# Primitives Primitive blocks provide direct access to device hardware. Use these blocks to read sensors, control digital and analog outputs, handle button inputs, and create custom hardware interactions. ## Understanding Registers Primitives use [register](https://docs.uniot.io/general-concepts/primitives#registers) indices instead of physical pin numbers. The register system organizes pins into categories (digital input/output, analog input/output) and assigns each an index. View your device's pin-to-register mapping in the Registers tab on the Uniot Platform. ## Return Values in Primitives Primitives can either return a value or perform an action without returning meaningful data. The visual blocks for default primitives (like `analog read`, `digital read`, etc.) are predefined and correctly reflect whether they return values or not. For user-defined primitives, the visual editor cannot automatically determine whether a primitive should return a value or just perform an action. Since primitives are C++ functions called by the UniotLisp Interpreter, every function must return something—even action-only primitives return a boolean value. Therefore, user primitive blocks are rendered with a return value by default. If your primitive doesn't need to return a meaningful value, you can disable the return value field in the block configuration. ## analog read

Reads an analog value from the specified register. Use this for sensors that output variable voltage levels. **Parameters:** * **Register** (Number): The analog input register index **Returns:** * **Number**: Value between 0-1023 (10-bit ADC resolution) **Example:**

Check light level and push event if it's too low

## analog write

Writes an analog (PWM) value to the specified register. Use this to control LED brightness, motor speed, or other variable outputs. **Parameters:** * **Register** (Number): The analog output register index * **Value** (Number): Value between 0-1023 (PWM duty cycle) **Example:**

Receive brightness value from dashboard and set LED accordingly

## digital read

Reads the digital state from the specified register (HIGH or LOW). **Parameters:** * **Register** (Number): The digital input register index **Returns:** * **Boolean**: `#t` for HIGH, `()` for LOW **Example:**

Read switch state and push event when changed

## digital write

Sets a digital register to HIGH or LOW. Use this to control LEDs, relays, or other on/off outputs. **Parameters:** * **Register** (Number): The digital output register index * **Value** (Boolean): `#t` for HIGH, `()` for LOW **Example:**

## button clicked

Detects button press events on the specified register. Returns true when a press-and-release cycle is detected, then resets until the next click. **Parameters:** * **Register** (Number): The digital input register index **Returns:** * **Boolean**: `#t` if clicked since last check, `()` otherwise **Example:**

## user primitive (template)

Creates a custom primitive definition that you'll implement in your firmware. Click the gear icon to configure parameters and return type. **Configuration:** * **Primitive Name**: Unique identifier for your custom primitive * **Parameters**: Define input values the primitive accepts * **Return Type**: Configure what type of value it returns (boolean by default, or none for action-only primitives) {% hint style="info" %} **Note on return values**: Since primitives are C++ functions called by the UniotLisp Interpreter, every function must return a value. If your primitive performs an action without needing to return a meaningful value, it should still return a boolean. The visual block is rendered with a return value by default. If you don't need to use the return value, you have to disable the corresponding field in the block configuration. {% endhint %} **Use cases**: Creating custom sensor drivers, protocol handlers, complex hardware control, or specialized operations not covered by standard primitives. ## user primitive (autogenerated)

Calls a custom primitive already defined in your device firmware. These blocks are automatically generated based on primitives registered in your firmware code. **Parameters:** * **Arguments**: Values matching the primitive's defined parameters **Returns:** * **Type varies**: Depends on how the primitive was defined in firmware {% hint style="info" %} **Note on return values**: Since the visual editor cannot determine whether a user primitive is intended to return a value or just perform an action, autogenerated blocks are rendered with a return value by default. This is because all C++ functions must return something—even action-only primitives return a boolean value. If your primitive doesn't need to return a meaningful value, you have to disable the return value field in the block configuration. {% endhint %} **Example:**