USB : Overview    USB Is Like A Railroad  

Once the device passes Enumeration, it can receive OUT data from the PC, or make IN data available for the PC to take.

Please re-read that last sentence carefully.

It’s important to understand that USB is a Master/Slave technology. The PC or "Host" is the Master, and your peripheral device is the Slave. The PC polls the peripheral device: it can send OUT data to the device any time it wants, and the PC expects to be able to read IN data from the device any time it wants. The device can only wait with its data until the PC host decides to ask the device for it. Despite the fact that one of the USB transfer types is called “Interrupt” type, the slave device cannot actually interrupt the PC at all.


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