HIDIntervalTest – Illustrate human interface device sampling interval.

The routine queries first the keyboard, later on the mouse buttons for
changes in key press state or mouse button press state, recording the
timestamp of any change. This is done first for 10 seconds on the
keyboard, then for 10 seconds on the mouse.

Time difference between consecutive “state-change” samples is computed
and plotted as a histogram.

At least on USB-HID devices - keyboards and mice connected via USB - you
should see some characteristic distribution in the histograms. You’ll see
that clusters of samples are spaced in even time intervals, typically
separated by multiples of 8-10 msecs. This indicates that operating
systems only update mouse and keyboard state at a sampling frequency of
about 100 - 125 Hz and therefore measured mouse or keyboard response
times will be quantized with at least that granularity.

There are more complex interactions, especially inside keyboards, that
will increase latency and uncertainty about the timing of key presses
beyond the 8 msecs granularity of the USB-HID scan cycle.

Be very cautious when using normal keyboards for reaction time
measurements, or better use a dedicated response box.

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