Measure an analog voltage with the ADC of the Pi Pico in MicroPython
On the Raspberry Pi Pico, some pins are attached to the built-in ADC of the microcontroller. An ADC gives a digital value proportional to the measured input voltage.
ADC stands for Analog to Digital Converter.
On the Raspberry Pi Pico pins 26, 27 and 28 can be used as an analog input. The resolution of the measured values is on 16 bits, the values are therefore between 0 and 65535.
On the uPesy RP2040 DevKit board, there is an additional analog input on pin 29.
There is a “hidden” analog input in the RP2040 microcontroller which allows to measure directly the temperature of the chip. This is somewhat similar to temperature measurements from computer CPUs. It should not be used to get an accurate room temperature. This input corresponds to the channel
Unlike the Arduino, the Pico analog inputs should not exceed a 3.3V voltage. This could damage or even destroy the ADC inputs. Therefore, it is essential to check not to use a 5V supply voltage to measure the value of a sensor. There are solutions to limit the ADC input voltage to 3.3V.
The analog input voltage is measured using the
submodule of the
module. Like with PWM, we associate a physical pin with the object
from machine import Pin, ADC # Pin definitions adc_pin = Pin(26, mode=Pin.IN) adc = ADC(adc_pin)
Remember to import the ADC submodule.
You can also create an
object by specifying the ADC channel digit you want to use.
from machine import ADC adc = ADC(0) # Select the ADC_0 ()
Once you have configured an analog input in MicroPython, you can read the analog value using the
function on the Raspberry Pi Pico.
from machine import Pin, ADC adc = ADC(Pin(26, mode=Pin.IN)) print(adc.read_u16())
It will be necessary to apply a formula to have value measured in volts:
The Raspberry Pi Pico has an internal temperature sensor to measure the temperature of the RP2040 chip. Although not very reliable, it does provide insight into the temperature of the CPU (just like on computers). This sensor is directly connected to the
input of the ADC.
- To measure the temperature, you must:
Retrieve the raw analog value of the temperature sensor available on channel 4 of the ADC
Convert this value to voltage
Use a formula given in the RP240 documentation to convert this voltage to temperature (in ° C)
from machine import ADC import time temp_sensor = ADC(4) while True: adc_voltage = temp_sensor.read_u16() * 3.3 / 65535 cpu_temp = 27 - (adc_voltage - 0.706)/0.001721 # Formula given in RP2040 Datasheet print(cpu_temp) time.sleep_ms(1_000)
You can see the temperature change by putting your finger on the RP2040 chip.
REPL console output:
32.19402 32.19402 32.66216 31.72584 33.13027 33.59842 34.06656 34.06656 34.53474 34.06656
We will use PWM and ADC with MicroPython to control the LED brightness.
Consider putting a resistor in series with the LED to prevent burning. (330 Ω, for example)
For reading the knob position of the potentiometer only :
from machine import Pin, ADC import time # Create an ADC object linked to pin 26 adc = ADC(Pin(26, mode=Pin.IN)) while True: # Read ADC and convert to voltage val = adc.read_u16() val = val * (3.3 / 65535) print(round(val, 2), "V") # Keep only 2 digits # Wait a bit before taking another reading time.sleep_ms(100)
To read knob position and change the brightness:
from machine import Pin, ADC, PWM import time # Create a PWM object linked to pin 15 pwm_led = PWM(Pin(15,mode=Pin.OUT)) pwm_led.freq(1_000) # Create an ADC object linked to pin 26 adc = ADC(Pin(26, mode=Pin.IN)) while True: val = adc.read_u16() pwm_led.duty_u16(val) time.sleep_ms(10)