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# Air Quality Monitor
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**Air Quality Monitor** is a lightweight web interface that displays real-time data from two environmental sensors: **HTU21D** and **BMP180**. The system uses **NGINX** to serve an HTML page that reads live data from JSON files.
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All the system is runing on a beagle bone black.
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## Project Description
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This project was designed to visualize environmental parameters in a simple, fast, and efficient way using an embedded graphical interface. The data is obtained from the following sensors:
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- **HTU21D**: Temperature and relative humidity.
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- **BMP180**: Atmospheric pressure and temperature.
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The data is stored in two JSON files, updated by the embedded system and displayed through an HTML/JavaScript-based frontend.
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---
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## Project Structure
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UI directory
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- BMP180.json # Pressure and temperature data from BMP180
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- HTU21D.json # Temperature and humidity data from HTU21D
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- index.html # Main web interface
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- index.js # JavaScript logic to fetch and display JSON data
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- style.css # Custom CSS styles
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> The `.json` files are automatically updated by C programs that communicate with the sensors via I2C witch the librarys integrated on the repository.
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---
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##
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Both sensors that we are using are conected to the I2C port 2 of the beagle bone
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## 🚀 Installation & Deployment with NGINX
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### Requirements
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- Linux server with `nginx` installed.
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- Root access or permission to modify NGINX configuration.
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### Steps
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1. Install NGINX (if not already installed):
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```bash
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sudo apt update
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sudo apt install nginx
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```
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2. Copy the project files to NGINX’s public directory (e.g., /var/www/html):
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``` bash
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```
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3. Redirect the configuration file to our repository folder:
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```bash
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sudo vi /etc/nginx/sites-available/default
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```
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4. Replace the line:
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```bash
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root /var/www/html;
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```
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with:
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```bash
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root /home/debian/path/to/your/repository;
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```
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5. Apply chages with:
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```bash
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sudo systemctl restart nginx
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```
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## User interface
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This is a simple web page that displays real-time sensor data from two devices:
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- BMP180: Shows temperature and pressure.
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- HTU21D: Shows humidity and temperature.
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It’s styled with a separate CSS file (style.css) and fetches live data using JavaScript (index.js), which likely reads values from two JSON files (BMP180.json and HTU21D.json).
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### HTML
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#### Complete code
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```html
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<!DOCTYPE html>
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<html lang="en">
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<head>
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<meta charset="UTF-8" />
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<meta name="viewport" content="width=device-width, initial-scale=1.0"/>
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<title>Sensor Dashboard</title>
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<link rel="stylesheet" href="style.css" />
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</head>
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<body>
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<div class="background"></div>
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<div class="container">
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<h1>Sensor Dashboard</h1>
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<div class="sensor-card">
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<img src="https://cdn-icons-png.flaticon.com/512/1126/1126741.png" alt="Temperature & Pressure Icon">
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<h2>BMP180</h2>
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<div class="data">
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<span id="temperature">Temperature: -- °C</span>
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<span id="pressure">Pressure: -- hPa</span>
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</div>
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</div>
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<div class="sensor-card">
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<img src="https://cdn-icons-png.flaticon.com/512/728/728093.png" alt="Humidity Icon">
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<h2>HTU21D</h2>
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<div class="data">
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<span id="humidity">Humidity: -- %</span>
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<span id="tempHTU">Temperature: -- °C</span>
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</div>
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</div>
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</div>
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<script src="index.js"></script>
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</body>
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</html>
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```
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#### Explanation
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```html
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<!DOCTYPE html>
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<html lang="en">
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```
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- Declares this document as HTML5.
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- lang="en" sets the document language to English, which is useful for accessibility and SEO.
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##### HEAD
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```html
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<head>
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<meta charset="UTF-8" />
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<meta name="viewport" content="width=device-width, initial-scale=1.0"/>
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<title>Sensor Dashboard</title>
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<link rel="stylesheet" href="style.css" />
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</head>
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```
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- charset="UTF-8" ensures proper text encoding.
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- viewport makes the layout responsive on mobile devices.
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- title is what appears in the browser tab.
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- link imports the external CSS file (style.css) for styling.
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##### BODY
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```html
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<body>
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<div class="background"></div>
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<div class="container">
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...
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</div>
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<script src="index.js"></script>
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</body>
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```
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- The background div might be used for a visual effect like a background image or color gradient (defined in your CSS).
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- container holds the main content (dashboard).
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- index.js is loaded at the end to ensure the DOM is ready before scripts run.
