1. Inter-Integrated Circuit (I2C)

1.1 Overview

I2C is a synchronous, multi-master, multi-slave, packet-switched, single-ended, serial communication protocol. It was developed by Philips Semiconductor (now NXP Semiconductors) in 1982 for connecting low-speed peripherals to processors and microcontrollers in short-distance, intra-board communication.

1.2 Physical Layer

I2C uses two bidirectional open-drain lines: Serial Data Line (SDA) and Serial Clock Line (SCL). These lines are connected to a positive supply voltage via pull-up resistors. When the bus is idle, both lines are pulled high.

Signal	Description
SDA	Serial Data Line, bidirectional
SCL	Serial Clock Line, bidirectional

1.3 Protocol

I2C communication is initiated by a master device, which generates the clock signal and addresses the slave device. Multiple master devices can coexist on the same bus, but only one can be active at a time. The I2C protocol defines the following terms:

• Transmitter: The device sending data to the bus
• Receiver: The device receiving data from the bus
• Master: The device initiating a transfer, generating clock signals, and terminating the transfer
• Slave: The device addressed by the master

1.3.1 Addressing

Each slave device on the I2C bus has a unique 7-bit or 10-bit address. The master initiates communication by sending a START condition followed by the slave address and a read/write bit. If a slave recognizes its address, it responds with an ACK bit.

1.3.2 Data Transfer

Data is transferred in 8-bit bytes, with the most significant bit (MSB) first. Each byte is followed by an ACK/NACK bit from the receiver. The master generates a STOP condition to end the communication.

1.4 Speed Modes

I2C defines several speed modes:

Mode	Max. Bit Rate	Max. Bus Capacitance
Standard-mode (Sm)	100 kbit/s	400 pF
Fast-mode (Fm)	400 kbit/s	400 pF
Fast-mode Plus (Fm+)	1 Mbit/s	550 pF
High-speed mode (Hs-mode)	3.4 Mbit/s	400 pF
Ultra Fast-mode (UFm)	5 Mbit/s	400 pF

1.5 Advantages and Disadvantages

Advantages:
– Simple and easy to implement
– Requires only two wires (SDA and SCL)
– Supports multiple masters and multiple slaves
– Widely supported by many devices

Disadvantages:
– Slower data rates compared to other protocols
– Limited bus capacitance and cable length
– No error checking or correction mechanism
– Requires pull-up resistors, which can consume power

2. Serial Peripheral Interface (SPI)

2.1 Overview

SPI is a synchronous, full-duplex, master-slave, serial communication protocol. It was developed by Motorola in the late 1980s for high-speed, short-distance communication between a microcontroller and peripheral devices.

2.2 Physical Layer

SPI uses four wires:

Signal	Description
MOSI	Master Output, Slave Input
MISO	Master Input, Slave Output
SCLK	Serial Clock (generated by the master)
SS/CS	Slave Select/Chip Select (active low)

2.3 Protocol

SPI communication is always initiated by the master device. The master selects a slave device by pulling its SS/CS line low and generates the clock signal on the SCLK line. Data is transferred in both directions simultaneously on the MOSI and MISO lines.

2.4 Advantages and Disadvantages

Advantages:
– High-speed data transfer (up to tens of Mbit/s)
– Full-duplex communication
– Simple hardware interface

Disadvantages:
– Requires more wires than I2C
– No acknowledgment mechanism
– No standard for addressing slaves
– Limited cable length due to high frequencies

3. Universal Asynchronous Receiver-Transmitter (UART)

3.1 Overview

UART is an asynchronous, full-duplex, point-to-point, serial communication protocol. It is widely used for communication between microcontrollers and peripherals, such as GPS modules, Bluetooth modules, and serial consoles.

3.2 Physical Layer

UART uses two wires:

Signal	Description
TX	Transmit Data
RX	Receive Data

3.3 Protocol

UART communication does not require a separate clock signal. Instead, the transmitter and receiver agree on a baud rate (bits per second) and frame format (start bit, data bits, parity bit, stop bit). Data is sent one byte at a time, with the least significant bit (LSB) first.

