Wired communication uses a physical conductor to transmit signals.

MCU_A  ======= Copper/Fiber =======  MCU_B

The physical medium can take several forms:

• Copper wire
• Twisted pair cables
• Coaxial cables
• Optical fiber

These media define the electrical and physical characteristics of the communication channel, directly influencing speed, noise immunity, and distance.

Characteristics of Wired Communication

Wired systems are widely used because they offer predictable and controlled environments compared to wireless systems.

Advantages:

• High reliability due to stable physical connections
• High data rates, especially with modern interfaces
• Low latency with deterministic timing
• More secure, as physical access is required for interception

Disadvantages:

• Requires physical installation and infrastructure
• Limited mobility and flexibility
• Increased complexity in cable routing and management

These trade-offs make wired communication the preferred choice in systems where performance, determinism, and security are critical.

From this general understanding, wired communication can be further categorized based on how data is physically transmitted: parallel and serial communication.

Parallel vs Serial Communication

The first design decision in wired systems is how bits are transmitted across the medium. This leads to two fundamental approaches.

Parallel Communication

In parallel communication, multiple bits are transmitted simultaneously over multiple data lines.

D0 ───────────────
D1 ───────────────
D2 ───────────────
D3 ───────────────
CLK───────────────

Each line carries one bit, and all bits are synchronized using a shared clock.

Characteristics:

• High throughput due to simultaneous transmission
• Requires precise timing alignment across all lines
• Sensitive to skew (timing differences between lines)

Advantages:

• High speed for short distances

Disadvantages:

• Large number of pins required
• Crosstalk between adjacent lines
• Complex PCB routing and layout constraints

Typical use cases:

• Legacy microprocessor buses
• External memory interfaces (e.g., SDRAM, parallel flash)

Despite its speed advantage, parallel communication does not scale well with distance and complexity. These limitations led to a major industry shift toward serial communication.

Serial Communication

In serial communication, bits are transmitted sequentially over one or a few lines.

Data: 1 0 1 1 0 0 1
Line: ──▁▔▔▁▁▔────

Instead of multiple wires, a single line carries all bits in sequence.

Key benefits:

• Reduced pin count
• Simpler PCB routing
• Lower electromagnetic interference (EMI)
• Better scalability over longer distances

Because of these advantages, modern embedded systems rely almost entirely on serial communication protocols.

This leads us to the practical implementations of serial communication used in real systems.

Serial Communication Protocols in Embedded Systems

Serial communication is implemented through standardized protocols, each designed for specific requirements such as speed, topology, and reliability.

UART (Universal Asynchronous Receiver Transmitter)

UART is one of the simplest and most widely used communication protocols. It operates using asynchronous communication and is typically used for point-to-point connections.

MCU_A TX ───────── RX MCU_B
MCU_A RX ───────── TX MCU_B
GND     ───────── GND

Characteristics:

• Asynchronous (no clock line)
• Requires baud rate agreement between devices
• Typically full-duplex using separate TX and RX lines

Advantages:

• Simple implementation
• Minimal hardware requirements
• Widely supported

Limitations:

• Lower speed compared to synchronous protocols
• Limited scalability (point-to-point only)

Typical use cases:

• Debug consoles
• GPS modules
• Bluetooth modules

UART directly reflects the concepts introduced earlier:

• Asynchronous synchronization
• Full-duplex communication
• Point-to-point topology

To overcome UART’s limitations in speed and scalability, synchronous protocols are used.

SPI (Serial Peripheral Interface)

SPI is a high-speed synchronous protocol designed for short-distance communication between a master and one or more slaves.

        Master
         |
   MOSI ──────────┐
   MISO ──────────┤
   SCLK ──────────┤
   CS   ──────┐   │
               │   │
            +-------+
            | Slave |
            +-------+

Signal lines:

• MOSI (Master Out Slave In)
• MISO (Master In Slave Out)
• SCLK (Clock)
• CS (Chip Select)

Characteristics:

• Synchronous communication
• Full-duplex data transfer
• Master-driven clock

Advantages:

• High speed
• Deterministic timing
• Simple protocol structure

Limitations:

• Requires more pins than some alternatives
• No built-in addressing (requires separate CS lines)

Typical use cases:

• Displays
• ADC/DAC converters
• Flash memory

SPI maps to:

• Synchronous communication
• Full-duplex mode
• Star-like topology

For systems requiring fewer wires and support for multiple devices on the same bus, another protocol is used.

I2C (Inter-Integrated Circuit)

I2C is a two-wire synchronous communication protocol designed for connecting multiple devices on the same bus.

SDA ─────────┬────────┬────────
SCL ─────────┬────────┬────────
             |        |
           Slave1   Slave2

Signal lines:

• SDA (Data line)
• SCL (Clock line)

Characteristics:

• Address-based communication
• Supports multiple masters and slaves
• Half-duplex communication

Advantages:

• Minimal wiring (only two lines)
• Built-in addressing mechanism
• Easy device expansion

Limitations:

• Lower speed compared to SPI
• Shared bus can become a bottleneck
• Requires pull-up resistors

Typical use cases:

• Sensors
• EEPROM
• Real-time clocks (RTC)

I2C represents:

• Synchronous communication
• Half-duplex mode
• Bus topology

For environments requiring higher robustness and fault tolerance, especially in noisy conditions, more advanced protocols are needed.

CAN (Controller Area Network)

CAN is designed for high-reliability communication in harsh environments such as automotive and industrial systems.

MCU1 ----\
MCU2 ----- CAN_H / CAN_L ----- MCU3
MCU4 ----/

Key features:

• Differential signaling (CAN_H and CAN_L)
• Built-in error detection and correction
• Message-based arbitration

Characteristics:

• Multi-node bus system
• High noise immunity
• Deterministic communication

Advantages:

• Robust against electrical noise
• Scalable network design
• Fault-tolerant communication

Limitations:

• More complex protocol stack
• Requires dedicated CAN controller

Typical use cases:

• Automotive ECUs
• Industrial automation systems

CAN combines:

• Synchronous-like coordination
• Half-duplex communication
• Bus topology with arbitration

Connecting Back to Communication Fundamentals

All wired protocols are direct implementations of the core concepts discussed in the communication fundamentals article:

• Synchronization
• UART → Asynchronous
• SPI/I2C/CAN → Synchronous
• Duplex
• UART → Full duplex
• SPI → Full duplex
• I2C/CAN → Half duplex
• Topology
• UART → Point-to-point
• SPI → Star
• I2C/CAN → Bus

This structured understanding allows you to analyze any protocol based on first principles rather than memorization.

Transition to Wireless Communication

Wired communication operates in a controlled and predictable environment where the medium is fixed and stable. However, modern systems increasingly require mobility, scalability, and remote connectivity.

This leads to wireless communication, where:

• The physical medium (copper or fiber) is replaced by air
• Signals are transmitted using electromagnetic waves
• Noise, interference, and security challenges increase significantly

The same foundational concepts still apply:

• Synchronization becomes more complex due to lack of a fixed medium
• Duplex often becomes time-scheduled rather than truly simultaneous
• Topology evolves into dynamic structures such as mesh networks

Understanding wired communication provides the necessary baseline to explore wireless systems, where the same principles must operate under far less predictable conditions.