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##### Dashboard Title
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```html
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<h1>Sensor Dashboard</h1>
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```
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- This is the main heading of your web page.
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##### Sensor Card: BMP180
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```hmtl
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<div class="sensor-card">
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<img src="https://cdn-icons-png.flaticon.com/512/1126/1126741.png" alt="Temperature & Pressure Icon">
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<h2>BMP180</h2>
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<div class="data">
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<span id="temperature">Temperature: -- °C</span>
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<span id="pressure">Pressure: -- hPa</span>
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</div>
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</div>
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```
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- This block represents the BMP180 sensor.
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- The icon (from flaticon.com) visually represents pressure/temperature.
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- The id attributes (temperature, pressure) are hooks used by JavaScript to insert real values from BMP180.json.
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##### Sensor Card: HTU21D
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```html
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<div class="sensor-card">
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<img src="https://cdn-icons-png.flaticon.com/512/728/728093.png" alt="Humidity Icon">
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<h2>HTU21D</h2>
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<div class="data">
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<span id="humidity">Humidity: -- %</span>
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<span id="tempHTU">Temperature: -- °C</span>
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</div>
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</div>
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```
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- This block represents the HTU21D sensor.
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- The image is a humidity icon.
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- The id attributes (humidity, tempHTU) are also updated by JavaScript using HTU21D.json.
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##### JavaScript Integration
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```hmtl
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<script src="index.js"></script>
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```
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- This line loads your script, which is responsible for:
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- Fetching the JSON files.
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- Parsing their content.
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- Replacing the placeholder values (-- °C, -- hPa, etc.) with live sensor data.
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### Index.js
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The script fetches data from two local JSON files:
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- BMP180.json — contains temperature and pressure.
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- HTU21D.json — contains temperature and humidity.
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Then it updates the HTML every 3 seconds so the page always shows live sensor values.
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#### Full code
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```js
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function updateBMP180() {
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fetch('BMP180.json')
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.then(res => res.json())
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.then(data => {
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document.getElementById('temperature').textContent = `Temperature: ${data.temperature.toFixed(2)} °C`;
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document.getElementById('pressure').textContent = `Pressure: ${data.pressure.toFixed(2)} hPa`;
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})
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.catch(err => console.error("Error BMP180:", err));
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}
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function updateHTU21D() {
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fetch('HTU21D.json')
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.then(res => res.json())
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.then(data => {
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document.getElementById('humidity').textContent = `Humidity: ${data.humidity.toFixed(1)} %`;
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document.getElementById('tempHTU').textContent = `Temperature: ${data.temperature.toFixed(1)} °C`;
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})
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.catch(err => console.error("Error HTU21D:", err));
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}
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setInterval(() => {
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updateBMP180();
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updateHTU21D();
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}, 3000);
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// Carga inicial
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updateBMP180();
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updateHTU21D();
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```
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### Code explanation
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#### Function: updateBMP180
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```js
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function updateBMP180() {
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fetch('BMP180.json')
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.then(res => res.json())
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.then(data => {
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document.getElementById('temperature').textContent = `Temperature: ${data.temperature.toFixed(2)} °C`;
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document.getElementById('pressure').textContent = `Pressure: ${data.pressure.toFixed(2)} hPa`;
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})
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.catch(err => console.error("Error BMP180:", err));
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}
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```
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- This defines a new function named updateBMP180. You’ll call this function when you want to update the BMP180 sensor data on the page.
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- fetch('BMP180.json'): Loads the JSON file from the same directory.
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- .then(res =\>json()): Parses the response as JSON
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- .then(data +\> {}): Accesses the data inside the file.
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- document.getElementById(...) updates the corresponding <span> elements in your HTML.
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#### Function: updateHTU21D
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```js
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function updateHTU21D() {
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fetch('HTU21D.json')
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.then(res => res.json())
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.then(data => {
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document.getElementById('humidity').textContent = `Humidity: ${data.humidity.toFixed(1)} %`;
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document.getElementById('tempHTU').textContent = `Temperature: ${data.temperature.toFixed(1)} °C`;
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})
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.catch(err => console.error("Error HTU21D:", err));
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}
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```
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Works the same way as updateBMP180(), but:
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- Fetches from HTU21D.json.
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- Displays humidity and temperature.
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- Uses .toFixed(1) for 1 decimal place (common for humidity values).
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#### Auto-update every 3 seconds
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```js
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setInterval(() => {
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updateBMP180();
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updateHTU21D();
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}, 3000);
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```
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- Calls both update functions every 3,000 milliseconds (3 seconds).
