USB : Overview    USB Facts of Life  

It’s important to understand that USB is a Master/Slave technology. The PC 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.)

This Master/Slave relationship between the PC Master and the peripheral Slave means that the USB hardware on each end is very different. A USB peripheral device does not have the right kind of USB hardware to command another USB peripheral, so please don’t expect to be able to turn your inexpensive microcontroller-based USB peripheral device into something that can control another USB device like a printer.

It can’t happen, unless you buy a more complicated micro that contains special hardware that can enable it to act as a USB "Host," and go through all the steps to program it to do all the extra work that a Host needs to do.


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