easytarget-MQ-Pro-IO/GPIO-examples.md

GPIO usage examples (for the MQ-Pro)

This guide assumes you have a correctly installed and set up board, with the correct device tree (plus overlays) to expose the pins you want to use.

Caveat: notes here are biased towards Python usage, since that is what I will be using in my projects.

General Purpose GPIO (digital read/write)

You do not need to use a custom Device Tree in order to use digital IO

• The 'default' device tree for the MQ pro has 26 free pins to use!

Look at the great guide here: https://worldbeyondlinux.be/posts/gpio-on-the-mango-pi/

It does a better job of explaining this than I can do in a short guide.

PWM

working, There are eight PWM timers available and GPIO pins can be mapped to these in a custom device tree

• Note: lgpio provides a software (bit-bang) PWM solution that can be used on any free pin, and does not need a device tree modification. This may be a better solution for many projects.

The available hardware PWM mappings are somewhat limited, see the diagram in the main README to determine which pins on the GPIO connector can be used.

• The example below uses (legacy) /sys/class control, which in turn needs root access. PWM control from userland seems like a WIP for linux at present.

The following needs to be run as root. It uses pwm2 (the lora example device tree attaches this to pin 31 on the GPIO connector).

First, export the PWM interface:

# echo 2 > /sys/class/pwm/pwmchip0/export

• The node for the interface wil appear at /sys/class/pwm/pwmchip0/pwm2/

Set a default period (10μs) and duty cycle (5μs, 50%):

# echo 10000 > /sys/class/pwm/pwmchip0/pwm2/period`
# echo 5000 > /sys/class/pwm/pwmchip0/pwm2/duty_cycle`

After setting the default you can enable it with:

# echo 1 > /sys/class/pwm/pwmchip0/pwm2/enable`

You can stop and detach the interface with: # echo 2 > /sys/class/pwm/pwmchip0/unexport

The following is a shell script that implements a crude LED fader:

#!/bin/bash
# PWM silly fader
#
pwm="/sys/class/pwm/pwmchip0/pwm2"

echo normal > $pwm/polarity
echo 10000 > $pwm/period
echo 1 > $pwm/enable
while true ; do
    for p in 40 100 400 1000 4000 10000 6000 1200 600 120 60 0 ; do
        echo -n "."
        echo $p > $pwm/duty_cycle
        sleep 0.25
    done
    echo
done

See the kernel guide for the parameters we set to assign and control the pin.

I2C

Working: I have read temperature, pressure and humidity from a BME280 sensor connected to pins 3 and 5.

Install pypi:bme280 and it's requirement smbus-cffi.

• I am using a virtual environment, rather than installing globally.
• Add the user to the group 'i2c' and re-login.

$ sudo apt install python3-venv python3-dev i2c-tools

$ sudo usermod -a -G i2c <username>
# Log out then in again so that your user now has the `i2c` group membership

# Create virtualenv in a directory './bme-env' and activate it (exit with `deactivate`, removing the directory+contents deletes the venv)
$ python3 -m venv bme-env
$ source bme-env/bin/activate

# Install the sensor library and dependencies
(bme-env) $ pip install --upgrade pip
(bme-env) $ pip install --upgrade smbus-cffi bme280

# The bme280 library provides a python API, and a commandline tool
(bme-env) $ which read_bme280
<cwd>/env/bin/read_bme280

# My bme280 defaults to address 0x76, and I'm using I2C0
(bme-env) $ read_bme280 --i2c-bus 0 --i2c-address 0x76
1024.85 hPa
  56.84 %
  21.59 C

SPI

Working?: When I enable SPI1 in the device tree a device is registered at /sys/devices/platform/soc/4026000.spi/

• It lists it's driver (correctly) as sun6i-spi and is a bus master.
• Kernel drivers that use SPI via pinctl should be able to use this.
• But no block device appears at /dev/spi*.
  • Normally spi-tools provides userland support via the /dev/spi* device.
• I do not plan to use SPI so I have not tested further.

---

Extra!

Status LED:

The onboard (blue) status LED is attached to gpio PD18, and can be controlled via the sys tree:

$ sudo sh -c "echo 1 > /sys/devices/platform/leds/leds/blue\:status/brightness" to turn on

$ sudo sh -c "echo 0 > /sys/devices/platform/leds/leds/blue\:status/brightness" to turn off

You can make it flash as network traffic is seen with:

$ sudo sh -c "echo phy0rx > /sys/devices/platform/leds/leds/blue\:status/trigger"

Other control options are available, $ sudo cat /sys/devices/platform/leds/leds/blue\:status/trigger shows a list and the current selection. Most do not work or are not very useful; ymmv.

• PD18 can also be re-mapped to pwm-2 in a modified device tree if you want to manually control the LED and vary it's brightness.
• PD18 is also used as the LED_PWM pin on the DSI/LVDS output

Onboard CPU temperature sensor:

$ sudo apt install lm-sensors
$ sensors
cpu_thermal-virtual-0
Adapter: Virtual device
temp1:        +19.4°C

HOWEVER: This is nonsense.. I'm testing with the board in an enclosure; and the attached BME280 sensor is showing room temp outside the enclosure as 22°C.

• The CPU is definately running hotter than 19° 🤦
• ¿Check out the device tree, maybe a bad offset. Or some kind of calibration/reference voltage needed?