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- Keeps the UI in sync with new sensor readings, assuming the .json files are being updated continuously.
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#### Initial load
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```js
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updateBMP180();
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updateHTU21D();
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```
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- Ensures the data is shown immediately on page load, before the 3-second interval kicks in.
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### CSS
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#### Full code
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```css
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* {
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margin: 0;
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padding: 0;
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box-sizing: border-box;
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font-family: "Segoe UI", sans-serif;
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}
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body, html {
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height: 100%;
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background-color: #0d1117;
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color: #ffffff;
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position: relative;
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overflow: hidden;
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}
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.background {
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background-image: url('https://wallpapers.com/images/hd/blue-circuit-board-traces-zn0xezd4t8axj9r6.webp');
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background-size: cover;
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background-position: center;
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opacity: 0.1;
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filter: blur(3px);
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position: absolute;
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top: 0; left: 0;
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width: 100%;
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height: 100%;
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z-index: -1;
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}
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.container {
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max-width: 800px;
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margin: 40px auto;
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background-color: #161b22;
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padding: 30px;
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border-radius: 20px;
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box-shadow: 0 0 30px rgba(0, 255, 255, 0.2);
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animation: fadeIn 1s ease-in;
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}
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h1 {
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text-align: center;
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margin-bottom: 30px;
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font-size: 2.5rem;
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color: #58a6ff;
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}
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.sensor-card {
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background-color: #1f2937;
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padding: 20px;
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border-radius: 15px;
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margin-bottom: 20px;
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box-shadow: 0 0 20px rgba(100, 255, 255, 0.1);
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animation: slideIn 1s ease;
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display: flex;
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flex-direction: column;
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align-items: center;
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text-align: center;
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}
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.sensor-card img {
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width: 64px;
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height: 64px;
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margin-bottom: 10px;
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}
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.sensor-card h2 {
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margin-bottom: 10px;
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color: #90cdf4;
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}
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.data span {
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display: block;
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margin: 5px 0;
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font-size: 1.2rem;
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color: #ffffff;
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}
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@keyframes fadeIn {
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from { opacity: 0; transform: translateY(-20px); }
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to { opacity: 1; transform: translateY(0); }
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}
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@keyframes slideIn {
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from { opacity: 0; transform: scale(0.9); }
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to { opacity: 1; transform: scale(1); }
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}
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```
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#### Explaining
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Global Styling
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- *: Resets default browser spacing (margin, padding) and sets a consistent font and box model across the entire page.
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- body, html:
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- Sets full height layout.
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- Applies a dark background (#0d1117) with white text.
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- Hides overflow and enables positioning for internal layers.
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⸻
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Background Layer
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- .background:
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- Adds a faint, blurred circuit board image as the background.
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- Uses opacity: 0.1 and filter: blur(3px) to give it a soft tech feel.
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- Positioned absolutely behind everything (z-index: -1).
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⸻
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Main Container
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- .container:
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- A centered, card-like section with:
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- A dark background (#161b22)
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- Rounded corners and soft glowing shadow.
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- Padding and a fade-in animation on load.
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⸻
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Title
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- h1:
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- Large, centered header in light blue (#58a6ff).
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- Styled to stand out at the top of the dashboard.
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⸻
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Sensor Cards
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- .sensor-card:
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- Styled boxes for each sensor.
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- Darker background (#1f2937) with a soft shadow.
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- Rounded corners, padding, and a slide-in animation when they load.
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- Contents are centered vertically and horizontally.
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- .sensor-card img:
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- Sensor icons sized to 64×64 pixels with margin for spacing.
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- .sensor-card h2:
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- Sub-headers for each sensor card in a soft blue (#90cdf4).
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⸻
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Data Text
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- .data span:
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- Each line of sensor data (e.g., temperature, humidity).
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- Displayed as blocks with spacing and larger font for visibility.
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⸻
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Animations
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- @keyframes fadeIn:
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- Smooth slide-down + fade-in for the container.
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- @keyframes slideIn:
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- Subtle zoom-in effect for each sensor card.
