Differences Between I2C and SPI Protocols

In this blog, we are going to look at the differences between I2C and SPI. This is something that is usually asked in interviews as well. So, let’s understand the differences in a logical manner rather than simply memorizing everything.

First, a very obvious difference is in the number of pins. In SPI, we have got four pins which are MOSI, MISO, SCLK, and CS.

In the case of I2C, there are only two pins and they are SDA and SCLK. So, that’s the less number of pins as compared to SPI which means that I2C can be implemented at a low cost as compared to SPI.

SPI has full-duplex communication, which means in SPI the data can be transferred from master to slave and vice versa.

In the case of I2C, there is only one data line thus the communication will always be half duplex.

In the case of SPI, the chip select line allows the master to select the desired slave for communication thus the number of slaves is equal to the number of chip select lines which means more space and more cost.

Now, due to the absence of such a pin in I2C. The I2C master transfers either 7 bits or 10 bits long address over the SDA line to communicate with a particular slave. So, it has got its own addressing scheme due to the address bits. Start and stop bit and acknowledgment bit there is more overhead in the case of I2C for point-to-point communication.

In SPI there is no such overhead also one can transfer as many data bits as required. In I2C, one can send one data byte at a time.

Now let’s consider the acknowledgment bit. The acknowledgment bit is sent after every transaction in I2C which makes it more reliable.

SPI doesn’t have that feature inherently included in it what you gain in the simplified electronics in SPI you lose on the software. So you need to write your own error handling and handshaking code since there is no way of knowing if the slave received the messages sent to it nor is there any error checking in the protocol itself.

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Now, let’s shift our focus to the hardware and understand its implications. First of all, SPI uses a push-pull design for MOSI and MISO pins.

I2C uses an open drain design with a pull-up resistor. Open drain has its advantages but it sets a limit on the maximum achievable speed SPI offers more speed in the range of 10 MHz or more. There is no official limit on the speed in SPI and this speed is due to the push-pull configuration.

The maximum speeds in I2C are in the range of 5 MHz for ultra-fast mode now there are different modes and different modes have different speeds.

Now, what is the implication of this high speed due to this high-speed SPI is also more susceptible to noise thus it is used if the distance is less than 20 centimeters and for distances of around one meter I2C is used.

let’s come back to the open drain configuration due to the open drain configuration I2C supports a multi-master system. SPI-based design can only have one master.

Also, clock stretching is another exclusive feature of I2C. Where the slave can stretch the clock or in other words, it can pull the SCL line low thus it can control the flow of data this is another implication of open drain config. SPI doesn’t have that feature. Now, due to the open drain devices with different supply voltages for example 5 volts, and 3.3 volts can co-exist on the same bus such a thing is not possible in SPI. I2C draws more power as compared to SPI due to the presence of a load resistor in its design.

SPI is a de-facto standard it’s not an official standard thus several variants and customizations can create compatibility problems.

I2C is an official standard thus it provides compatibility. I2C implementations and backward compatibility.

Conclusion

There are a lot of differences between SPI and I2C. So, as a simplified rule,

SPI is a better choice if you want to use a small number of peripherals and transfer a large amount of data or data streams at a high speed with low power consumption.

I2C is the best option if you want to control several peripherals with the intermittent transfer of a few bytes of data or where speed is not much of a concern also I2C is preferred when your design requires more than one master.

Where are SPI and I2C used?

Now, where is I2C usually used it is used in RTC in combination with your obvious controller. You can use it with EEPROMs.

SPI is used for devices that provide data streams for example with ADC or with a data logger. When you have to do continuous data logging for audio, video signals, or software flashing devices thus you should select the option that best fits your system’s requirements.

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