The Core Embedded System Protocols
These protocols dictate how microcontrollers, sensors, and computers talk to one another.
1. UART — Universal Asynchronous Receiver-Transmitter
Wiring Topology
UART Wiring Topology
Device A Device B
┌──────────┐ ┌──────────┐
│ TX ├──────────────────────────┤ RX │
│ │ │ │
│ RX ├──────────────────────────┤ TX │
└──────────┘ └──────────┘
(e.g. MCU) point-to-point (e.g. GPS)
2 wires only
NO clock wire
The Data Frame (What One Byte Looks Like on the Wire)
UART Data Frame
Idle START D0 D1 D2 D3 D4 D5 D6 D7 PARITY STOP Idle
───┐ ┌────────
│ ┌───┐ ┌───┐ ┌───┐ ┌───┐ ┌───┐ ┌───┐ ┌───┐ │
└──┘ └──┘ └────────┘ └──┘ └────────┘ └──┘ └──┘ └──┘
│ │ 8 data bits │ │ │
│ │ (e.g. 0x55 = 0101 0101) │ │ │
│ │ │ │ │
│ └─ "Get ready!" error check ┘ └──┘
│ Line goes LOW "Done!"
│ to signal start Line goes HIGH
Both sides MUST pre-agree on baud rate (e.g. `9600`, `115200` bps)
No clock = any mismatch → garbled data ("framing error")
How it Works
UART is one of the oldest and simplest methods. It uses just two wires: a transmit line (TX) and a receive line (RX). To connect two devices, the TX of the first device connects to the RX of the second, and vice versa. It is strictly a point-to-point connection — only two devices can talk to each other at once.
It is asynchronous, meaning there is no shared clock wire. Instead, both devices must agree beforehand on a specific speed, the baud rate (e.g., 9600 bits per second). Because there is no clock to signal when a message starts, UART wraps data in a start bit, then 8 data bits, an optional parity bit, and a stop bit.
Where you will find it: Arduino boards, GPS modules, and basic Bluetooth adapters.
2. SPI — Serial Peripheral Interface
Wiring Topology
SPI Wiring Topology
Master (MCU)
┌─────────────────┐
│ SCLK MOSI MISO│
└───┬──────┬────┬──┘
│ │ │
┌─────────────┼──────┼────┼──────────────────┐
│ SCLK │ MOSI │ MISO │ shared bus
│ │ │ │ │
┌────┴───────┐ ┌───┴──────┴────┴───┐ ┌────────────┴───┐
│ CS ◄──┐ │ │ CS ◄──┐ │ │ CS ◄──┐ │
│ │ │ │ │ │ │ │ │
│ Slave 1│ │ │ Slave 2│ │ │ Slave 3│ │
└────────┘ │ └────────┘ │ └────────┘ │
│ │ │
CS1 ┘ CS2 ┘ CS3 ┘
(unique per slave — "tap on the shoulder")
Full-Duplex Data Flow
SPI Full-Duplex Data Flow
Master ──SCLK──► Slave Clock ticks on every bit
Master ──MOSI──► Slave Master → Slave ("instructions")
◄─MISO── Slave Slave → Master ("response")
↑ both directions simultaneously
= FULL DUPLEX, like a two-way road
CS LOW = "I'm talking to YOU"
CS HIGH = "You're dismissed"
How it Works
Think of SPI as a boss (the “Master”) managing one or more workers (the “Slaves”). It is incredibly fast and allows data to flow in both directions at the exact same time (full-duplex). A shared SCLK clock wire keeps all devices in perfect synchronous time.
Each Slave shares the same MOSI and MISO wires, but gets its own dedicated Chip Select (CS) wire — the Master pulls one CS LOW to address exactly one Slave. Adding more devices means one more CS wire per device.
Where you will find it: SD cards, LCD screens, and high-speed flash memory.
3. I2C — Inter-Integrated Circuit
Wiring Topology (All Devices Share 2 Wires)
I2C Wiring Topology
VCC
│
┌┴┐ ┌┴┐ pull-up resistors
└┬┘ └┬┘ (hold lines HIGH when idle)
│ │
├────┴──────────────────────────────── SDA (data)
│
└───────────────────────────────────── SCL (clock)
│ │ │
┌──┴──────┐ ┌────┴────┐ ┌─────┴────┐
│ Master │ │Slave │ │ Slave │ ... up to 127 devices
│ (MCU) │ │Addr=0x48│ │Addr=0x3C │ on the same 2 wires
└─────────┘ └─────────┘ └──────────┘
(temp sensor) (OLED display)
The “Group Chat” Address Protocol
I2C Address Protocol
Master wants to talk to Slave at address 0x48:
SDA: ─ START ─[ 0x48 ]─[ W ]─[ ACK ]─[ data... ]─[ ACK ]─ STOP ─
│ │ │
│ │ └─ Slave 0x48 pulls SDA low:
│ │ "I'm here! Go ahead."