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## Sensor library
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### HTU21D library
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The library contains 3 files, HTU21D.c, htu21d.h and main.c
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#### htu21d.h
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```h
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#ifndef HTU21D_H
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#define HTU21D_H
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// HTU21D i2c address
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#define HTU21D_ADDR 0x40
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//commands for readings
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#define HTU21D_TEMP 0xE3
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#define HTU21D_HUM 0xE5
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#define HTU21D_RESET 0xFE
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//funtion declarations
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//Temp:
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int getTemp(int fd, double *temp);
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//HUM
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int getHum(int fd, double *hum);
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//RESET
|
|
|
int getReset(int fd);
|
|
|
#endif
|
|
|
```
|
|
|
##### Explanation
|
|
|
|
|
|
- HTU21D_ADDR: The I2C address of the sensor (0x40).
|
|
|
- HTU21D_TEMP: Command to read temperature (0xE3).
|
|
|
- HTU21D_HUM: Command to read humidity (0xE5).
|
|
|
- HTU21D_RESET: Command to reset the sensor (0xFE).
|
|
|
- Declares three functions:
|
|
|
1. getTemp() – for reading temperature,
|
|
|
2. getHum() – for reading humidity,
|
|
|
3. getReset() – for resetting the sensor.
|
|
|
|
|
|
#### HTU21D.c
|
|
|
```c
|
|
|
#include <stdio.h>
|
|
|
#include <stdlib.h>
|
|
|
#include <string.h>
|
|
|
//Aditional librarys
|
|
|
#include <errno.h>
|
|
|
#include <fcntl.h>
|
|
|
#include "htu21d.h"
|
|
|
|
|
|
#define I2C_PATH "/dev/i2c-%d"
|
|
|
#define I2C_PORT 2
|
|
|
int main(){
|
|
|
char filePath[20];
|
|
|
snprintf(filePath, sizeof(filePath), I2C_PATH, I2C_PORT );
|
|
|
int fd = open(filePath, O_RDWR);
|
|
|
|
|
|
if(fd<0){
|
|
|
fprintf(stderr, "Error: Unable to access HTU21D sensor: %s",strerror(errno));
|
|
|
exit(-1);
|
|
|
}
|
|
|
//measurements
|
|
|
double temperature=0;
|
|
|
double humidity=0;
|
|
|
if((getTemp(fd, &temperature)<0)||(getHum(fd, &humidity)<0)){
|
|
|
fprintf(stderr,"Error -404: Measurments not read");
|
|
|
exit(-1);
|
|
|
}
|
|
|
|
|
|
|
|
|
//printf("HTU21D Module \n");
|
|
|
//printf("%5.2fC \n", temperature);
|
|
|
//printf("%5.2fC \n", humidity);
|
|
|
printf("{");
|
|
|
printf("\"temperature\": %5.2f,", temperature);
|
|
|
printf("\"humidity\": %5.2f", humidity);
|
|
|
printf("}");
|
|
|
return 0;
|
|
|
|
|
|
}
|
|
|
```
|
|
|
##### Explanation
|
|
|
###### Librarys
|
|
|
```c
|
|
|
#include <unistd.h>
|
|
|
#include <sys/ioctl.h>
|
|
|
#include <linux/i2c-dev.h>
|
|
|
#include <i2c/smbus.h>
|
|
|
#include <stdio.h>
|
|
|
#include "htu21d.h"
|
|
|
```
|
|
|
These headers provide:
|
|
|
- unistd.h: basic system calls (like read, write, etc.)
|
|
|
- sys/ioctl.h: lets you control devices (e.g., set I2C slave address)
|
|
|
- linux/i2c-dev.h: allows interaction with the I2C bus
|
|
|
- i2c/smbus.h: provides high-level SMBus/I2C functions like i2c_smbus_read_i2c_block_data
|
|
|
- stdio.h: for printing error messages
|
|
|
- "htu21d.h": includes your own header file (constants and function declarations)
|
|
|
###### getTemp() Function
|
|
|
```c
|
|
|
int getTemp(int fd, double *temp) {
|
|
|
getReset(fd); // Resets the sensor before reading
|
|
|
|
|
|
char buffer[3]; // Buffer to hold 3 bytes of data from sensor
|
|
|
__s32 res = i2c_smbus_read_i2c_block_data(fd, HTU21D_TEMP, 3, buffer);
|
|
|
```
|
|
|
- Calls getReset(fd) to ensure the sensor is in a clean state before reading.
|
|
|
- Uses i2c_smbus_read_i2c_block_data to read 3 bytes from the sensor using the temperature command 0xE3.
|
|
|
- These 3 bytes contain raw temperature data + CRC (which you ignore here).
|
|
|
```c
|
|
|
if (res < 0) {
|
|
|
perror("ERROR-1: Failed to read Temperature");
|
|
|
return -1;
|
|
|
}
|
|
|
```
|
|
|
- If the read fails (e.g., I2C communication issue), it prints an error and returns -1.
