SPI vs. UART: Which Protocol Should You Use?


The SPI and UART are used in everything from simple microcontrollers to complex industrial equipment. If you’re looking to connect two devices, you’ll need to choose one or the other.

We’ll compare SPI and UART in this article to help you determine which protocol is appropriate for your project.



What is an SPI (Serial Peripheral Interface)?

It stands for Serial Peripheral Interface. This is a communication protocol that allows two devices to communicate with each other.

Commonly used for communication between a microcontroller and a memory chip. It’s also commonly used to connect peripheral devices such as sensors or displays.

Because SPI is a serial protocol, it uses only one wire (or line) to transmit data. This means that the data sent over the wire is only being sent one bit at a time. In most cases, this serial transmission happens very quickly.

It’s also versatile enough that it can be used in a variety of different applications—for example, you might use SPI to connect a flash drive to your computer or perhaps connect an LCD display to your Arduino board.

SPI Wires

SPI Wires

SPI Wires

This protocol allows two devices to send information to each other. The SPI protocol consists of four wires:


MISO stands for master in slave out. This is a way to communicate between two SPI devices.

The MISO wire is a bi-directional signal line. Provides data exchange between the master and slave devices in a synchronous communication system. This wire is used to transmit data from the master device to the slave device, and it allows the reception of data by the master device from the slave device.


This controls whether or not the device being communicated with is currently selected. The SS/CS line is one of these shared lines. It can be used to select which device on the bus you want to communicate with.

The SS pin is normally pulled high, while the CS pin is pulled low. In the case of SPI, these signals are open-drain. They must be pulled up with resistors in order to be able to drive them low. For example, if you have a 4-bit chip select that you want to use on an 8-bit bus, you can combine all 4 of those chip selects into one 8-bit chip select by tying them together and pulling them low with a single pull-up resistor.


The serial clock line, also known as the shift clock, is used to synchronize the data transfer between the microcontroller and SPI device. It is a continuous clock signal that determines the speed at which data is shifted in and out of the device.

This clock signal is used to synchronize all other signals in an SPI bus. It is usually connected to a dedicated pin on the master device. The frequency of this clock signal must be greater than or equal to 1 MHz.


The MOSI signal lets the microcontroller in a computer know that it should send data to the SPI peripheral. This will then read it and send it on to its own peripherals.

In the world of data transmission, a serial interface means that one bit of information is transmitted at a time, in sequential order. It transfers and takes data from the master device and sends it over to the slave device.



What is a UART (Universal Asynchronous Receiver Transmitter)?

A standard way of sending data between two devices. It’s used in many different types of electronic devices, from consumer devices to industrial equipment.

It’s called “universal” because it can be used to communicate with many different types of devices and protocols, and it’s called “asynchronous” because it doesn’t require the sender and receiver to be synchronized with each other—that is, it doesn’t require them to send bits of data at regular intervals.

The UART takes input from a computer or other device and sends it out to another device using the same protocol. This means that if you have a computer with a serial port, it is possible to connect it to another computer or device using this port and send data between them over wires or wirelessly as long as both devices support the same protocol.

SPI and UART Similarities

Both are used to communicate between microcontrollers and peripheral devices. Both protocols are based on the same principles:

Serial Communication

Serial Communication

Duplex Serial Communication

These protocols both use duplex serial communication. This means that they can send and receive data at the same time, which makes them ideal for devices that need to be able to talk to each other while doing other things at the same time—like a laptop talking to its printer.

This allows both SPI and UART protocols to operate at higher speeds than other serial interfaces, since they don’t have to wait for a response before sending another signal.

Short-Distance Communication

Both SPI and UART are used for short-distance communication. This means that they transmit data over a few feet, rather than over long distances like Ethernet. They’re both used in microcontrollers, which are tiny computers that perform tasks like controlling motors or sensors.

Data Processing

Both SPI and UART have a similar concept of data processing. Both are receiver-transmitter devices that use a serial communication interface to send and receive data. In SPI, the receiver is connected to the microprocessor, while in UART it is connected to a peripheral device. The microprocessor can send bytes of data through the SPI bus or UART line at high speeds.

SPI and UART Differences

They’re similar in the way that they enable data to be sent between devices, but there are some key differences between them.


SPI uses four data wires to transfer data at high speeds, while UART only uses two, that’s why SPI protocols can transfer data to and from a device much faster compared to UART protocols.

Data Rates

SPI sends and receives data at a high-speed of up to 100 MHz, while UART sends and receives data at a much lower speed of up to 20 KBps. This means that UART can be used for low-speed applications such as sending text messages, while SPI is more appropriate for high-speed applications such as controlling motors or other hardware devices.

Wire Interface

Wire Interface

Wire Interface

SPI is a master/slave communication protocol that operates in full duplex mode. The master provides information to the slave, who processes it and returns a response. With SPI, it’s possible to have multiple slaves connected on the same bus, allowing them to all communicate with one another simultaneously.

