Difference Between UART and SPI

What is SPI (Serial Peripheral Interface)?

SPI stands for Serial Peripheral Interface (SPI). Although it resembles I2C, it is a different form of serial-communications protocol designed exclusively for connecting microcontrollers. Data can be transmitted and received simultaneously with the full-duplex operation. Operate at 8Mbits or higher data transfer speeds. It is usually faster than I2C due to the simple protocol. Each device will require its own address wire, even if data and clock lines are shared. Used in situations where speed is critical. (For example, SD cards, display modules, or when information is updated and changes often, such as thermometers)

Differences between UART vs SPI

While both UART and SPI offer full-duplex communication, they both use "serial" transmission in some fashion and are only ideal for short-distance applications; there aren't many other similarities. What are the distinctions between UART and SPI? There are a lot of them, and we'll go through a few of them here.

Hardware vs protocol

One of the most significant distinctions is that UART is a hardware type, whereas SPI is a protocol. It's easy to miss this when working with the subtleties of getting things to function in an embedded system. UART, on the other hand, is a piece of hardware (a microchip), whereas SPI is a communication protocol or specification.

Number of pins

UARTs only require two pins, whereas SPI devices require at least four. This means that if pins/traces are sparse when constructing a system, SPI may not be the best option.

Number of devices that can be connected

UARTs and SPI devices support a varied number of devices as a corollary to the number of pins. UART is practically confined to 1 to 1 communication because it only uses Tx and Rx for outbound communication. SPI, on the other hand, can use its master/slave paradigm to allow for an excessive number of communications.

Communications speed

SPI devices require at least four pins, whereas UARTs only require two. This means that if pins/traces are limited while developing a system, SPI may not be the best option.

Price

The cost of a given solution is a major deciding factor in every engineering project. In general, SPI is less expensive than UART.

Asynchronous vs Synchronous

Another significant difference between UART and SPI, as you can see from the previous descriptions, is that UART communication is asynchronous while SPI communication is synchronous.

Size

SPI devices, on average, take considerably less area than UART chips. As a result, SPI may be better suited to use cases with limited board space.

UART Interface

How does it work?

• It can operate between devices in three ways:
  • Simplex = one-way data transmission
     
  • Full-duplex = data is transmitted in both directions simultaneously
     
  • Half-duplex = data is transmitted in both directions, but not simultaneously.
     
• After the connection is established, data flows from the TX of the transmitting UART to the RX of the receiving UART.
   
• UART is an asynchronous serial transmission protocol, which means it has no clocks
   
• The transmitting UART translates parallel data from the master device (such as the CPU) into serial data and sends it to the receiving UART in serial. The serial data will subsequently be converted back to parallel data for the receiving device.
   
• Because UART lacks clocks, start and stop bits are sent to denote the beginning and conclusion of a message.
   
• This tells the receiving UART when to start reading bits and when to stop. When a start bit is detected by the receiving UART, the bits are read at the defined BAUD rate.
   
• BAUD Rate is the UART data transfer speed, which is set at 115,200 by default (BAUD rate is based on symbol transmission rate but is similar to bit rate).
   
• Both UARTs must communicate at a similar baud rate. If the BAUD rate differential is greater than 10%, the bit timing may be wrong, rendering the data useless. The user must ensure that UARTs are set up to send and receive data from the same packet.

UART Working Protocol

• A data-transmitting UART will first receive data from another component's data bus (e.g., CPU).
   
• To generate the data packet, it will take the data from the data bus and add a start bit, a parity bit, and a stop bit.
   
• The data packet is subsequently transferred via the TX pin, where it is read by the receiving UART via the RX pin. Data is transmitted until the transmitting UART is empty.

Advantages of Using UART

• It is simple to use and highly documented, as it is a frequently used procedure with a wealth of web resources.
   
• No clock is required.
   
• The parity bit is used to check for errors.

Disadvantages of Using UART

• The data frame size is limited to 9 bits.
   
• Multiple master and slave systems are not possible.
   
• To avoid data loss, each UART's Baud rate must be within 10% of one another.
   
• Data transfer speeds are slow.
   

Also see, Difference Between Verilog and Vhdl

SPI Interface

How does it work?

• It communicates in two ways:
  
  
  1. A Chip Select line is used to select each device. For each device, a separate Chip Select line is required. SPI is currently used in the most prevalent way by RPIs.
  2. Daisy-chaining where each device is connected to the other through its data out to the data in line with the next.
      
• The number of SPI devices that can be linked is unlimited. However, the chip select approach has practical limitations due to the amount of hardware select lines accessible on the primary device. The daisy-chaining method has functional limitations due to the complexity of sending data via devices.
   
• The SPI interface is simple and efficient in point-to-point communication since it does not require addressing procedures and offers full-duplex communication. 

SPI Working Protocol

• The SPI communicates via four ports which are:
  
  • MOSI – Master Output, Slave Input
     
  • MISO – Master input, Slave output
     
  • Clock (SPI CLK, SCLK)
     
  • Chip select (CS)
     
• In a multi-slave system, each slave has its own enable signal, which is slightly more difficult for hardware than the I2C system.
   
• The SPI interface is two simple shift registers in the inside hardware. The data sent is 8 bits long. The slave enables signal and shift pulse generated by the master device are used to send it bit by bit. The front portion is higher, while the back portion is lower.
   
• SPI stands for synchronous serial data transmission between the CPU and a low-speed peripheral device. The data is transferred bit by bit under the master device's shift pulse. The front portion is higher, while the back portion is lower. It's full-duplex communication, and the data transmission speed is far quicker than the I2C bus, reaching several megabits per second.

Advantages of using SPI

• Because there is no sophisticated slave addressing mechanism like I2C, the protocol is straightforward.
   
• In comparison to UART and I2C, it is the fastest protocol.
   
• There is no start and stop bits, unlike UART, which means data can be transmitted continuously without interruption.
   
• Separate MISO and MOSI lines allow data to be broadcast and received simultaneously.

Disadvantages of using SPI

• More Pin ports are occupied, the practical limit to several devices.
   
• No flow control is specified, and no acknowledgment mechanism confirms whether data is received, unlike I2C.
   
• Uses four lines – MOSI, MISO, SPI CLK, CS.
   
• There is no form of an error check, unlike in UART.
   
• Only one master.
  You can also read 8051 Microcontroller Pin Diagram here.

SPI vs UART: What should I use?

The question of "UART vs. SPI: Which Should I Use?" does not have a simple answer. However, now that you have the knowledge we've provided, you can make an informed decision on which solution is ideal for a certain application you're working on.

SPI is recommended in applications that require faster communication between chips or applications that require communication between several devices. On the other hand, UART may be more suited for applications that must traverse a longer distance, such as diagnostic screens or other RS-232-compatible applications.

Frequently Asked Questions

Can SPI be used as UART?

No, SPI (Serial Peripheral Interface) and UART (Universal Asynchronous Receiver/Transmitter) are different communication protocols with distinct hardware requirements and timing characteristics.

Why use UART over SPI?

UART is simpler, consumes fewer resources, and is more suitable for point-to-point communication or interfacing with legacy devices lacking SPI compatibility.

Is SPI better than UART?

UART is substantially slower than SPI. An SPI solution can be three times faster than a UART solution in some cases.

Is UART a protocol?

UART is a hardware communication mechanism that uses asynchronous serial transmission at a variable speed. The output bits from the transmitting device travelling to the receiving end are asynchronous since there is no clock signal to synchronize them.

Conclusion

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We hope that this blog has helped you enhance your knowledge regarding the difference between SPI and UART.

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