|
|
|
```c
|
|
|
*temp = -46.85 + 175.72 * (buffer[0] * 256 + buffer[1]) / 65536.0;
|
|
|
return 0;
|
|
|
}
|
|
|
```
|
|
|
- The first two bytes in buffer are combined to make a 16-bit raw temperature.
|
|
|
- The formula from the HTU21D datasheet is applied to convert raw data into actual temperature in Celsius.
|
|
|
- The result is stored in the variable pointed to by *temp.
|
|
|
- Returns 0 to indicate success.
|
|
|
|
|
|
###### getHum() Function
|
|
|
This function is nearly identical to getTemp(), but it reads humidity instead.
|
|
|
```c
|
|
|
int getHum(int fd, double *hum) {
|
|
|
getReset(fd);
|
|
|
char buffer[3];
|
|
|
__s32 res = i2c_smbus_read_i2c_block_data(fd, HTU21D_HUM, 3, buffer);
|
|
|
```
|
|
|
- Uses the command HTU21D_HUM (value 0xE5) to read humidity data.
|
|
|
```c
|
|
|
if (res < 0) {
|
|
|
perror("ERROR -3: Failed to read Humidity");
|
|
|
return -1;
|
|
|
}
|
|
|
*hum = -6 + 125 * (buffer[0] * 256 + buffer[1]) / 65536.0;
|
|
|
return 0;
|
|
|
}
|
|
|
```
|
|
|
- Applies the formula from the datasheet to convert the raw bytes to % humidity.
|
|
|
###### getReset() Function
|
|
|
```c
|
|
|
int getReset(int fd) {
|
|
|
if (0 > ioctl(fd, I2C_SLAVE, HTU21D_ADDR)) {
|
|
|
perror("ERROR -2: Failed in reset");
|
|
|
return -2;
|
|
|
}
|
|
|
i2c_smbus_write_byte(fd, HTU21D_RESET);
|
|
|
return 0;
|
|
|
}
|
|
|
```
|
|
|
- ioctl() tells the I2C driver which slave device you want to talk to (address 0x40).
|
|
|
- Then i2c\_smbus\_write\_byte() sends the reset command 0xFE to the HTU21D.
|
|
|
- Resets the sensor, which is sometimes necessary to avoid bad reads or stuck states.
|
|
|
|
|
|
### BMP180 library
|
|
|
The library contains 3 files, bmp180.c, bmp180.h and main.c
|
|
|
#### bmp180.c
|
|
|
```c
|
|
|
#include "bmp180.h"
|
|
|
#include <unistd.h>
|
|
|
#include <fcntl.h>
|
|
|
#include <sys/ioctl.h>
|
|
|
#include <linux/i2c-dev.h>
|
|
|
#include <stdio.h>
|
|
|
|
|
|
#define BMP180_ADDR 0x77
|
|
|
|
|
|
// Lectura de 16 bits de un registro (dos bytes)
|
|
|
static int read16(int fd, uint8_t reg, int16_t *value) {
|
|
|
uint8_t buf[2];
|
|
|
if (write(fd, ®, 1) != 1) return -1;
|
|
|
if (read(fd, buf, 2) != 2) return -1;
|
|
|
*value = (buf[0] << 8) | buf[1];
|
|
|
return 0;
|
|
|
}
|
|
|
|
|
|
// Escritura de 8 bits a un registro
|
|
|
static int write8(int fd, uint8_t reg, uint8_t value) {
|
|
|
uint8_t buf[2] = {reg, value};
|
|
|
if (write(fd, buf, 2) != 2) return -1;
|
|
|
return 0;
|
|
|
}
|
|
|
|
|
|
// Leer calibración desde el sensor
|
|
|
int bmp180_init(int fd, bmp180_calib_data_t *calib) {
|
|
|
if (read16(fd, 0xAA, &calib->AC1) < 0) return -1;
|
|
|
if (read16(fd, 0xAC, &calib->AC2) < 0) return -1;
|
|
|
if (read16(fd, 0xAE, &calib->AC3) < 0) return -1;
|
|
|
if (read16(fd, 0xB0, (int16_t*)&calib->AC4) < 0) return -1;
|
|
|
if (read16(fd, 0xB2, (int16_t*)&calib->AC5) < 0) return -1;
|
|
|
if (read16(fd, 0xB4, (int16_t*)&calib->AC6) < 0) return -1;
|
|
|
if (read16(fd, 0xB6, &calib->B1) < 0) return -1;
|
|
|
if (read16(fd, 0xB8, &calib->B2) < 0) return -1;
|
|
|
if (read16(fd, 0xBA, &calib->MB) < 0) return -1;
