Many displays for Raspberry Pi / Beaglebone have missing connections between the HDMI controller and the backlight driver / MOSFET. Below is a modification for this 7-Inch-1024x600 Capacitive Touch ...

Raspberry Pi has introduced a second generation of its 7inch touch display, increasing resolution from 800 x 480 to 1,280 x 720. Unlike the original which used an external board, the new display ...

If you open /dev/input/event0 directly, repeatedly read from it, and measure the interval between individual reads, you'll find that this interval is around 40ms, so 25 touch events per second / 25Hz.