> 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/digital-pins.md). # Digital Pins {% embed url="" %} {% hint style="info" %} :man\_mage: **Important:** Most MCUs possess a number of digital pins, capable of measuring and/or authoring digital (binary) voltages. In this context, the word ***digital*** essentially means ***binary*** - i.e. `ON` or `OFF`, `HIGH` or `LOW`, `1` or `0`. We will use `HIGH` and `LOW` to describe the two binary states that MCU digital pins can take in this module. {% endhint %} ## Logic Levels MCUs vary in the way they assign binary states (i.e. `HIGH` or `LOW`) to voltages, depending on their built-in hardware. While there is much variety in this space, most MCU digital pins operate at one of two so-called 'logic levels': 1. Defining `LOW` as `GND` (i.e. $$0;\text{V}$$), and `HIGH` as $$3.3;\text{V}$$, or, 2. Defining `LOW` as `GND` (i.e. $$0;\text{V}$$), and `HIGH` as $$5;\text{V}$$. It is important to know what logic level your MCU operates at - not least because you might be using the pin to drive other circuits. In terms of inputs, feeding in voltages higher than the MCU tolerance can lead to damage to a pin or the entire board! {% hint style="info" %} :man\_mage: **Important:** The Arduino Mega boards we use for this module utilise $$5;\text{V}$$ logic - i.e. they can accept up to $$5;\text{V}$$ as input and can produce $$5;\text{V}$$ signals as outputs. Many other boards, including some Arduino products, the Teensy line of MCUs, and many industrial-grade MCUs (e.g. STM32), utilise $$3.3;\text{V}$$ logic. {% endhint %} ## Hardware This module uses an [Arduino Mega](https://store.arduino.cc/products/arduino-mega-2560-rev3), which provides: * 22 digital-only pins (labelled `D0-D21`) on the long-edge of the board with the button, * 32 digital-only pins (labelled `D22-D53`) along the short-edge of the board without the USB, and, * 16 'digital' pin functionalities (`D54-D69`, labelled `A0-A15`) via the multi-purpose analog pins on the long-edge of the board without the button, for a total of 70 individual digital I/O pin functionalities. Note that (at least with Arduino), there is no hardware difference between digital *inputs* and digital *outputs* - any digital pin is capable of reading or writing digital signals.

Arduino digital pins. Remixed from https://content.arduino.cc/assets/Pinout-Mega2560rev3_latest.pdf (CC-BY-SA).

## Practical Considerations ### Input/Output Currents MCU digital pins are only equipped to deliver/absorb (sometimes called *source* and *sink*) very small currents - sufficient to drive small LEDs, or utilise communication protocols. The Arduino Mega, for instance, is capable of sourcing or sinking$$20;\text{mA}$$ per pin continuously. Many MCUs are only capable of $$10;\text{mA}$$ per pin. {% hint style="danger" %} :zap:**Warning:** Trying to drive large loads (e.g. motors, solenoids, contractors, etc., that can draw $$>1000;\text{mA}$$ of current) from an MCU digital pin will cause damage to pin, and likely to the board and it's power supply also. **DO NOT try to drive large currents directly from your Arduino, especially if it is being powered by your computer via USB. You might kill the board, your laptop, or both!** {% endhint %} It *is* possible to control large loads using an MCU - for such heavy-duty applications, see [Relays](/zerotohero/arduino-zero-to-hero/advanced-skills/relays.md). ### Special Pins Note that some digital pins have additional special/reserved duties - typically for communication protocols. These duties do not prohibit these pins from being used as 'normal' digital pins, but note that not every digital pin can perform every function - you may want to be strategic about which pins you use for which applications. For instance, on Arduino, `PIN 1` and `PIN 0` are used for serial `TX` and `RX` communications, and can exhibit *weird* behaviour if you e.g. plan to use the USB serial monitor at the same time as reading or writing these pin states!