|
|
|
if (read16(fd, 0xBC, &calib->MC) < 0) return -1;
|
|
|
if (read16(fd, 0xBE, &calib->MD) < 0) return -1;
|
|
|
return 0;
|
|
|
}
|
|
|
|
|
|
// Leer temperatura sin procesar (raw temp)
|
|
|
static int bmp180_read_raw_temperature(int fd, int32_t *raw_temp) {
|
|
|
if (write8(fd, 0xF4, 0x2E) < 0) return -1; // start temp measurement
|
|
|
usleep(4500); // esperar 4.5 ms
|
|
|
int16_t value;
|
|
|
if (read16(fd, 0xF6, &value) < 0) return -1;
|
|
|
*raw_temp = value;
|
|
|
return 0;
|
|
|
}
|
|
|
|
|
|
// Leer presión sin procesar (raw pressure)
|
|
|
static int bmp180_read_raw_pressure(int fd, int32_t *raw_press, int oss) {
|
|
|
if (write8(fd, 0xF4, 0x34 + (oss << 6)) < 0) return -1; // start pressure measurement
|
|
|
usleep(25000); // esperar 25 ms para oss=0 (simple oversampling)
|
|
|
uint8_t buf[3];
|
|
|
uint8_t reg = 0xF6;
|
|
|
if (write(fd, ®, 1) != 1) return -1;
|
|
|
if (read(fd, buf, 3) != 3) return -1;
|
|
|
*raw_press = ((buf[0] << 16) | (buf[1] << 8) | buf[2]) >> (8 - oss);
|
|
|
return 0;
|
|
|
}
|
|
|
|
|
|
int bmp180_read_temperature(int fd, bmp180_calib_data_t *calib, double *temperature) {
|
|
|
int32_t UT;
|
|
|
if (bmp180_read_raw_temperature(fd, &UT) < 0) return -1;
|
|
|
|
|
|
int32_t X1 = ((UT - calib->AC6) * calib->AC5) >> 15;
|
|
|
int32_t X2 = (calib->MC << 11) / (X1 + calib->MD);
|
|
|
int32_t B5 = X1 + X2;
|
|
|
*temperature = ((B5 + 8) >> 4) / 10.0;
|
|
|
return 0;
|
|
|
}
|
|
|
|
|
|
int bmp180_read_pressure(int fd, bmp180_calib_data_t *calib, double *pressure) {
|
|
|
int oss = 0; // oversampling setting 0..3
|
|
|
int32_t UP;
|
|
|
if (bmp180_read_raw_pressure(fd, &UP, oss) < 0) return -1;
|
|
|
|
|
|
// Recalcular B5 para temperatura, necesario para presión
|
|
|
int32_t UT;
|
|
|
if (bmp180_read_raw_temperature(fd, &UT) < 0) return -1;
|
|
|
|
|
|
int32_t X1 = ((UT - calib->AC6) * calib->AC5) >> 15;
|
|
|
int32_t X2 = (calib->MC << 11) / (X1 + calib->MD);
|
|
|
int32_t B5 = X1 + X2;
|
|
|
|
|
|
int32_t B6 = B5 - 4000;
|
|
|
X1 = (calib->B2 * ((B6 * B6) >> 12)) >> 11;
|
|
|
X2 = (calib->AC2 * B6) >> 11;
|
|
|
int32_t X3 = X1 + X2;
|
|
|
int32_t B3 = (((calib->AC1 * 4 + X3) << oss) + 2) >> 2;
|
|
|
|
|
|
X1 = (calib->AC3 * B6) >> 13;
|
|
|
X2 = (calib->B1 * ((B6 * B6) >> 12)) >> 16;
|
|
|
X3 = ((X1 + X2) + 2) >> 2;
|
|
|
|
|
|
uint32_t B4 = (calib->AC4 * (uint32_t)(X3 + 32768)) >> 15;
|
|
|
uint32_t B7 = ((uint32_t)UP - B3) * (50000 >> oss);
|
|
|
|
|
|
int32_t p;
|
|
|
if (B7 < 0x80000000) {
|
|
|
p = (B7 << 1) / B4;
|
|
|
} else {
|
|
|
p = (B7 / B4) << 1;
|
|
|
}
|
|
|
|
|
|
X1 = (p >> 8) * (p >> 8);
|
|
|
X1 = (X1 * 3038) >> 16;
|
|
|
X2 = (-7357 * p) >> 16;
|
|
|
|
|
|
p = p + ((X1 + X2 + 3791) >> 4);
|
|
|
|
|
|
*pressure = p / 100.0; // Pa a hPa (mbar)
|
|
|
return 0;
|
|
|
}
|
|
|
```
|
|
|
##### explanation
|
|
|
###### Headers and defines
|
|
|
```c
|
|
|
#include "bmp180.h"
|
|
|
#include <unistd.h>
|
|
|
#include <fcntl.h>
|
|
|
#include <sys/ioctl.h>
|
|
|
#include <linux/i2c-dev.h>
|
|
|
#include <stdio.h>
|
|
|
|
|
|
#define BMP180_ADDR 0x77
|
|
|
```
|
|
|
- Includes standard and I2C Linux headers.
