RP Pico / Pico2 - ADC shows different behaviour #4936

mietzen · 2025-06-27T11:08:18Z

mietzen
Jun 27, 2025

Hi,

this is my demo code for measuring the temperature in °C from a NTC 10K B3450 sensor:

Wiring:

ADC_VRef -> 10k_Thermistor -> GPIO28 -> 10k_Resitor -> GND

Code:

package main

import (
	"fmt"
	"machine"
	"math"
	"time"
)

func main() {
	time.Sleep(time.Millisecond * 1000)
	fmt.Println("Starting...")

	var adc machine.ADC = machine.ADC{Pin: machine.GPIO28}
	machine.InitADC()
	adc.Configure(machine.ADCConfig{})

	const (
		r25   = 10000.0
		beta  = 3450.0
		rHigh = 10000.0
	)

	for {
		adcValue := float64(adc.Get())
		println(adcValue)

		// Thermistor to GND
		//r := rHigh / (65535.0/adcValue - 1.0)
		// Thermistor to VCC
		r := rHigh * (65535.0/adcValue - 1.0)

		lnr := math.Log(r / r25)
		tsC := -274.15 + 1.0 / (1.0 / 298.15 + lnr / beta)
		fmt.Printf("%5.1f °C\n", tsC)
		time.Sleep(2 * time.Second)
	}
}

This works for a RP2040 (Pico) but with a RP2350 (Pico2) the temperature reading has a ~ 17K offset.

Is there a difference between RP2350 and RP2040 when it comes to ADC measurements?

Here are the outputs of pico and pico2:

Pico

$ tinygo flash -target=pico -monitor read_temp.go                               
Connected to /dev/cu.usbmodem111101. Press Ctrl-C to exit.
Starting...
+3.168000e+004
 22.3 °C
+3.153600e+004
 22.0 °C
+3.142400e+004
 21.9 °C
+3.132800e+004
 21.7 °C
+3.128000e+004
 21.6 °C
+3.116800e+004
 21.4 °C

Pico2

$ tinygo flash -target=pico2 -monitor read_temp.go
Connected to /dev/cu.usbmodem2101. Press Ctrl-C to exit.
Starting...
+3.966400e+004
 35.7 °C
+4.000000e+004
 36.3 °C
+3.956800e+004
 35.5 °C
+3.977600e+004
 35.9 °C
+3.984000e+004
 36.0 °C

Edit: I also checked ADC_VRef voltage on each board, both are a 3.28V

Answered by eliasnaur

Jun 27, 2025

I glanced at the Pico SDK, and it doesn't seem to differentiate between rp2350 and rp2040 either.

I did find #4938, but that's an unlikely cause.

I also found #4939, which is more juicy.

View full answer

eliasnaur · 2025-06-27T13:02:26Z

eliasnaur
Jun 27, 2025
Collaborator

I don't see any difference in the datasheets, and the TinyGo implementation is the same for the two chips. I'd try with a known voltage (say, the reference voltage) and see whether the raw ADC readings still differ.

3 replies

mietzen Jun 27, 2025
Author

You mean just a pull up like this?

ADC_VREF -> 10k Resistor -> ADC2 (GPIO28)

Edit:

Average of 5 ADC readings:

	Pull down	Pull up
RP2350	457.6	63993.6
RP2040	608.0	63955.2

Pull up seems not really to differ that much, Pull down differs a bit, is this significant?

Pico2 Pull down

$ tinygo flash -target=pico2 -monitor read_temp.go
Connected to /dev/cu.usbmodem1101. Press Ctrl-C to exit.
Starting...
+4.640000e+002
-66.7 °C
+4.640000e+002
-66.7 °C
+4.800000e+002
-66.3 °C
+4.480000e+002
-67.2 °C
+4.320000e+002
-67.6 °C

Pico2 Pull up

$ tinygo flash -target=pico2 -monitor read_temp.go
Connected to /dev/cu.usbmodem1101. Press Ctrl-C to exit.
Starting...
+6.396800e+004
169.6 °C
+6.398400e+004
170.2 °C
+6.398400e+004
170.2 °C
+6.403200e+004
172.1 °C
+6.400000e+004
170.9 °C

Pico Pull down

$ tinygo flash -target=pico -monitor read_temp.go
Connected to /dev/cu.usbmodem211101. Press Ctrl-C to exit.
Starting...
+6.080000e+002
-63.2 °C
+6.080000e+002
-63.2 °C
+6.080000e+002
-63.2 °C
+6.080000e+002
-63.2 °C
+6.080000e+002
-63.2 °C

Pico Pull up

$ tinygo flash -target=pico -monitor read_temp.go 
Connected to /dev/cu.usbmodem211101. Press Ctrl-C to exit.
Starting...
+6.406400e+004
173.4 °C
+6.392000e+004
167.8 °C
+6.395200e+004
169.0 °C
+6.392000e+004
167.8 °C
+6.392000e+004
167.8 °C