│ └─ Write (0) or Read (1) flag
└─ 7-bit address broadcast to ALL devices
Only 0x48 responds — rest stay silent
Half-duplex: only ONE direction at a time (like a walkie-talkie)
Open-drain lines: devices can only PULL LOW, never push HIGH
Pull-up resistors: naturally float back to HIGH when released
How it Works
I2C is the smart middle ground for connecting many parts without creating a wire mess. Every Slave device has a unique 7-bit address. The Master broadcasts an address on the shared SDA line — only the matching device responds with an ACK bit. It is half-duplex (one direction at a time), and devices use open-drain outputs with pull-up resistors to prevent short circuits.
Where you will find it: Temperature sensors, OLED displays, and gyroscopes.
4. CAN — Controller Area Network
Wiring Topology (Two-Wire Differential Bus)
CAN Wiring Topology
Node 1 Node 2 Node 3 Node 4
(Engine) (Brakes) (Airbag) (Dashboard)
┌───────┐ ┌───────┐ ┌───────┐ ┌───────┐
│ CAN │ │ CAN │ │ CAN │ │ CAN │
│ ctrl │ │ ctrl │ │ ctrl │ │ ctrl │
└───┬───┘ └───┬───┘ └───┬───┘ └───┬───┘
│ │ │ │
──────┴─────────────┴─────────────┴─────────────┴────── CAN High
──────┬─────────────┬─────────────┬─────────────┬────── CAN Low
[R] [R]
120Ω terminator 120Ω terminator
(prevents signal reflections)
Differential Signalling — Noise Cancellation
CAN Differential Signalling
CAN High: ─────┐ ┌────┐ ┌─────
└────┘ └────┘
CAN Low: ──┐ ┌────┐ ┌──────
└────┘ └────┘
Receiver reads: CAN_High − CAN_Low = clean signal
Noise spike hits BOTH wires equally → cancels out → data intact
Arbitration — No Collision, No Data Loss
CAN Arbitration
Node A (ID=0x100) and Node B (ID=0x050) transmit simultaneously:
Bit position: 10 9 8 7 6 5 4 3 2 1 0
Node A sends: 1 0 0 0 0 0 0 0 0 0 0 (0x100)
Node B sends: 0 0 0 0 0 0 0 0 0 0 0 (0x050)
↑
First bit differs here
Node A reads the bus and sees a 0 (not its 1)
Node A immediately STOPS transmitting and waits
Node B wins — lower ID = higher priority
No data corrupted. No retransmission needed.
How it Works
Invented by Bosch in 1983 to reduce copper wiring in cars. Instead of addressing devices, CAN addresses messages — each message has an ID, and every node decides whether it cares. Nodes share a differential pair (CAN High / CAN Low) which cancels electrical noise. If two nodes transmit simultaneously, the one with the lower ID wins arbitration bit-by-bit without corrupting data.
Where you will find it: Every modern car (OBD-II port), trucks, industrial robots, and medical equipment.
5. RS-232
Voltage Levels — Why It Survives Noise
RS-232 Voltage Levels
UART / TTL Logic RS-232 (the tough older brother)
───────────────── ────────────────────────────────
Logic 1 = 3.3V Logic 1 = +3V to +15V (typically +12V)
Logic 0 = 0V Logic 0 = −3V to −15V (typically −12V)
Noise immunity: Noise immunity:
┌──── 3.3V (logic 1) ┌──── +15V (logic 1)
│ │
│ Any noise > 1.65V │ Noise must be > 15V swing to flip a bit
│ can flip the bit ←BAD │ Virtually impossible in practice ←GOOD
│ │
└──── 0V (logic 0) └──── −15V (logic 0)
↑
27V total swing vs 3.3V = 8× tougher
The MAX232 Translator
The MAX232 Translator
MCU (3.3V/5V TTL) MAX232 chip RS-232 cable
┌────────────────┐ ┌──────────┐ ┌──────────────┐
│ │ │ │ │ │
│ TX (0–3.3V) ─┼──────►│ step up ├────────►│ TX (+12/−12V)│
│ │ │ │ │ │
│ RX (0–3.3V) ◄┼───────┤ step down│◄────────┤ RX (+12/−12V)│
│ │ │ │ │ │
└────────────────┘ └──────────┘ └──────────────┘
"translator"
NEVER connect MCU directly to RS-232 — it will fry!