3.4 Advantages and Disadvantages

Advantages:
– Simple and widely supported
– Requires only two wires (TX and RX)
– Supports full-duplex communication

Disadvantages:
– Slower data rates compared to SPI
– No standard for baud rate and frame format
– No built-in error checking or correction

4. Controller Area Network (CAN)

4.1 Overview

CAN is a multi-master, message-based, serial communication protocol designed for automotive applications. It was developed by Bosch in the 1980s to reduce wiring complexity and increase reliability in vehicle control systems.

4.2 Physical Layer

CAN uses a twisted pair of wires:

Signal	Description
CAN_H	CAN High
CAN_L	CAN Low

4.3 Protocol

CAN communication is based on message broadcasting. Each message has a unique identifier that defines its priority and content. All nodes on the bus receive every message, but only the nodes interested in a particular message will process it.

4.4 Advantages and Disadvantages

Advantages:
– High reliability and error detection
– Real-time deterministic communication
– Multi-master support
– Broadcast communication

Disadvantages:
– Limited data rate (up to 1 Mbit/s)
– Relatively complex protocol stack
– Requires a CAN controller on each node

5. 1-Wire

5.1 Overview

1-Wire is a half-duplex, master-slave, serial communication protocol developed by Dallas Semiconductor (now Maxim Integrated). It is designed for low-speed, low-cost communication with small devices, such as temperature sensors and memory chips.

5.2 Physical Layer

1-Wire uses a single wire for data and power:

Signal	Description
DQ	Data and Power

5.3 Protocol

1-Wire communication is initiated by the master device, which sends reset pulses and issues commands to the slave devices. Each slave device has a unique 64-bit ROM code that identifies it on the bus.

5.4 Advantages and Disadvantages

Advantages:
– Requires only one wire for data and power
– Low cost and simple implementation
– Unique device addressing

Disadvantages:
– Slow data rate (up to 16.3 kbit/s)
– Limited power delivery to slave devices
– Requires strict timing and pull-up resistor

6. RS-485

6.1 Overview

RS-485 is a differential, multi-drop, half-duplex, serial communication protocol. It is widely used in industrial automation, building automation, and other applications requiring long-distance, noise-immune communication.

6.2 Physical Layer

RS-485 uses a twisted pair of wires:

Signal	Description
A	Non-inverting signal
B	Inverting signal

6.3 Protocol

RS-485 communication is typically implemented using higher-level protocols, such as Modbus or Profibus. These protocols define the message format, addressing, and error checking.

6.4 Advantages and Disadvantages

Advantages:
– Long-distance communication (up to 1200 m)
– High noise immunity due to differential signaling
– Support for multiple devices on the same bus

Disadvantages:
– Requires a separate protocol for data link and application layers
– Half-duplex communication requires direction control
– termination resistors are required at both ends of the bus

FAQ

Q1: Which serial communication protocol is best for my application?

A1: The choice of serial communication protocol depends on your specific requirements, such as data rate, distance, number of devices, and cost. Consider the advantages and disadvantages of each protocol and select the one that best fits your needs.

Q2: Can I use multiple serial communication protocols in the same system?

A2: Yes, it is common to use multiple serial communication protocols in a single system. For example, you can use I2C for on-board communication between a microcontroller and sensors, while using RS-485 for long-distance communication with other devices.

Q3: What is the maximum number of devices that can be connected to an I2C bus?

A3: The maximum number of devices on an I2C bus is limited by the addressing space and the total bus capacitance. With 7-bit addressing, up to 128 devices can be connected, while 10-bit addressing allows for up to 1024 devices. However, the total bus capacitance should not exceed 400 pF for standard and fast modes.

Q4: How can I increase the reliability of serial communication in noisy environments?

A4: To increase reliability in noisy environments, consider using differential signaling protocols like RS-485, proper shielding and grounding techniques, and error detection and correction mechanisms. Additionally, ensure that the data rate and cable length are within the specifications of the chosen protocol.

Q5: Can I mix 3.3V and 5V devices on the same serial bus?

A5: Mixing 3.3V and 5V devices on the same serial bus is possible but requires careful design considerations. Use level shifters or voltage translators to match the voltage levels between devices. Also, ensure that the current-sinking and current-sourcing capabilities of the devices are compatible.

In conclusion, understanding the characteristics and trade-offs of various serial communication protocols is essential for designing efficient and reliable embedded systems. By selecting the appropriate protocol for your application and following best practices for implementation, you can ensure robust data transfer between devices in your system.