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如图中所示，分别为BeagleBone和StartKid的原理图的USB1_VBUS连接，其中一个接入了1.2K的电阻，另一个却没有。查阅手册没有找到这部分的相关资料，请问这里的1.2K电阻的作用是什么，怎么计算得到？