Edit: had a mistake in my code and had to redo the measurements, still the same result.

eliasnaur Jun 27, 2025
Collaborator

Yeah, the pull down and pull up values don't say anything conclusive. How about ADC_VREF -> 10k Resistor -> ADC2 (GPIO28) -> 10k -> GND to generate VREF/2 and to rule out the thermistor? Do the other ADC pins behave similarly?

mietzen Jun 27, 2025
Author

Both does give the same results. Below I tested against similar C Code, with that code I get the same results for RP2040 and RP2350.

mietzen · 2025-06-27T17:01:33Z

mietzen
Jun 27, 2025
Author

Ok I did some more debugging, I wrote (with some help of Claude) a Program in C. That works on RP2040 and RP2350, I then ported it to GO:

C

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

// Thermistor parameters
#define B_VALUE 3450.0              // B-value of the NTC thermistor
#define NOMINAL_RESISTANCE 10000.0  // Resistance at 25°C (10k ohm)
#define NOMINAL_TEMP 25.0           // Temperature for nominal resistance (25°C)

// Hardware parameters
#define THERMISTOR_ADC_CHANNEL 2    // GPIO28 is ADC channel 2
#define SERIES_RESISTOR 10000.0     // 10k ohm series resistor
#define ADC_RESOLUTION 4096.0       // 12-bit ADC (2^12)
#define VREF 3.3                    // Reference voltage

// Function to convert ADC reading to resistance
float adc_to_resistance(uint16_t adc_value) {
    // Convert ADC value to voltage
    float voltage = (adc_value / ADC_RESOLUTION) * VREF;
    
    // Calculate thermistor resistance using voltage divider
    // In your wiring: VREF -> THERMISTOR -> GPIO28 -> 10k_RESISTOR -> GND
    // Vout = Vref * (R_series / (R_thermistor + R_series))
    // Solving for R_thermistor:
    // R_thermistor = R_series * (Vref - Vout) / Vout
    float resistance = SERIES_RESISTOR * (VREF - voltage) / voltage;
    
    return resistance;
}

// Function to convert resistance to temperature using B-parameter equation
float resistance_to_temperature(float resistance) {
    // B-parameter equation:
    // 1/T = 1/T0 + (1/B) * ln(R/R0)
    // Where:
    // T = temperature in Kelvin
    // T0 = nominal temperature in Kelvin (25°C = 298.15K)
    // R = measured resistance
    // R0 = nominal resistance at T0
    // B = B-value of the thermistor
    
    float temp_kelvin = 1.0 / ((1.0 / (NOMINAL_TEMP + 273.15)) + 
                              (1.0 / B_VALUE) * log(resistance / NOMINAL_RESISTANCE));
    
    // Convert from Kelvin to Celsius
    float temp_celsius = temp_kelvin - 273.15;
    
    return temp_celsius;
}

int main() {
    // Initialize stdio for USB output
    stdio_init_all();
    
    // Initialize ADC
    adc_init();
    
    // Enable ADC input on GPIO28 (ADC channel 2)
    adc_gpio_init(28);
    
    // Wait for USB connection (optional, remove if not needed)
    sleep_ms(2000);
    
    printf("Thermistor Temperature Monitor\n");
    printf("==============================\n");
    
    while (1) {
        // Select ADC channel 2 (GPIO28)
        adc_select_input(THERMISTOR_ADC_CHANNEL);
        
        // Read ADC value
        uint16_t adc_value = adc_read();
        
        // Convert ADC reading to resistance
        float resistance = adc_to_resistance(adc_value);
        
        // Convert resistance to temperature
        float temp_celsius = resistance_to_temperature(resistance);
        
        // Debug: show what voltage this represents
        float voltage = (adc_value / ADC_RESOLUTION) * VREF;
        printf("Debug - ADC: %d, Voltage: %.3fV\n", adc_value, voltage);
        
        // Display ADC value and temperature
        printf("ADC: %4d, %5.1f °C\n", adc_value, temp_celsius);
        
        // Wait 2 seconds before next reading
        sleep_ms(2000);
    }
    
    return 0;
}

Pico RP2040

Thermistor Temperature Monitor
==============================
Debug - ADC: 1949, Voltage: 1.570V
ADC: 1949,  22.5 °C
Debug - ADC: 1942, Voltage: 1.565V
ADC: 1942,  22.4 °C
Debug - ADC: 1944, Voltage: 1.566V
ADC: 1944,  22.4 °C
Debug - ADC: 1946, Voltage: 1.568V
ADC: 1946,  22.5 °C
Debug - ADC: 1945, Voltage: 1.567V
ADC: 1945,  22.4 °C

Pico RP2040

Thermistor Temperature Monitor
==============================
Debug - ADC: 1955, Voltage: 1.575V
ADC: 1955,  22.7 °C
Debug - ADC: 1951, Voltage: 1.572V
ADC: 1951,  22.6 °C
Debug - ADC: 1952, Voltage: 1.573V
ADC: 1952,  22.6 °C
Debug - ADC: 1953, Voltage: 1.573V
ADC: 1953,  22.6 °C

Voltages and ADC Values are in the same range for both boards.

