb例程 usb_ex4_dev_bulk 下载后 usb不识别 一直是未识别USB设备(设备描述符请求失败)

Part Number: TMS320F28388D

使用自己做的板子,下载例程usb_ex4_dev_bulk后,usb不识别 一直是未识别USB设备(设备描述符请求失败),请问这是什么原因?