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SPI vs I2C: How to Choose the Right Protocols?

spi vs i2c: how to choose the right protocol?

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Ever felt confused about when to use SPI or I2C in your embedded systems project? Do you know the main differences between these two ubiquitous communication protocols? If not, you’re not alone – many engineers struggle to distinguish between SPI and I2C and when each is the right choice.

This article will provide a clear and compelling comparison to help you decide. We’ll explore how each protocol works, break down its key features, analyze the tradeoffs, and discuss the criteria for selection. In the end, you’ll have a comprehensive understanding of SPI and I2C.

Introduction to SPI and I2C

SPI and I2C are ubiquitous communication protocols that serve as the lifeblood of embedded systems and IoT devices. These serial interfaces allow integrated circuits and microcontrollers to talk to peripheral chips, sensors, and actuators within a system. Both protocols were conceived in the 1980s to enable short-distance data transfer on circuit boards.

SPI, created by Motorola, uses four wires for full-duplex communication. This allows simultaneous sending and receiving, supporting faster speeds. I2C, developed by Philips, uses only two wires for half duplex signaling. This simplifies the connections, but means data can only travel in one direction at a time. While slower, I2C enables multiple masters and slaves on one bus.

where is spi/i2c/uart in an arduino?

Today, SPI and I2C pervade modern electronics. Tiny microcontrollers integrate one or both to effortlessly converse with components like memory, converters, touch controllers, and displays. Engineers lean on these robust, mature interfaces to quickly develop functional prototypes and products, from DIY hobby boards to commercial appliances. Without SPI and I2C, many embedded and IoT devices would be far more complex or simply not feasible.

How SPI Works?

This versatile protocol helps systems transfer data rapidly between microcontrollers and peripherals. Let’s explore what makes SPI tick.

SPI and I2c and UART and MEM and Epp All in one PCB

Four-Wire Interface

SPI uses just four wires, keeping things simple. The serial clock line (SCLK) drives timing. The master output, slave input line (MOSI) carries data from master to slave. Its counterpart, master input, slave output (MISO), sends data from slaves back to the master.

Finally, the slave select (SS) line allows the master to activate a specific slave device. This efficient interface allows fast two-way data transfer between chips with minimal connections. In systems with multiple slaves, each gets its own SS line.

Master-Slave Architecture

SPI employs a single-master, multi-slave architecture. The master device oversees all timing and data exchange across the serial bus. It generates the SCLK clock pulses and toggles the SS line to choose which slave to talk to. Slave devices follow the master’s lead, transmitting back over MISO when requested.

This centralized control enables reliable high-speed transfers. Without the bus contention issues of multi-master protocols like I2C, SPI can scream along at MHz transmission rates to exchange data rapidly. This performance makes SPI well-suited for speed-critical applications.

SPI Master and Slave

Efficient Communication Processes and Protocols

To initiate an SPI data transmission, the master pulls the desired slave’s SS line low, activating it. The master then drives clock pulses on SCLK while sending 8-bit data frames over MOSI and receiving bits on MISO simultaneously.

Common SPI modes define details like clock polarity, edge triggering, and bit order. For example, Mode 0 clocks data on the rising edge with the most significant bit first. The master and slave must use compatible settings.
Once all data is exchanged, the master stops pulsing SCLK and deactivates the slave by setting SS high again. This completes the quick and effective SPI transaction.

Some of SPI’s important characteristics include:

  • Full duplex transfers for fast sending and receiving
  • No built-in handshaking – slaves operate independently
  • Support for multiple slave devices
  • High speed clock rates, often over 10 MHz
  • Different polarity and clock options (modes)
  • Simple to implement in hardware and software
  • Wide adapter compatibility and minimal latency

With its lean four-wire interface, speed-optimized architecture, and lightweight protocol, SPI delivers raw performance for demanding applications like video displays, sensors, and SD cards. It moves data rapidly and efficiently.

How I2C Works?

Invented by Philips in the 1980s, I2C (Inter-Integrated Circuit) has become a widely used serial bus thanks to its simplicity and flexibility. With only two bidirectional wires, I2C enables easy communication between multiple integrated circuits and peripherals. Let’s unravel how this savvy protocol operates.


I2C and I²C

Both names, I2C and I²C, are correct and commonly used to refer to the Inter-Integrated Circuit protocol. I2C stands for “Inter-Integrated Circuit,” indicating the intercommunication between integrated circuits. The name I2C is widely recognized and used in documentation, datasheets, and technical literature.

On the other hand, I²C is an alternative representation of the protocol’s name, where the superscript “2” indicates the square of the number 2. This notation is used to emphasize the pronunciation of “I-squared-C,” which is a common way to pronounce the acronym. The use of the superscript “²” is often seen in mathematical or technical contexts to denote square values.

So, both I2C and I²C refer to the same protocol and are used interchangeably. The choice to use one over the other is largely a matter of personal preference or adherence to specific style guidelines.

Two-Wire Interface

I2C requires just two shared lines for communication – serial data (SDA) and serial clock (SCL). Devices connect to these wires via open-drain or open-collector outputs paired with pull-up resistors. The SCL line provides the timing reference, while the SDA carries the data.

This two-wire scheme minimizes connections, great for small systems. However, it means SDA and SCL are shared buses, so data flows in half duplex mode. Still, two wires do it all – a mark of elegant engineering.

I2C Protocol

Multi-Master Multi-Slave Architecture

A key advantage of I2C is its built-in multi-master, multi-slave capability. One or more master devices can control and exchange data with one or more slave chips on the same bus.

Each slave has a fixed 7-bit address, allowing up to 112 nodes. Some slaves have user-configurable extra address bits, expanding the possibilities. Master devices initiate all data transfers by sending the desired slave address and reading/writing data to it. Slave nodes simply send or receive data as requested.