How it Works
Think of RS-232 as UART's tough older brother. It uses the same asynchronous TX/RX communication style, but survives harsh environments over long cables (up to 50 feet). Modern MCUs whisper at 0–3.3V. RS-232 shouts at ±15V. A translator chip (MAX232) always sits between the MCU and the cable to safely convert voltages.
Where you will find it: Industrial machinery, CNC routers, factory automation, and legacy point-of-sale systems. USB replaced it for consumers, but it refuses to die in professional settings.
6. 1-Wire
Wiring Topology — Power AND Data on One Wire
1-Wire Wiring Topology
Master (MCU)
┌──────────┐
│ │ Single data wire
│ GPIO ───┼────────────────────────────────────────────────
│ │ │ │ │ │
└──────────┘ ┌┴┐ ┌┴┐ ┌┴┐ ┌┴┐
└┬┘ └┬┘ └┬┘ └┬┘
│ 4.7kΩ │ │ │
pull-up GND GND GND
to VCC
┌──────┐ ┌──────┐ ┌──────┐ ┌──────┐
│DS18B20│ │DS18B20│ │DS18B20│ │DS18B20│
│ ID: │ │ ID: │ │ ID: │ │ ID: │
│ A3F2..│ │ 9B1C..│ │ E72A..│ │ 0048..│
└───────┘ └───────┘ └───────┘ └───────┘
(all share the exact same single wire)
Parasitic Power — Energy Sipping
1-Wire Parasitic Power
Normal (idle):
Wire ──── HIGH (VCC via pull-up) ───────────────────────────
↓ Slave capacitor charges up
[C] ← stored energy
Transmitting (wire pulled LOW briefly):
Wire ──┐ LOW ┌─ HIGH ──────────────────────────────────
└─────────┘
│ Slave survives on [C] stored charge during LOW pulse
│ Master reads the pulse duration to decode bit value
64-bit ROM ID burned at factory — mathematically unique, forever:
┌────────┬───────────────────────────────────┬────────┐
│ 8-bit │ 48-bit serial number │ 8-bit │
│ family │ (unique per device, ever) │ CRC │
└────────┴───────────────────────────────────┴────────┘
How it Works
1-Wire uses a single data wire for both power (parasitic power) and communication. Slave devices charge a tiny internal capacitor from the idle HIGH line, then survive on that stored energy while transmitting. Every device has a permanent 64-bit ID burned at the factory — no address conflicts possible. The Master can search the bus to discover every connected device.
Where you will find it: DS18B20 temperature sensors, laptop battery gauges, and iButton security keys. Unbeatable for running one cable to dozens of sensors across a large building.
Protocol Comparison
Protocol Comparison Table
Protocol Wires Devices Speed Clock? Duplex Best For
──────── ───── ─────── ──────────── ─────── ──────── ─────────────────────
UART 2 2 115200 bps None Full Simple point-to-point
SPI 4+ N* 50+ MHz Yes Full High-speed data (SD/LCD)
I2C 2 127 400 kHz Yes Half Many sensors, few pins
CAN 2 127 1 Mbps None Half Noisy environments, cars
RS-232 2 2 115200 bps None Full Long cables, industrial
1-Wire 1 N 15 kbps None Half Single cable, many sensors
* SPI: requires 1 extra CS wire per additional deviceThis table provides a quick comparison of common hardware communication protocols used in embedded systems and industrial applications.
| Protocol | Wires | Devices | Speed | Clock? | Duplex | Best For |
|---|---|---|---|---|---|---|
| UART | 2 | 2 | 115200 bps | None | Full | Simple point-to-point |
| SPI | 4+ | N* | 50+ MHz | Yes | Full | High-speed data (SD/LCD) |
| I2C | 2 | 127 | 400 kHz | Yes | Half | Many sensors, few pins |
| CAN | 2 | 127 | 1 Mbps | None | Half | Noisy environments, cars |
| RS-232 | 2 | 2 | 115200 bps | None | Full | Long cables, industrial |
| 1-Wire | 1 | N | 15 kbps | None | Half | Single cable, many sensors |
Note: SPI requires 1 extra Chip Select (CS) wire per additional device added to the bus.