> For the complete documentation index, see [llms.txt](https://monasheng.gitbook.io/zerotohero/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://monasheng.gitbook.io/zerotohero/arduino-zero-to-hero/core-skills/input-output-pins/analog-pins/analogread.md). # analogRead() {% embed url="" %} MCU analog *input* pins can be used to read variable voltages in a circuit, acting like rudimentary, discrete oscilloscopes. ## Analog to Digital Converters and Resolution MCUs typically achieve this using a built-in ['analog to digital' converter](https://learn.sparkfun.com/tutorials/analog-to-digital-conversion/all) circuit, or *ADC*. ADCs have a *resolution*, meaning that they can only identify finite, discrete steps between `LOW` and `HIGH`. Voltages between discrete steps are essentially rounded to the nearest ADC step before output. ADC resolution is normally talked about in terms of *bits* - i.e. binary states. A pretty bad ADC might be e.g. 2 bit - i.e. capable of representing $$2^2=4$$ unique steps over it's input range.

With only 2^2=4 steps of resolution, the underlying input voltage trace (blue) is approximated poorly by the stepped/discretised ADC input (purple).

The ADCs on most Arduino MCUs have 10-bit resolution, meaning they can identify $$2^{10}=1024$$ voltage steps in their $$0-5;\text{V}$$ input range - a very respectable level of precision!

With 2^{10}=1024 steps of resolution, the underlying input voltage trace (blue) is approximated well by the stepped/discretised ADC input (purple), though a close analysis reveals this input is still stepped.

### `pinMode()` To use an analog input pin for measurement, you should first set it to `INPUT` mode. While on some MCUs, pins will default to `INPUT` mode, it is best practice to be explicit in your code. Mode definition is typically achieved in the `setup()` function, as it only needs to be done once per boot. The `pinMode()` function expects a pin number and mode as arguments: ```arduino void setup() { // Set PIN A0 as an input. pinMode(A0, INPUT); // Setup the Serial Monitor for later Serial.begin(9600); } ``` {% embed url="" %} ### `analogRead()` After the mode is set, the pin voltage can be read. The `analogRead()` function expects a pin number as the only argument: ```arduino void loop() { // Read the state of PIN A0, and print this to the serial monitor Serial.println(analogRead(A0)); // Wait for 0.25 seconds delay(250); } ``` You can `analogRead()` in the `setup()` function to query the initial/boot state of a pin, but this is usually done programmatically in the `loop()` function - often as a function of time/on a cycle. {% embed url="" %} {% hint style="info" %} :man\_mage: **Important:** The `analogRead()` function returns a raw ADC value, which depends on the resolution of your MCU. It **does not** return a measured voltage! For the Arduino Mega, this ADC value ranges between `0-1023`. {% endhint %} ## Complete Sample Code {% code overflow="wrap" %} ```arduino void setup() { // Set PIN A0 as an input. pinMode(A0, INPUT); // Setup the Serial Monitor for later Serial.begin(9600); } void loop() { // Read the state of PIN A0, and print this to the serial monitor Serial.println(analogRead(A0)); // Wait for 0.25 seconds delay(250); } ``` {% endcode %}

A potentiometer (blue) provides a proportional output voltage on it's centre pin. This proportional output varies the voltages on the left- and right-hand-side pin voltages (GND and 5 V respectively in this case), depending on how far the knob is turned.

## Converting to Voltage `analogRead()` returns the raw ADC value, which maps between `HIGH` and `LOW` depending on the resolution of the board. To convert from an ADC value to a voltage, you can use the following formula: $$ V\_\text{in}=\texttt{adcIn}\times \frac{V\_\text{nom}}{\texttt{res}} $$ Where: * $$V\_\text{in}$$ is the voltage seen by the input pin, * $$\texttt{adcIn}$$ is the measured ADC value, * $$V\_\text{nom}$$ is the operating voltage range of the input pin (e.g. $$5;\text{V}$$ for most Arduino boards), and, * $$\texttt{res}$$ is the operating resolution of the MCU ADC (e.g. $$2^{10}=1024$$ for most Arduino boards). | Board | Vnom | res | Practical resolution | | ------------ | ---------------- | -------------------------------- | -------------------- | | Arduino Mega | $$5;\text{V}$$ | $$\texttt{10 bit: }2^{10}=1024$$ | $$4.9;\text{mV}$$ | | Teensy 4.0 | $$3.3;\text{V}$$ | $$\texttt{10 bit: }2^{10}=1024$$ | $$3.2;\text{mV}$$ | | STM 32 | $$3.3;\text{V}$$ | $$\texttt{12 bit: }2^{12}=4096$$ | $$1.2;\text{mV}$$ | ```arduino void setup() { // Set PIN A0 as an input. pinMode(A0, INPUT); // Setup the Serial Monitor for later Serial.begin(9600); } void loop() { // Read the state of PIN A0 and put it in a variable int adcIn = analogRead(A0); // Define the board parameters float vNom = 5.0; float res = 1024.0; // Calculate the voltage float Vin = adcIn * vNom/res; // Print this to the serial monitor Serial.println(Vin); // Wait for 0.25 seconds delay(250); } ```