GO

Same program in tinygo:

package main

import (
	"fmt"
	"machine"
	"math"
	"time"
)

const (
	// Thermistor parameters
	B_VALUE            = 3450.0  // B-value of the NTC thermistor
	NOMINAL_RESISTANCE = 10000.0 // Resistance at 25°C (10k ohm)
	NOMINAL_TEMP       = 25.0    // Temperature for nominal resistance (25°C)

	// Hardware parameters
	SERIES_RESISTOR = 10000.0 // 10k ohm series resistor
	ADC_RESOLUTION  = 65536.0 // TinyGo returns 16-bit values (2^16)
	VREF            = 3.3     // Reference voltage
)

// Function to convert ADC reading to resistance
func adcToResistance(adcValue uint16) float64 {
	// Convert ADC value to voltage
	voltage := (float64(adcValue) / ADC_RESOLUTION) * VREF

	// Calculate thermistor resistance using voltage divider
	// In your wiring: VREF -> THERMISTOR -> GPIO28 -> 10k_RESISTOR -> GND
	// R_thermistor = R_series * (Vref - Vout) / Vout
	resistance := SERIES_RESISTOR * (VREF - voltage) / voltage

	return resistance
}

// Function to convert resistance to temperature using B-parameter equation
func resistanceToTemperature(resistance float64) float64 {
	// B-parameter equation:
	// 1/T = 1/T0 + (1/B) * ln(R/R0)
	// Where T is in Kelvin, T0 = 298.15K (25°C)

	tempKelvin := 1.0 / ((1.0 / (NOMINAL_TEMP + 273.15)) +
		(1.0/B_VALUE)*math.Log(resistance/NOMINAL_RESISTANCE))

	// Convert from Kelvin to Celsius
	tempCelsius := tempKelvin - 273.15

	return tempCelsius
}

func main() {
	// Wait for initialization
	time.Sleep(time.Second * 2)
	fmt.Println("Thermistor Temperature Monitor")
	fmt.Println("==============================")

	// Initialize ADC
	machine.InitADC()

	// Configure ADC on GPIO28
	adc := machine.ADC{Pin: machine.GPIO28}
	adc.Configure(machine.ADCConfig{})

	for {
		// Read ADC value
		adcValue := adc.Get()

		// Debug: show what voltage this represents
		voltage := (float64(adcValue) / ADC_RESOLUTION) * VREF
		fmt.Printf("Debug - ADC: %d, Voltage: %.3fV\n", adcValue, voltage)

		// Convert ADC reading to resistance
		resistance := adcToResistance(adcValue)

		// Convert resistance to temperature
		tempCelsius := resistanceToTemperature(resistance)

		// Display ADC value and temperature
		fmt.Printf("ADC: %4d, %5.1f °C\n", adcValue, tempCelsius)

		// Wait 2 seconds before next reading
		time.Sleep(2 * time.Second)
	}
}

Pico2 RP2350

Thermistor Temperature Monitor
==============================
Debug - ADC: 40240, Voltage: 2.026V
ADC: 40240,  37.5 °C
Debug - ADC: 40096, Voltage: 2.019V
ADC: 40096,  37.2 °C
Debug - ADC: 39584, Voltage: 1.993V
ADC: 39584,  36.3 °C
Debug - ADC: 39792, Voltage: 2.004V
ADC: 39792,  36.7 °C
Debug - ADC: 39600, Voltage: 1.994V
ADC: 39600,  36.3 °C

Pico RP2040

Thermistor Temperature Monitor
==============================
Debug - ADC: 31312, Voltage: 1.577V
ADC: 31312,  22.7 °C
Debug - ADC: 31280, Voltage: 1.575V
ADC: 31280,  22.7 °C
Debug - ADC: 31296, Voltage: 1.576V
ADC: 31296,  22.7 °C
Debug - ADC: 31280, Voltage: 1.575V
ADC: 31280,  22.7 °C
Debug - ADC: 31280, Voltage: 1.575V
ADC: 31280,  22.7 °C

The voltages and ADC values are different, between the boards. So something is definitely different between pico & pico2 tinygo implementations.

