arduino uno 下载程序出错

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avrdude: ser_open(): can't open device "\\.\COM3": 拒绝访问。


processing.app.debug.RunnerException: 下载程序到板卡时出现问题，请在http://www.arduino.cc/en/Guide/Troubleshooting#upload寻找解决办法
        at processing.app.debug.BasicUploader.uploadUsingPreferences(BasicUploader.java:126)
        at processing.app.Sketch.upload(Sketch.java:1664)
        at processing.app.Sketch.exportApplet(Sketch.java:1623)
        at processing.app.Sketch.exportApplet(Sketch.java:1595)
        at processing.app.Editor$DefaultExportHandler.run(Editor.java:2393)
        at java.lang.Thread.run(Thread.java:619)

有时候可以有时候就有毛病，不知道为什么