Why voltage from Raspberry Pi Pico ADC is not equal 0, when nothing is connected to the pin?

Question

I was trying to read some voltages from a sensor, but before doing that, I checked how readings would look like, when nothing is connected to pins.

Here's my code based on examples:

#include <stdio.h>
#include "pico/stdlib.h"
#include "hardware/gpio.h"
#include "hardware/adc.h"

int main() {
    stdio_init_all();
    const float conversion_factor = 3.3f / (1 << 12);

    adc_init();    
    gpio_set_dir_all_bits(0);

    for (int i = 2; i < 30; ++i) {
        gpio_set_function(i, GPIO_FUNC_SIO);
        if (i >= 26) {
            gpio_disable_pulls(i);
            gpio_set_input_enabled(i, false);
        }
    }    
    gpio_init(PICO_DEFAULT_LED_PIN);
    gpio_set_dir(PICO_DEFAULT_LED_PIN, GPIO_OUT);
    adc_set_temp_sensor_enabled(true);

    bool enabled = 0;

    while (1) {
        float result[3];
        for (int i=0; i<3; i++)
        {
            adc_select_input(i);
            sleep_ms(10);
            result[i] = adc_read() * conversion_factor;
        }
        adc_select_input(4);
        sleep_ms(10);
        float temp_adc = (float)adc_read() * conversion_factor;
        float tempC = 27.0f - (temp_adc - 0.706f) / 0.001721f;

        printf("Voltage 0-2: %f,%f,%f V, temp: %f\r\n", result[0], result[1], result[2], tempC);
        enabled = (enabled + 1) % 2;
        gpio_put(PICO_DEFAULT_LED_PIN, enabled);
        sleep_ms(2000);
    }
}

I got the output in which only onboard temperature seems reasonable and not changing:

Voltage 0-2: 1.145654,0.374634,1.025610 V, temp: 23.861492
Voltage 0-2: 0.407666,1.086035,0.441504 V, temp: 23.393347
Voltage 0-2: 1.136792,0.359326,1.046558 V, temp: 23.393347
Voltage 0-2: 0.558325,0.605859,0.579272 V, temp: 23.861492
Voltage 0-2: 0.645337,0.504346,0.696094 V, temp: 23.861492

In addition, I have two Raspberry Pi Pico's, and got similar result's on both.

Why values from ADC 0 to 2 are not equal/near 0, and they're changing so rapidly, when completely nothing is connected to the Pico?

I'm not an expert in that field, but measuring an open pin will not give meaningful results. Try it again with the pin explicitly connected to 0V and if that works you should search for pull down resistor and open circuit. — gerum, Jul 24 '22 at 22:00

score 3 · Answer 1 · answered Jul 24 '22 at 22:03

3

This is happens when pins aren't connected to anything - it is called noise. elegant explanation here

answered Jul 24 '22 at 22:03

Oscar

96
1
9

score 2 · Accepted Answer · answered Jul 25 '22 at 12:13

Pins are never not connected. They have a pullup/down resistor that can be switched on or off and they are connected to the read and write circuitry that can be switched between.

Those aren't physical switches. When you switch the pullup/down resistor off what that really means is that you make that a really high resistor. A bit of current will always leak across the switch.

You also have current running through nearby pins that might induce a current in the floating pin.

You might also simply have residual charge left on the pin from when it was connected or was an output pin. If a floating pin was output HIGH and you switch it to read the next value you read is probably going to be HIGH. Reading that drains a bit of the left over charge so if you wait a bit the next read might be LOW.

Measuring a floating pin is basically meaningless but might be a source for random bits. For anything else that's what the pullup/down resistors are for. [If the Pico doens't have those you have to connect one externally].

Why voltage from Raspberry Pi Pico ADC is not equal 0, when nothing is connected to the pin?

2 Answers2