When your project involves interfacing peripherals with a microcontroller, there is a good chance you’ll encounter either I2C (or I²C—Inter-Integrated Circuit) or SPI (Serial Peripheral Interface). Both protocols define communications between a master and one or more slave devices. While interchangeable in some scenarios, each protocol has its specific advantages and drawbacks, making the choice of which to use is important for an elegant and highly functional device.
SPI vs I2C
SPI and I2C were both conceived in the 1980s—SPI by Motorola and I2C by Philips (now NXP). SPI exists as a de facto standard, while I2C is more formalized. Both are synchronous protocols, appropriate for short distance communications, and they generally operate in the 3.3 or 5V range.
I2C Advantages
Users may prefer the I2C protocol for specific applications for two main reasons.
1. Simplicity. The most apparent difference between I2C and SPI is that I2C works as a 2-wire bus, needing only serial data (SDA) and serial clock (SCK) lines for data transmission and synchronization. SPI, on the other hand, requires four wires to control a single slave: SCK, master out slave in (MOSI), master in slave out (MISO), and slave select (SS).
2. Easy add-ons. When users need more than one slave device, SPI implements an additional SS pin for each one. When an I2C system needs to implement new slave devices, they can simply “clip on” to the existing bus using a 7-bit addressing system to identify each module. This I2C scheme requires a proper address configuration but avoids the burden of extra wiring for each device.
SPI Advantages
While I2C’s wiring simplicity and standardization are significant advantages, SPI has some attractive features as well.
1. Communication. Separate MISO/MOSI data lines mean that it’s capable of full-duplex communication, as opposed to I2C’s half-duplex operation, meaning that data send and receive transmissions must alternate.
2. Speed. I2C originally defined data transfer rates at 100kbps, though we have seen it bump up to 400kbps or even up to 5Mbps in Ultra Fast-mode. SPI, however, does not define a top—or any—communications speed, and can be implemented at speeds of 10 Mbps or more.
SPI’s advantages make it appropriate for applications like reading/writing data to an SD card, or any other application where data transfer speed is essential. Conversely, I2C tends to be strong when sending rather simple control signals to multiple peripherals, such as reading from a real-time clock or adjusting the volume of a remote speaker. Users can also utilize SPI for remote speaker control, though SPI’s more complex wiring requirements may make it a less attractive option.
As a very simplified rule of thumb, you might say that SPI is a better choice for the small number of peripherals that need to transfer a large amount of data. I2C is the best option if you need to control many peripherals, especially if you are transferring a small amount of data to each one. Your application, of course, may be different, and designers should also consider what devices are available that support only one protocol or the other.
