DHT22 Temperature and Humidity Sensor - Beaglebone tools

Pinout

Using pin P8_15 as input. Using pin P8_11 as output.

Both pins connect directly to the DHT22's data pin, but the output pin is protected by a 10k resistor. In theory a single pin is required rather than an input and an output, but this approach has the advantage of protecting the output pin against shorts. The input pin remains directly connected to the DHT22.

[ Vcc ]-----[ 10k ohm pull-up ]---------.
                                        |
[ BBB P8_15 (in) ]----------------------.
                                        |--------[ DHT22 data ]
                                        |
[ BBB P8_11 (out) ]----[ 620 ohm ]------`

Changing the pin modes

The input pin is mapped to an internal PRU register for ultra fast access. This requires changing the pin mode with the following procedure.

Build the device tree overlay:

cd dht
make
make install

The install part copies the device tree fragment into /lib/firmware.

Enable PRU on BBB:

echo BB-BONE-PRU-01 >/sys/devices/bone_capemgr.9/slots

At this point, the PRU should be detected in dmesg. The kernel module used to interface with it is uio_pruss. It gets loaded automatically.

echo BB-DHT22-Pins >/sys/devices/bone_capemgr.9/slots