Are you working on an electronics project with a Raspberry Pi or a microcontroller and peripherals like sensors or LCDs? Then you will need to establish a reliable connection between your microcontroller and the sensors or other peripherals.
I2C (Inter-Integrated Circuit) and SPI (Serial Peripheral Interface) are two of the most common communication protocols in electronics projects. But what are their differences, and how do you decide which one is best for your project?
Keep reading to discover more about I2C vs. SPI!
Why do we Need Communication Protocols Like I2C and SPI?
As already mentioned, we need communication protocols for establishing the communication between a computer or microcontroller and peripherals like sensors or LCDs. More precisely, we need a shared and structured language to ensure data is transmitted in the right way.
Let’s say you are building a weather station using a Raspberry Pi. Every weather station includes some kind of sensors like temperature and humidity sensors, air quality sensors, wind speed sensors, etc.
Of course, we don’t want to connect 8 separate cables to each sensor to transmit 8 bits of data. That would be a whole mess.
Therefore, we must establish a protocol that uses fewer cables while still being able to receive the data from all our sensors.
For instance, using I2C we only need two cables to connect all our sensors to the Raspberry Pi.
How does I2C Work?
I2C stands for Inter-Integrated Circuit and is pronounced as “I squared C”.
This protocol allows for simple communication between multiple devices using only two wires:
SDA (Serial Data): This connection is used to send data between master and slave
SCL (Serial Clock): This connection is used to synchronize communication.
Using I2C we can have multiple slaves as well as multiple masters on a single bus. This can be handy if you want to use multiple microcontrollers.
Each slave has a unique address, which allows the master to communicate with one specific device. I2C can handle addresses that are 7 Bits or 10 Bits long. With 7-bit addressing, we can have 128 devices on one bus, and with 10-bit addressing, we can even have 1024 devices.
All the data is sent via the SDA (Serial Data) line, while the SCL (Serial Clock) line synchronizes the timing of data transmission, ensuring that all devices on the bus operate in sync.
If no data is being transmitted (idle), the SDA and SCL lines are pulled high (on a 3.3V system high means ~3.3V). When data is being transmitted, however, the SCL line alternates between high and low with a frequency of 100kHz in standard mode and up to 3.4Mhz in high-speed mode.
In the following part, we are also going to look at what the SDA line does while data is transmitted.
The I2C Message Frame
Let’s have a look at what the communication using I2C looks like.
Above, you can see how an I2C message frame is structured. This structure is standardized so all I2C devices follow the same message style.
Every message begins with the start condition. The start condition is initiated by the master by pulling the SDA line low (~0V) while the SCL line is still high.
Now, the master specifies which slave he wants to address by sending a 7-bit or 10-bit address via the SDA line. A high signal indicates a binary 1 and a low signal indicates a binary 0. Also, note that I2C sends data with the MSB (Most Significant Bit) first.
After transmitting the address, the master indicates whether it wants to read data from the slave or send data to it using the R/W bit. If the master wants to send data to the slave, it pulls the SDA line low.
Next, the slave with the specified address pulls the SDA line low to send an ACK signal. This means that it is ready to receive or send data. If it is not ready or the address does not exist on the BUS, the SDA line remains high, signaling a NACK (NoACK).
After the slave responds with an ACK, the actual data transfer begins. Depending on the R/W bit, the data is transmitted either by the master or the slave. The data is divided into 8-bit frames. After each frame, the receiving device pulls the SDA line low for one bit to send an ACK signal, indicating a successful transfer.
If all the information has been transferred successfully, the sending device (master or slave) ends the message with a stop condition. This condition is met when the SDA line transitions from low to high while the SCL line remains high.
How does SPI Work?
SPI (Serial Peripheral Interface) is a communication protocol, similar to I2C, designed for high-speed data transfer between a master device, such as a microcontroller, and one or more peripheral slave devices.
SPI mainly uses four connections for communication:
MOSI (Master Out, Slave In): Data transfer from the master to the slave
MISO (Master In, Slave Out): Data transfer from the slave to the master
SCK (Serial Clock): Used to synchronize communication
SS/CS (Slave Select/Chip Select): Select the target device.
Because there are two separate connections for sending and receiving, SPI supports full-duplex communication, meaning that data can be sent and received simultaneously.
Some slaves, like 7-segment-displays for instance, don’t send any data, they just need to receive data from the master. Hence, they may have no MISO connection.
Additionally, if you want to connect multiple slaves, you’ll need a separate chip select (SS/CS) pin for each slave. The number of slaves on one BUS only depends on how many SS/CS pins there are.
The SS/CS pin is active low, meaning that when a slave is not selected, the SS/CS line is high (on a 3.3V system, the SS/CS pin is at ~3.3V).
To select a slave, the master pulls its SS/CS pin low (~0V), while leaving the other SS/CS pins high. This tells the selected slave that it should listen for SPI data or send SPI data on the MOSI/MISO pins.
Compared to I2C, SPI does not have a standardized communication structure for the MISO and MOSI pins. Each device can have its own protocol for how data is formatted and exchanged. As a result, the master device needs to handle the data structure for each device separately. This information is typically detailed in the device’s documentation.
I2C vs. SPI: The Key Differences
Here’s a brief side-by-side comparison of I2C and SPI to highlight their differences:
Feature
I2C
SPI
I2C or SPI? Deciding the Right Protocol for Your Project
Choosing between I2C and SPI depends on your project’s specific requirements, such as communication speed, the number of devices, and the complexity of the wiring.
Amount of Peripherals and Wiring Complexity
If your project uses many peripherals like sensors or even other masters, I2C is probably the best choice, as it only uses two pins on each device. This reduces wiring complexity by a lot compared to SPI where every slave needs a separate SS/CS pin.
Protocol Complexity and Data Structure
Also, I2C’s standardized data transfer protocol makes communication between multiple devices a bit easier, compared to SPI where probably every device uses a different data structure.
Transfer Speed
On the other hand, if speed matters to you a lot, SPI is better suited because it supports full-duplex data transfer, meaning that data can be sent and received at the same time. For example, the speed is relevant if you’re dealing with displays or memory cards.
Interference
Additionally, SPI is better suited than I2C in an environment with heavy interference. That’s because SPI doesn’t rely on only one data line and start/stop conditions where exact timing matters a lot.
Of course, it doesn’t necessarily have to be “I2C vs. SPI”. Instead, you can use both protocols for different parts of your project. For example, you could use I2C to gather data from different sensors and then use SPI to save the data to a storage device.
Conclusion
I2C and SPI are both very efficient methods of transmitting data between a master and one or multiple slaves.
The question “I2C vs. SPI” can be answered by weighing the pros and cons depending on the needs of your project.
I2C’s standardized protocol and scalability make it a great choice for projects with many peripheral devices, while SPI’s speed and reliability are most useful when speed and reliability matter the most.
Whether you’re connecting sensors, displays, or memory cards, you won’t get around these two popular communication protocols.
If this post inspired you to try out I2C and SPI, you can have a look at some project ideas using the Raspberry Pi (Pico): https://coolplaydev.com/raspberry-pi-projects
Thanks for Reading!