---
title: Communication buses
lang: en
type: notion
tags:
  - eee
  - notion
prerequis:
  - microcontroleur-en
aa:
  - RA-EEE-C03-2/EEE/4
phases:
  - concept
  - dossier-technique
draft: false
source_fr: embarque/mcu/bus/bus-de-communication.md
source_sha256: 830c23b601213b546670fe89ae9ba3b0805fbc9b6c4858ac58a5887788c382f7
---

A **communication bus** is a set of lines and rules that lets two digital components exchange data — typically a [[microcontroleur-en|microcontroller]] and its sensors, memories or displays. This page is the **entry hub** to the three protocols most common on a project: [[uart-en|UART]], [[i2c-en|I2C]] and [[spi-en|SPI]]. It places them relative to one another and gives the method for choosing, without detailing each one (that is what the child pages are for).

![Three bus topologies. UART: a point-to-point link on two crossed wires (TX to RX). I2C: two shared wires (SDA, SCL) linking a controller, historically called the master, to several addressed peripherals, with pull-up resistors. SPI: three shared wires (MOSI, MISO, SCK) plus one CS select line per peripheral.](/ressources/img/bus-de-communication/topologies.svg)

## What is it for?

A microcontroller rarely works alone: it reads sensors, writes to a display, stores on a memory card, and sometimes talks to another microcontroller. Rather than a dedicated wire per piece of information, a bus **standardises** the way data travels over a few wires, which saves pins and makes components interchangeable.

Choosing a bus means weighing up several parameters:

- the **number of wires** used, and so of pins taken up on the microcontroller;
- the **data rate** you can reach;
- the **distance** over which the link stays reliable;
- the **number of components** you can hang on the same wires;
- the **complexity** of putting it to work, in wiring and in code.

This choice is made during the [[concept-en|concept]] phase (when the architecture is settled) and frozen in the [[dossier-technique-en|technical design file]] (at wiring and layout time).

## How do you choose a bus?

Four distinctions shape the landscape. A bus is **serial** (the bits go one after another, over few wires, the case in embedded work) or parallel. It is **synchronous** when a **clock** line paces the exchanges, **asynchronous** when both sides have to agree on a data rate in advance. It is **point-to-point** (two components only) or **multi-node** (several components on the same wires). Finally, it exchanges in one direction at a time, or **full-duplex** (both directions at once).

The three common protocols line up like this:

| Protocol | Wires | Clock | Components | Data rate | Distance | Typical use |
| --- | --- | --- | --- | --- | --- | --- |
| [[uart-en\|UART]] | 2 (TX/RX) | no (asynchronous) | 2 (point-to-point) | moderate | a few metres | debug console, GPS, serial modules |
| [[i2c-en\|I2C]] | 2 (SDA/SCL) | yes (synchronous) | several (addresses) | moderate | on the board (< 1 m) | sensors, displays, real-time clock |
| [[spi-en\|SPI]] | 4 (MOSI/MISO/SCK/CS) | yes (synchronous) | several (1 CS per peripheral) | high | very short | fast display, SD card, converter |

Three reflexes for settling it. For a **simple link** to a single peripheral, a commissioning console or a serial module, UART is enough. To hang **several sensors** without multiplying wires, I2C gathers them on two lines thanks to addressing. For a **fast** peripheral (graphic display, SD card, converter), SPI gives you the data rate, at the price of one select wire per component added.

On any of these buses, the [[niveaux-de-tension-en|logic level]] compatibility (3.3 V / 5 V) between components still has to be checked before wiring. And to *see* what really travels on the wires — and decode the exchanges frame by frame — the dedicated instrument is the [[analyseur-logique-en|logic analyser]].

Beyond these three protocols, the **fieldbuses** — RS-485 (a rugged, multi-node serial link over tens of metres) and above all **CAN** (the bus of the automotive and industrial worlds, everywhere in robotics) — carry the same principles over distances and reliability demands of another order.

## See also

- [[uart-en|UART]] — asynchronous link, 2 wires, point-to-point
- [[i2c-en|I2C]] — synchronous bus, 2 wires, several components by address
- [[spi-en|SPI]] — fast synchronous bus, 4 wires, selection by CS
- [[microcontroleur-en|Microcontroller]] — the bus links the microcontroller to its peripherals
- [[niveaux-de-tension-en|Logic levels]] — level compatibility comes up on every bus
- [[analyseur-logique-en|Logic analyser]] — seeing and decoding what travels on a bus