|
|
|
- Defines the I2C address of the BMP180 sensor.
|
|
|
###### read16() – Read 16-bit (2-byte) value from a register
|
|
|
```c
|
|
|
static int read16(int fd, uint8_t reg, int16_t *value)
|
|
|
```
|
|
|
- Sends the register address via write().
|
|
|
- Reads 2 bytes and stores the result as a big-endian 16-bit signed integer.
|
|
|
- Used to read calibration values and sensor data.
|
|
|
###### write8() – Write 8-bit value to a register
|
|
|
```c
|
|
|
static int write8(int fd, uint8_t reg, uint8_t value)
|
|
|
```
|
|
|
- Sends 2 bytes: register address and value.
|
|
|
- Used to trigger temperature or pressure measurements by writing command codes to control register 0xF4
|
|
|
###### bmp180\_init() – Read all calibration data from sensor
|
|
|
```c
|
|
|
int bmp180_init(int fd, bmp180_calib_data_t *calib)
|
|
|
```
|
|
|
- Reads 11 calibration values from registers 0xAA to 0xBF.
|
|
|
- These are factory-set and unique to each sensor.
|
|
|
- Needed to apply temperature and pressure compensation formulas correctly.
|
|
|
###### bmp180\_read\_raw\_temperature() – Trigger and read unprocessed temperature
|
|
|
```c
|
|
|
static int bmp180_read_raw_temperature(int fd, int32_t *raw_temp)
|
|
|
```
|
|
|
- Writes 0x2E to control register 0xF4 to start temperature conversion.
|
|
|
- Waits 4.5 ms (conversion time).
|
|
|
- Reads result from 0xF6 and 0xF7.
|
|
|
###### bmp180\_read\_raw\_pressure() – Trigger and read unprocessed pressure
|
|
|
```c
|
|
|
static int bmp180_read_raw_pressure(int fd, int32_t *raw_press, int oss)
|
|
|
```
|
|
|
- Writes 0x34 + (oss << 6) to register 0xF4 to start pressure conversion.
|
|
|
- Waits 25 ms for conversion (OSS=0).
|
|
|
- Reads 3 bytes from 0xF6, 0xF7, 0xF8 and shifts them to get a 19-bit value.
|
|
|
- oss (oversampling setting) affects resolution and delay.
|
|
|
###### bmp180\_read\_temperature() – Convert raw temperature to °C
|
|
|
```c
|
|
|
int bmp180_read_temperature(int fd, bmp180_calib_data_t *calib, double *temperature)
|
|
|
```
|
|
|
- Uses raw temperature and calibration data.
|
|
|
- Follows Bosch’s datasheet compensation formula:
|
|
|
X1 = ((UT - AC6) * AC5) >> 15
|
|
|
X2 = (MC << 11) / (X1 + MD)
|
|
|
B5 = X1 + X2
|
|
|
T = (B5 + 8) >> 4 → gives temperature in 0.1°C
|
|
|
- Final result is divided by 10.0 to get °C.
|
|
|
###### bmp180\_read\_pressure() – Convert raw pressure to hPa
|
|
|
```c
|
|
|
int bmp180_read_pressure(int fd, bmp180_calib_data_t *calib, double *pressure)
|
|
|
```
|
|
|
- Uses raw pressure and temperature (needed for B5).
|
|
|
- Applies long compensation formula from datasheet:
|
|
|
1. Many intermediate steps using calibration values.
|
|
|
2. Computes B3, B4, B6, B7, X1, X2, X3, and finally p.
|
|
|
- Final pressure p is in Pa; divide by 100.0 to get hPa (mbar).