UART is a simplified serial protocol that just requires one pair of wires for sending and receiving data. It’s possible to have multiple devices connected on the same bus using UART, but there’s no way for them all to talk at once like you can with SPI.

Data Transmission

Typically used for high-speed data transmission between two devices, such as a microcontroller and another chip or peripheral. The microcontroller acts as the master device, controlling the data flow between itself and the slave device. SPI can transmit multiple bits at once, which makes it more efficient than UART, but it requires more complex hardware on both ends of the connection.

UART is not good for high-speed transmission because each bit must be sent individually; there’s no way to send multiple bits at once.

SPI Advantages

SPI has various benefits over other interface types. This is much faster than serial communication, which requires one-at-a-time data transmission. Other advantages include:

Low Signal/Pin Count

SPI offers a low signal/pin count, which means that you can use fewer wires to connect your system than you would if you used another type of interface. This makes SPI particularly useful when you’re looking at small-form-factor devices or when space is limited.

Supports Several Masters

Allows several masters to communicate with one another. This makes it ideal for use in microcontrollers, where multiple devices can share the same bus and each send data to and from the bus.

For example, an SPI-based device could send data to another SPI-based device (such as a display or sensor) without having to worry about whether the other device has completed its task or not.

Two wires for Communication

This protocol uses two wires to create communication among several devices. SPI uses separate clock and data lines to send information between devices. The SPI bus has a master device and one or more slave devices. The master can communicate with each of the slaves by sending commands in serial format over the MOSI line, while simultaneously sending data to the slave on the MISO line.

Adapt to the Demands of Different Slave

SPI is well suited for applications where you need to connect multiple slave devices to a host controller, such as in an embedded system or a computer system. The protocol allows the slave devices to adapt to the demands of the host controller, which means that it can easily be used for applications like connecting multiple sensors that have different communication requirements.

SPI Disadvantages

While SPI has many benefits, including its ability to support multiple devices, it also has a few disadvantages.


SPI is a serial interface that allows for communication between multiple devices. The problem with SPI is that it can be complicated to set up, especially when there are a lot of devices in the mix.

When you’re connecting SPI devices, you need to make sure that the lines don’t interfere with each other and that they don’t get crossed up. This can result in problems such as data loss or incorrect readings from sensors.

Slower Speed

SPI is slower than other communication protocols because it requires more clock cycles to transfer data than they do. This can be an issue if you’re trying to transmit a large amount of data, since the slower speed will mean that it takes longer for all of the information to transfer.

Half-Duplex Interface

The SPI interface is a half-duplex interface, which means that if a device is sending data, it can’t receive any. If the device isn’t ready to send or receive data, it must be told to wait until it is ready to do so.

UART Advantages

UART has several other benefits over other serial interfaces:

No Need for Clock

UARTs don’t need any kind of clock. This means that the data is sent at a constant rate, and there’s no need for synchronization. This makes UARTs extremely useful for sending data over long distances, or through noisy environments.

With other kinds of serial communication, such as SPI or I2C, you need to synchronize the sender and receiver so they can talk to each other. This means that if there’s any kind of delay in either device, it can cause problems with communication—for example, if one device sends its data a little bit later than another device expects, this can cause errors in reception.

Easy to operate

UARTs are simple to set up and use because they don’t require any configuration beyond connecting the data pin to the appropriate I/O pin on your microcontroller. They also have few pins, so you can have many of them in a single circuit board without having to worry about using up too much space or power.

Parity Bit for Error Checking

When you’re working with a serial interface, it’s important to have error checking. UARTs come with a parity bit that enables error checking, which can help ensure that your data is being correctly transmitted.

UART Disadvantages

While UART is an effective solution for several applications, there are some disadvantages to using the UART protocol:

Frame Size 9 Bits Only

This means that the frame size, or the amount of data that can be sent over a single wire, is limited to nine bits per frame. If you try to send any more than 9 bits in one frame, it will be truncated.

Low Data Transmission Speeds

The biggest disadvantage of UART is that it has low data transmission speeds. While this may not be a problem for simple applications, it can be a major drawback for high-speed data transmission.

The reason for this is that UART uses only one wire for communication and does not utilize any error-correction techniques.

Because of this, UART is not well suited for applications where there is a need to transmit large amounts of data quickly between two devices.

Can’t Use Several Master or Slave Systems

If you want to use several Master or Slave systems, you need to make sure that they are all on the same clock frequency. If they’re not, your data will be corrupted.

You also need to be careful about using different baud rates for the same master and slave system. If you do so, the data won’t be processed properly.




So, what’s the verdict? Which bus protocol should you use? Is it SPI or UART? Well, there is no simple answer to that. The best choice will likely depend on your project needs and familiarity with these protocols. However, if you have a system that already uses one of these protocols, it’s best to stick with it in order to keep communication consistent between multiple systems.

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