USB : USB Classes    A Closer Look at HID Class  

HID devices use only Control and Interrupt transfer types.

For these transfer types, USB rules say that a full speed USB device can send at most one packet (whose size can be up to 64 bytes) per 1 msec frame from any single Interrupt endpoint. That's a transfer rate of 64,000 bytes per second per USB Interface, equivalent to a serial port speed of over half a million Baud. That's more than fast enough for many, many applications.

The USB rules are much more restrictive for low speed devices: only one 8 byte packet can be sent every 10 msec from a single Interrupt endpoint, for a throughput of 800 bytes/sec.

Although many mouse, keyboard, and joystick devices happen to be low speed devices, there is no rule that says they have to be.


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