|
|
|
#### bmp180.h
|
|
|
```c
|
|
|
#ifndef BMP180_H
|
|
|
#define BMP180_H
|
|
|
|
|
|
#include <stdint.h>
|
|
|
|
|
|
typedef struct {
|
|
|
int16_t AC1, AC2, AC3;
|
|
|
uint16_t AC4, AC5, AC6;
|
|
|
int16_t B1, B2;
|
|
|
int16_t MB, MC, MD;
|
|
|
} bmp180_calib_data_t;
|
|
|
|
|
|
int bmp180_init(int fd, bmp180_calib_data_t *calib);
|
|
|
int bmp180_read_temperature(int fd, bmp180_calib_data_t *calib, double *temperature);
|
|
|
int bmp180_read_pressure(int fd, bmp180_calib_data_t *calib, double *pressure);
|
|
|
|
|
|
#endif
|
|
|
```
|
|
|
##### Explanation
|
|
|
###### Header
|
|
|
```h
|
|
|
#ifndef BMP180_H
|
|
|
#define BMP180_H
|
|
|
|
|
|
#include <stdint.h>
|
|
|
```
|
|
|
- Header guard to prevent multiple inclusion.
|
|
|
- Includes standard integer types (uint8_t, int16_t, etc.).
|
|
|
###### Structure: bmp180\_calib\_data\_t
|
|
|
```h
|
|
|
typedef struct {
|
|
|
int16_t AC1, AC2, AC3;
|
|
|
uint16_t AC4, AC5, AC6;
|
|
|
int16_t B1, B2;
|
|
|
int16_t MB, MC, MD;
|
|
|
} bmp180_calib_data_t;
|
|
|
```
|
|
|
- Holds all calibration constants (11 total).
|
|
|
- Types match datasheet: some are signed, others unsigned.
|
|
|
###### Function declarations
|
|
|
```h
|
|
|
int bmp180_init(int fd, bmp180_calib_data_t *calib);
|
|
|
int bmp180_read_temperature(int fd, bmp180_calib_data_t *calib, double *temperature);
|
|
|
int bmp180_read_pressure(int fd, bmp180_calib_data_t *calib, double *pressure);
|
|
|
```
|
|
|
#### main.c
|
|
|
```c
|
|
|
#include <stdio.h>
|
|
|
#include <fcntl.h>
|
|
|
#include <unistd.h>
|
|
|
#include <linux/i2c-dev.h>
|
|
|
#include <sys/ioctl.h>
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#include "bmp180.h"
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#define I2C_BUS "/dev/i2c-2"
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#define BMP180_ADDR 0x77
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int main() {
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int fd = open(I2C_BUS, O_RDWR);
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if (fd < 0) {
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perror("Error abriendo el bus I2C");
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return 1;
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}
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if (ioctl(fd, I2C_SLAVE, BMP180_ADDR) < 0) {
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perror("Error configurando la dirección I2C");
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close(fd);
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return 1;
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}
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bmp180_calib_data_t calib;
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if (bmp180_init(fd, &calib) < 0) {
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fprintf(stderr, "Error leyendo datos de calibración BMP180\n");
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close(fd);
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return 1;
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}
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double temperature, pressure;
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if (bmp180_read_temperature(fd, &calib, &temperature) < 0) {
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fprintf(stderr, "Error leyendo temperatura BMP180\n");
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close(fd);
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return 1;
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}
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if (bmp180_read_pressure(fd, &calib, &pressure) < 0) {
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fprintf(stderr, "Error leyendo presión BMP180\n");
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close(fd);
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return 1;
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}
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printf("{\"temperature\": %.2f, \"pressure\": %.2f}\n", temperature, pressure);
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close(fd);
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return 0;
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}
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```
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##### Explanation
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###### Open I2C Bus
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```c
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int fd = open("/dev/i2c-2", O_RDWR);
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```
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- Opens I2C device for read/write.
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###### Set I2C Slave Address
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```c
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ioctl(fd, I2C_SLAVE, BMP180_ADDR);
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```
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- Tells the kernel which I2C device address (0x77) to communicate with.
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###### Initialize BMP180
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```c
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bmp180_init(fd, &calib);
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```
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- Reads calibration constants from the sensor.
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###### Read Temperature and Pressure
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```c
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bmp180_read_temperature(fd, &calib, &temperature);
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bmp180_read_pressure(fd, &calib, &pressure);
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```
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- Uses previously stored calibration values to read and convert real temperature and pressure values.
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###### Output in JSON Format
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```c
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printf("{\"temperature\": %.2f, \"pressure\": %.2f}\n", temperature, pressure);
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```
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- Very useful becouse we are integrating with a frontend (e.g. web UI or logger).
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###### Cleanup
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```c
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close(fd);
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```
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- Closes the I2C device after finishing communication.
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## Results
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Here are some screenshots to prove the functionality of the UI.
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