This flexible topology allows diverse system designs with many integrated circuits cooperating efficiently on one bus.

Communication Processes

I2C communication is controlled via specific signaling protocols that all devices follow. First, the master generates a start condition. Then it transmits the 7-bit slave address and a read/write bit. The addressed slave sends an ACK bit to confirm.

Next, 8-bit data segments transfer between master and slave. Each piece is ACKed. When done, the master sends a stop condition, freeing the bus. Clock stretching allows slaves to halt transfers to catch up.

I2C offers many desirable features:

  • Just two bus lines keep wiring simple
  • No need for separate chip select signals like SPI
  • Built-in collision detection and arbitration for multi-master
  • Wide range of affordable components to choose from
  • Addressable slaves enable large networks
  • Error checking via ACK/NACK
  • Bus speeds range from 100 kbps to 5 Mbps
  • Low overhead bus protocol great for control

While slower than SPI, I2C wins on simplicity and flexibility. Its minimalist two-wire approach makes it a favorite for modest speed control applications.

SPI vs I2C - Which is Better?

SPI and I2C are both widely used in embedded systems and IoT devices thanks to their usefulness. But which interface is best for your application? Let’s compare these two serial communication protocols.

Speed and Throughput

If your application requires fast and efficient communication, especially for high-bandwidth data transfer, SPI might be the preferred choice. Its full duplex transfers facilitate rapid communication exceeding 10 Mbps. This throughput makes it ideal for data-intensive jobs.

Conversely, I2C has a maximum standard speed of 400 kbps, or 3.4 Mbps in High Speed mode. While respectable, this is substantially slower than SPI. I2C is better suited for undemanding applications like reading sensors. So for top speed, SPI wins. But I2C is “fast enough” for many uses. Choose wisely based on your speed requirements.

Number of Devices

SPI is better suited for point-to-point communication or communication with a limited number of devices. Each SPI device requires a separate select line, which can make it more suitable for applications with a smaller number of devices.

I2C, on the other hand, supports a bus architecture that allows multiple devices to be connected using shared data and clock lines, making it more scalable for applications with a larger number of devices.

Wiring and Hardware Requirements

SPI typically requires more wires (data, clock, select lines) compared to I2C (data and clock lines). If you have limited wiring options or space constraints, I2C may be more suitable due to its simpler wiring requirements. Additionally, some microcontrollers or integrated circuits may have built-in hardware support for one protocol over the other, which can influence your choice based on the available hardware.

I2C’s two-wire setup significantly reduces wiring complexity compared to SPI’s four-wire interface. However, I2C has extra protocol complexity from its built-in addressing and acknowledgment scheme. SPI has a simpler protocol but needs more wires for separate data lines and slave selects. This complicates PCB routing and interconnects.

Both offer hardware advantages. Two-wire I2C minimizes connections, while four-wire SPI enhances signal isolation. Pick the right bus for your system constraints.

Power Consumption

I2C typically consumes less power compared to SPI, which can be advantageous, especially in low-power or battery-operated applications. I2C’s slower clock and lower pin count reduce power consumption, a key strength. But its pull-up resistors can require more current, mitigating some savings.

SPI’s faster speeds and extra wires draw more current. Yet avoiding pull-up resistors on its output lines helps limit power needs. In practice, usage and implementation details matter more than the protocol choice itself. Well-designed boards in either bus can achieve low power operation.

i2c diy pcb project

Addressing and Compatibility

SPI does not have a standardized addressing scheme. Each device is individually selected using the select line. In contrast, I2C uses 7-bit or 10-bit addressing to identify devices on the bus, allowing for easy device identification and communication.

Consider the compatibility of your devices or components. Some devices may only support one of the protocols, which can impact your decision. Ensure that the devices you intend to use are compatible with the chosen protocol.

Noise Immunity

SPI tends to have better noise immunity due to its single-ended signaling and separate select lines for each device. I2C, with its open-drain signaling, may be more susceptible to noise interference.

I2C’s shared SDA/SCL lines are more susceptible to noise interference compared to SPI’s separate wires. Glitches can disrupt the SDA line for all I2C nodes. SPI offers better noise resilience and isolation thanks to its unshared MOSI/MISO connections. But neither interface is highly resistant to noise without careful PCB layout.

Engineers must take care to meet noise immunity requirements through proper routing and decoupling. If noise is a concern, SPI provides an advantage.


I2C’s 7-bit addressing supports over 100 nodes on one bus, expanding to thousands with 10-bit addressing. This enables large networks with minimal wires. SPI’s chip select pin requirement hampers scaling. Supporting each slave needs a dedicated CS line, complicating board layouts with multiple devices. For uncomplicated sensor and control networks, I2C is hard to beat. But properly designed boards can still achieve good scalability with SPI.

The Final Verdict

SPI and I2C both have merits depending on the application’s requirements. For simplicity and control, I2C is often best. When speed is critical, SPI delivers. Considering tradeoffs in speed, complexity, power, noise immunity, and scalability leads to the right pick.

Many systems blend both interfaces – using SPI for demanding tasks and I2C for peripherals. Get the best of both worlds by matching each protocol’s strengths to the job.


When choosing between SPI and I2C, engineers like us must weigh key factors like speed, complexity, power, noise immunity, and scalability. SPI excels at raw throughput while I2C simplifies wiring. Both serialize data for short-distance communication in embedded systems. Carefully evaluate hardware needs and application requirements when selecting an interface. SPI suits high-speed bulk data transfer needs. I2C fits simpler control-oriented tasks.

With their respective strengths, SPI and I2C will continue serving critical roles in electronics and IoT products. Understanding these widely used protocols helps developers pick the right serial bus for the job.

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Irene Shi
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