micropython analog inputs

MicroPython analog inputs (Raspberry Pi Pico)

Key concepts

• Analog vs digital
  • Analog: continuous measurements (infinite values between two points).
  • Digital: discrete states (e.g., on/off). Microcontrollers read GPIO pins as high/low using a threshold.
• ADC (Analog-to-Digital Converter)
  • Maps an analog voltage to a discrete integer value across N steps (resolution).
  • Resolution = 2^bits. Example: 8-bit → 256 steps; 16-bit → 65,536 steps (values 0..65,535).
  • ADC values range from 0 to (2^bits − 1). The voltage step size is Vref / (2^bits − 1); for a 16-bit ADC over 0–3.3 V this is 3.3 / 65,535.

Raspberry Pi Pico specifics

• The Pico has five ADC channels internally; three are exposed for external inputs:
  • ADC0 → GP26
  • ADC1 → GP27
  • ADC2 → GP28
• The remaining ADC channels are used for internal measurements (on‑chip temperature sensor and input/board voltage).
• Pico ADC range: 0–3.3 V. MicroPython returns 16-bit readings (integer values 0..65,535).

Hardware example: potentiometer

• Wiring
  • One outer pin of the potentiometer → 3.3 V
  • Other outer pin → GND
  • Middle/wiper pin → ADC input (e.g., GP26)
• Behavior
  • The potentiometer acts as a variable voltage divider. Turning the dial changes the voltage at the wiper, which the ADC reads.

MicroPython usage (quick guide)

• Typical imports:

from machine import Pin, ADC
import utime  # or: import time

• Create an ADC instance for GP26:

adc = ADC(26)   # ADC0 / GP26

• Read a raw ADC value (returns 0..65535):

value = adc.read_u16()

• Example: continuously print raw ADC value and computed voltage, and toggle an LED on Pin 0 based on the reading:

from machine import Pin, ADC
import utime

adc = ADC(26)            # GP26 / ADC0
led = Pin(0, Pin.OUT)

while True:
    raw = adc.read_u16()                  # 0..65535
    volts = raw * 3.3 / 65535             # convert to volts
    print(raw, "{:.3f} V".format(volts))
    if raw > 35000:
        led.on()
    else:
        led.off()
    utime.sleep_ms(200)

• Note: regular digital GPIO reads use a voltage threshold to decide high vs low; ADC provides a precise measurement across the whole range.

Practical tips / gotchas

• Double-check wiring: ensure the wiper is connected to the correct GP pin (GP26/GP27/GP28). Wrong pin → no or incorrect readings.
• Be aware of the small ambiguous threshold region around digital high/low switching when using GPIO as digital inputs.
• Watch for syntax/casing errors in code (class and variable names are case-sensitive in MicroPython).
• Remember the Pico ADC measures 0–3.3 V — don’t exceed that range without proper scaling/protection.

Source: hands-on MicroPython tutorial/demonstration (unnamed instructor) showing ADC use on the Raspberry Pi Pico.