Raspberry Pi Python IO

Enable Hardware

On Raspberry Pi OS-Desktop:

• access the start menu
• Preferences->Raspberry Pi Configuration

On the command line:

• sudo raspi-config
• enable/disable various IO in interface options

Check Pinout

Use pinout to identify GPIO pin numbers from terminal.

GPIO Zero

The GPIO Zero library provides an API to many common Raspberry Pi interfaces.

Install

On Raspberry Pi OS:

sudo apt update
sudo apt install python3-gpiozero

Some Examples

See GPIO Zero recipes for many more examples.

Turning on an LED

from gpiozero import LED
from time import sleep

led = LED(26)
led.on()
sleep(1)

Reading from a Button

Button is by default a switch which pulls the input low when active. See button docs for more options.

from gpiozero import Button

button = Button(21)

if button.is_pressed:
    print("<Pressed>")
else:
    print("<None>")

Commanding a Servo

• Servo has range -1.0 to 1.0
• AngularServo has range from -90 to 90 (but can be changed)

from gpiozero import AngularServo
from time import sleep

servo = Servo(18)

while True:
    servo.angle = 45
    sleep(2)
    servo.angle = -45
    sleep(2)

• If the servo.angle = None, then it receives no power

Reading from an Encoder

from gpiozero import RotaryEncoder
from time import sleep

# Note: For https://www.dfrobot.com/product-1431.html
encoder = RotaryEncoder(5, 6, max_steps=105, wrap=True)

while True:
    print("encoder position {}".format(encoder.value*180))
    sleep(1)

Generic Output Devices

from gpiozero import DigitalOutputDevice
from time import sleep

dout = DigitalOutputDevice(26)

while True:
    sleep(1)
    dout.off()
    sleep(1)
    dout.on()

Generic Input Devices

from gpiozero import DigitalInputDevice
from time import sleep

din = DigitalInputDevice(21, pull_up=True)
din.when_activated=lambda: print("HIGH")
din.when_deactivated=lambda: print("LOW")

while True:
    sleep(1)
    if din.value:
        print("{} seconds".format(din.active_time))
    else:
        print("{} seconds".format(din.inactive_time))

Reducing Servo Jitter

When running the servo code above, we get the warning:

PWMSoftwareFallback: To reduce servo jitter, use the pigpio pin factory.

We would like our PWM to be hardware driven, rather than be at the mercy of the OS.

There are two steps:

• Install pigpio
• Use GPIO Zero pin factories

Installing pigpio

On Raspberry Pi OS:

sudo apt update
sudo apt install pigpio
sudo apt install python3-pigpio

pigpio daemon

pigpio uses a daemon (that takes port :8888) to control IO pins.

The daemon must be running to control IO pins from your Raspberry Pi.

Start the pigpio daemon:

sudo systemctl start pigpiod

Enable pigpio daemon at boot:

sudo systemctl enable pigpiod

Using a Pin Factory

GPIO Zero uses Pin Factroies to provide various pin drivers.

Once pigpio is installed, we can use its pin factory.

from gpiozero import Servo
from gpiozero.pins.pigpio import PiGPIOFactory
from time import sleep

factory = PiGPIOFactory()
servo = Servo(18, pin_factory=factory)

while True:
    servo.min()
    sleep(2)
    servo.mid()
    sleep(2)
    servo.max()
    sleep(2)

This time, the pulses are very stable and the servo has no jitter.

Jitter Comparison

I wanted to compare pigpio to the default pin driver; so I put a scope on the two implementations:

• yellow is pigpio
  • on closer inspection only jitters about 0.2 microseconds; error (0.02%).
• blue is the default driver
  • it appears to put out two pulses per cycle
  • the second pulse seems to be asyncronous and the average of the two provide the servo pulse

pigpio

GPIO Zero doesn’t support some IO (such as serial).

In these cases, we can interface with the pigpio daemon directly.

See the pigpio python API for more details.

Interfacing with a serial port

Assuming pigpio is already installed and the daemon is running:

import pigpio
import time

pi = pigpio.pi()
if not pi.connected:
  exit()

s = pi.serial_open("/dev/ttyS0", 115200)
pi.serial_write(s, "j")

time.sleep(1)
len, data = pi.serial_read(s, 100)

if len > 0:
  print(data.decode("utf-8"))
else:
  print("no data")

• This interfaces with a bittle on a Raspberry Pi 3
  • Other models of Raspberry Pi may use /dev/ttyAMA0
• When debugging serial ports, consider PySerial’s miniterm