MicroPython on the Pico W

Everything so far was C++. Today you try MicroPython — Python for microcontrollers — on a Raspberry Pi Pico W. Same ideas (pins, loops, timing), different language, and no compile step.

No Pico W? MicroPython also runs on the ESP32 from week three; the ESP32 board page links the firmware. Only the LED pin changes (Pin(2, Pin.OUT)), and the ESP32 has no ADC 4 temperature sensor.

Install MicroPython (once)

1. Download the Pico W .uf2 file from micropython.org — make sure it’s the Pico W build, not plain Pico.
2. Hold the BOOTSEL button while plugging in USB. A drive called RPI-RP2 appears.
3. Drag the .uf2 onto it. The board reboots into MicroPython.
4. Install Thonny (thonny.org) and pick Tools → Options → Interpreter → MicroPython (Raspberry Pi Pico).

Drive doesn’t appear? See Pico RPI-RP2 drive not showing.

The REPL: talk to the board

The bottom pane in Thonny is the REPL (read–evaluate–print loop). Type a line and the board runs it at once:

>>> from machine import Pin
>>> led = Pin("LED", Pin.OUT)
>>> led.on()

The LED lights before you’ve written a program. This instant feedback is MicroPython’s best feature for learning.

Today’s program

Paste the code below into the editor and press Run. To make it start on its own at power-up, File → Save as → Raspberry Pi Pico and name it main.py.

• Pin("LED", Pin.OUT) — on the Pico W the LED is wired to the Wi-Fi chip, so it has a name instead of a number.
• ADC(4) is a temperature sensor inside the RP2040. read_u16() returns 0–65535; the formula from the RP2040 datasheet turns the voltage into °C. It measures the chip, so it reads a few degrees warm and isn’t a room thermometer.
• time.sleep(1) is Python’s delay(1000).
• Indentation is the code structure. Everything inside while True: is indented four spaces. A wrong indent is a real error, not just bad style.

C++ vs MicroPython in one line each

• C++: faster, tiny memory use, the most libraries, but compile → upload every change.
• MicroPython: type and run instantly, friendlier errors, but slower and needs more RAM.

MicroPython vs Arduino C++ goes deeper. Many FlashReady guides have both versions, e.g. DHT22 on the Pico W.

Try it

• Light the LED only when the chip is above 30 °C (hold your finger on the RP2040).
• Use Pin(15, Pin.IN, Pin.PULL_UP) to read day 3’s button on GP15.

You’re done when…

• The RPI-RP2 drive appeared, and vanished by itself after you copied the .uf2.
• Thonny’s Shell shows the >>> prompt, and led.on() lights the LED.
• Running the program prints tick 1 chip temperature: … C once a second while the LED toggles.
• Saved as main.py, it blinks on its own after unplugging and plugging in again — no Thonny needed.

Today's code

# Day 26 — MicroPython on the Pico W
# No wiring. Blinks the on-board LED and prints the RP2040's own temperature once a second.
# Save this as main.py on the Pico W (Thonny: File → Save as → Raspberry Pi Pico) to run it at power-up.
from machine import Pin, ADC
import time

led = Pin("LED", Pin.OUT)   # on the Pico W the LED hangs off the Wi-Fi chip, so it's "LED", not 25
sensor = ADC(4)             # ADC channel 4 = the temperature sensor inside the RP2040
TO_VOLTS = 3.3 / 65535      # read_u16() returns 0..65535

count = 0
while True:
    led.toggle()
    volts = sensor.read_u16() * TO_VOLTS
    temp_c = 27 - (volts - 0.706) / 0.001721   # formula from the RP2040 datasheet
    count += 1
    print("tick", count, "chip temperature: {:.1f} C".format(temp_c))
    time.sleep(1)

# Day 26 — MicroPython on the Pico W
# No wiring. Blinks the on-board LED and prints the RP2040's own temperature once a second.
# Save this as main.py on the Pico W (Thonny: File → Save as → Raspberry Pi Pico) to run it at power-up.
from machine import Pin, ADC
import time

led = Pin("LED", Pin.OUT)   # on the Pico W the LED hangs off the Wi-Fi chip, so it's "LED", not 25
sensor = ADC(4)             # ADC channel 4 = the temperature sensor inside the RP2040
TO_VOLTS = 3.3 / 65535      # read_u16() returns 0..65535

count = 0
while True:
    led.toggle()
    volts = sensor.read_u16() * TO_VOLTS
    temp_c = 27 - (volts - 0.706) / 0.001721   # formula from the RP2040 datasheet
    count += 1
    print("tick", count, "chip temperature: {:.1f} C".format(temp_c))
    time.sleep(1)