Day 10: (Mon Sep 29, Week 6) Observation Review; Pi Pico Take 2

Notes for 2025-09-29.

New Assignments

Due Wednesday: Please write a variation of the servo_step.py program (below) to produce a hobby servo motion pattern of your choice to be demonstrated at the start of the next class.

Administrative

Looking ahead:

• Wed Oct 1: in-class ideation

• Mon Oct 6: device 4

• Wed Oct 8: project proposals

I’d like us to have project groups formed and a clear project ideas articulated before the Fall Break. I prefer not to have any assignments due right after the break.

Agenda

1. Review observations from school visit.

2. Resume brief introduction to the Raspberry Pi Pico and CircuitPython.

3. Installing the Mu Python Editor.

4. Each person pick up a Pico and a USB cable.

5. Crash course in CircuitPython basics.

   • Introductory Examples - Raspberry Pi Pico

6. Introduction to hobby servos.

   • Hobby Servo Examples - Raspberry Pi Pico

Figures

Lecture code samples

Blink

# pico_blink.py

# Raspberry Pi Pico - Blink demo

# Blink the onboard LED and print messages to the serial console.

import board
import time
from digitalio import DigitalInOut, Direction, Pull

#---------------------------------------------------------------
# Set up the hardware: script equivalent to Arduino setup()
# Set up built-in green LED
led = DigitalInOut(board.LED)  # GP25
led.direction = Direction.OUTPUT

#---------------------------------------------------------------
# Run the main loop: script equivalent to Arduino loop()

while True:
    led.value = True
    print("On")
    time.sleep(1.0)

    led.value = False
    print("Off")
    time.sleep(1.0)

Servo Step

# servo_step.py
#
# Raspberry Pi Pico - hobby servo motion demo
#
# Demonstrates stepping a hobby servo back and forth.
#
# This assumes a tiny 9G servo has been wired up to the Pico as follows:
#   Pico pin 40 (VBUS)  -> servo red   (+5V)
#   Pico pin 38 (GND)   -> servo brown (GND)
#   Pico pin 1  (GP0)   -> servo orange (SIG)

# links to CircuitPython module documentation:
# time    https://circuitpython.readthedocs.io/en/latest/shared-bindings/time/index.html
# math    https://circuitpython.readthedocs.io/en/latest/shared-bindings/math/index.html
# board   https://circuitpython.readthedocs.io/en/latest/shared-bindings/board/index.html
# pwmio   https://circuitpython.readthedocs.io/en/latest/shared-bindings/pwmio/index.html

################################################################################
# load standard Python modules
import math, time

# load the CircuitPython hardware definition module for pin definitions
import board

# load the CircuitPython pulse-width-modulation module for driving hardware
import pwmio

#--------------------------------------------------------------------------------
# Define a function to issue a servo command by updating the PWM signal output.
# This function maps an angle specified in degrees between 0 and 180 to a servo
# command pulse width between 1 and 2 milliseconds, and then to the
# corresponding duty cycle fraction, specified as a 16-bit fixed-point integer.

def servo_write(servo, angle, debug=False):
    # calculate the desired pulse width in units of seconds
    pulse_width  = 0.001 + angle * (0.001 / 180.0)

    # fetch the current pulse repetition rate from the hardware driver
    pulse_rate = servo.frequency
    
    # calculate the duration in seconds of a single pulse cycle
    cycle_period = 1.0 / pulse_rate

    # calculate the desired ratio of pulse ON time to cycle duration
    duty_cycle   = pulse_width / cycle_period 

    # convert the ratio into a 16-bit fixed point integer
    duty_fixed   = int(2**16 * duty_cycle)

    # limit the ratio range and apply to the hardware driver
    servo.duty_cycle = min(max(duty_fixed, 0), 65535)

    # print some diagnostics to the console
    if debug:
        print(f"Driving servo to angle {angle}")
        print(f" Pulse width {pulse_width} seconds")
        print(f" Duty cycle {duty_cycle}")
        print(f" Command value {servo.duty_cycle}\n")

#--------------------------------------------------------------------------------
# Create a PWMOut object on Pin GP0 to drive the servo. The frequency argument
# specifies the pulse repetition rate in Hz (pulses per second).

servo = pwmio.PWMOut(board.GP0, duty_cycle=0, frequency=50)

# Begin the main processing loop.
while True:
    servo_write(servo, 0.0, debug=True)
    time.sleep(2.0)

    servo_write(servo, 180.0, debug=True)
    time.sleep(2.0)