This is a complete example on how to use the Beaglebone Black's PRU to poll for an input signal. The code can be found on GitHub.

This program runs on PRU1 and waits for P8.28 to go high and then pulls P8.29 high.

Circuit

A switch should be placed between the 3.3V and P8.28. An LED should be placed in series with a 1k resistor between P8.29 and ground.

Installation

An updated device tree compiler may be needed:

wget -c https://raw.githubusercontent.com/RobertCNelson/tools/master/pkgs/dtc.sh; chmod +x dtc.sh; ./dtc.sh

The PRU Drivers must also be installed.

The HDMI must be disabled by adding the following line to /boot/uEnv.txt

optargs=quiet capemgr.disable_partno=BB-BONELT-HDMI,BB-BONELT-HDMIN

The following will compile all the code for the PRU and main cpu:

make

Compile the device tree overlay and enable it:

cd device-tree
make enable

Running

./input

For the iLidar project we need a clock signal to drive the OmniVision camera. Originally the HDMI clock was used with success, but the HDMI framer had to be disabled. Disabling the HDMI allows for more inputs to be available to the Programmable Realtime Units (PRUs).

There are other timers ...