0 replies

eliasnaur · 2025-06-27T18:20:08Z

eliasnaur
Jun 27, 2025
Collaborator

I glanced at the Pico SDK, and it doesn't seem to differentiate between rp2350 and rp2040 either.

I did find #4938, but that's an unlikely cause.

I also found #4939, which is more juicy.

1 reply

mietzen Jun 28, 2025
Author

Thanks for looking into it @eliasnaur 👍

PR #4939 fixes the issue 🚀

$ ./tinygo_pr4939/tinygo/bin/tinygo flash -target=pico2 -monitor read_temp.go
Connected to /dev/cu.usbmodem1101. Press Ctrl-C to exit.
Thermistor Temperature Monitor
==============================
Debug - ADC: 30080, Voltage: 1.515V
ADC: 30080,  20.8 °C
Debug - ADC: 30112, Voltage: 1.516V
ADC: 30112,  20.9 °C
Debug - ADC: 30160, Voltage: 1.519V
ADC: 30160,  20.9 °C
Debug - ADC: 30112, Voltage: 1.516V
ADC: 30112,  20.9 °C
Debug - ADC: 30240, Voltage: 1.523V
ADC: 30240,  21.1 °C

I also tested it with the RP2040 and observed no regression.

$ ./tinygo_pr4939/tinygo/bin/tinygo flash -target=pico -monitor read_temp.go
Connected to /dev/cu.usbmodem211101. Press Ctrl-C to exit.
Thermistor Temperature Monitor
==============================
Debug - ADC: 30672, Voltage: 1.544V
ADC: 30672,  21.7 °C
Debug - ADC: 30656, Voltage: 1.544V
ADC: 30656,  21.7 °C
Debug - ADC: 30640, Voltage: 1.543V
ADC: 30640,  21.7 °C
Debug - ADC: 30656, Voltage: 1.544V
ADC: 30656,  21.7 °C
Debug - ADC: 30640, Voltage: 1.543V
ADC: 30640,  21.7 °C

PR #4938 didn't improve things:

$ ./tinygo_pr4938/tinygo/bin/tinygo flash -target=pico2 -monitor read_temp.go
Connected to /dev/cu.usbmodem1101. Press Ctrl-C to exit.
Thermistor Temperature Monitor
==============================
Debug - ADC: 39728, Voltage: 2.000V
ADC: 39728,  36.5 °C
Debug - ADC: 39632, Voltage: 1.996V
ADC: 39632,  36.4 °C
Debug - ADC: 39344, Voltage: 1.981V
ADC: 39344,  35.9 °C
Debug - ADC: 39424, Voltage: 1.985V
ADC: 39424,  36.0 °C
Debug - ADC: 39616, Voltage: 1.995V
ADC: 39616,  36.3 °C

RP2040, shows no regression:

$ ./tinygo_pr4938/tinygo/bin/tinygo flash -target=pico -monitor read_temp.go
Connected to /dev/cu.usbmodem211101. Press Ctrl-C to exit.
Thermistor Temperature Monitor
==============================
Debug - ADC: 30752, Voltage: 1.548V
ADC: 30752,  21.9 °C
Debug - ADC: 30672, Voltage: 1.544V
ADC: 30672,  21.7 °C
Debug - ADC: 30688, Voltage: 1.545V
ADC: 30688,  21.8 °C
Debug - ADC: 30672, Voltage: 1.544V
ADC: 30672,  21.7 °C
Debug - ADC: 30656, Voltage: 1.544V
ADC: 30656,  21.7 °C

TinyGo

RP Pico / Pico2 - ADC shows different behaviour #4936

Uh oh!

Uh oh!

mietzen Jun 27, 2025

Pico

Pico2

Replies: 3 comments · 4 replies

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eliasnaur Jun 27, 2025 Collaborator

Uh oh!

Uh oh!

mietzen Jun 27, 2025 Author

Pico2 Pull down

Pico2 Pull up

Pico Pull down

Pico Pull up

Uh oh!

eliasnaur Jun 27, 2025 Collaborator

Uh oh!

mietzen Jun 27, 2025 Author

Uh oh!

Uh oh!

mietzen Jun 27, 2025 Author

C

Pico RP2040

Pico RP2040

GO

Pico2 RP2350

Pico RP2040

Uh oh!

Uh oh!

eliasnaur Jun 27, 2025 Collaborator

Uh oh!

Uh oh!

mietzen Jun 28, 2025 Author

mietzen
Jun 27, 2025

Replies: 3 comments 4 replies

eliasnaur
Jun 27, 2025
Collaborator

mietzen Jun 27, 2025
Author

eliasnaur Jun 27, 2025
Collaborator

mietzen Jun 27, 2025
Author

mietzen
Jun 27, 2025
Author

eliasnaur
Jun 27, 2025
Collaborator

mietzen Jun 28, 2025
Author