SPI Protocol: The Speed Your Embedded Systems Need

Future trends in embedded systems will include revolutionary technologies such as embedded security, real-time data visualization, network connectivity, and the IoT, and deep learning capabilities. Whether you plan to leverage the latest technology in machine learning to build a self-driving car or you're building out a new security system for your embedded devices, you will need to choose a communication protocol that suits the specific needs of your device while providing appropriate versatility and speed.

Embedded systems engineers today have several communication protocols to choose from, but for those that value speed of communication and versatile applications, the SPI protocol presents as the best option.

What is the SPI Protocol?

In 1980, electronics manufacturer Motorola wanted to design a communication protocol for its microcontroller-operated embedded systems that would allow full-duplex synchronous serial communication between master and slave devices on the bus. The resulting innovation in embedded systems programming, which later became known as Serial Peripheral Interface (SPI) protocol, has become a leading de facto standard for short-distance communication in embedded systems.

The simple, intuitive, and efficient design of the SPI protocol makes it an ideal choice for embedded systems development. Sometimes called a four-wire serial bus, a basic SPI bus consists of an SPI master device and an SPI slave device connected by four wires. Two of the wires act as a signal line that can be used to transmit data, either from the master to the slave or in the opposite direction.  Another line is used for clock and the fourth line is used to designate the target slave device for communication. Each of the four wires in an SPI serial bus plays a specific role in the overall functioning of the system - here's how they are typically labeled:

SCLK - The Serial Clock wire, designated as SCLK, is a wire that transmits the clock data from the master device to slave devices. In an SPI system, the master devices set the clock frequency and configure the clock polarity and phase. Data is only sampled at specific frequencies, so it is crucial that the master device and slave device are properly in time with each other.

MOSI - The Master Out Slave In wire is used to transmit data from the master device to the slave device.

MISO - The Master In Slave Out wire is used to transmit data from the slave device to the master device. An SPI bus runs in full-duplex mode by default, meaning that data communication can flow between the master and slave device in both directions at the same time.

SS - The fourth wire used in the SPI protocol is known as the Slave Select wire, or "SS". Signaling on the SS wire is used to indicate which slave device the master device is talking to at any given time. SPI systems can include just one master device, but many different slave devices.

What are the Advantages of Using SPI Protocol?

For engineers working with embedded systems, there are a few different options as far as choosing a communication protocol. The most popular options, however, are the SPI and I2C protocols that were created by Motorola and Philips semiconductor divisions respectively. For many applications, SPI seems to offer the best features and the versatility that engineers need to perfect their application. Here are some of the key benefits of using the SPI protocol:

SPI Protocol is Faster than the Alternatives

When designing an embedded system, it is important to consider how the speed of the system will impact user experience. Slow data transfer can make the device seem sluggish and unresponsive for the end user, so it's important to choose a communication protocol that offers rapid data transfer.

SPI's major alternative, the I2C protocol, was originally designed for data transfer speeds of just 100 kHz - although improvements to data transmission modes have seen speed increases for I2C systems over the years. Meanwhile, systems that make use of the SPI protocol can achieve data transfer speeds in excess of 10+ MHz, making them must faster than systems that use I2C.

Part of the reason for the huge speed difference between I2C and SPI comes down to the complexity of the bus protocol that underlies I2C. I2C systems may support several masters on the bus, use intentional communication delays to arbitrate bus access and operate with a set maximum bus rate of between 10 kHz in low-speed mode and up to 5 MHz in high-speed mode. In contrast, the simple architecture of SPI minimizes bus overhead and there is no maximum bus rate, meaning no cap on communication speeds.

SPI Supports Full Duplex Communication

In addition to facilitating faster communication, SPI devices also run in full duplex by default, whereas I2C devices are by default in half-duplex. Here's how the difference works:

An I2C bus system has a single bi-directional line for data transmission between a master and slave. This means that while the master device is transmitting data to the slave, the slave must be receiving the data. Data can only travel in one direction at a time because there is just a single line for transmission. On the other hand, SPI systems have a MISO line and a MOSI line so the master and slave device can communicate bidirectionally at the same time.

Think about the difference between a telephone and a walkie-talkie. Telephones are full duplex, as you can listen to the other person and speak to them at the same time. Walkie-talkies are half duplex - you press the push-to-talk button to send data, and you can only receive data when you are not sending data.

Full-duplex communication is like a two-way highway - it allows the simultaneous transmission of data in both directions between the master and slave.

Image courtesy of Unsplash

SPI Devices Have Versatile Applications

SPI has earned a prominent role in embedded systems thanks to its high-speed capabilities, relatively low power consumption, and space-efficient design. There are many types of embedded systems that make use of the full-duplex communication capabilities offered by SPI devices, especially devices for digital signal processing or telecommunications.

SPI protocol has been used to design systems with many different types of peripherals, including temperature sensors, touch screen monitors, audio codecs, communication and networking devices, real-time clocks, and more. SPI is a diverse communication protocol that is suitable for many types of embedded systems.

Embedded Engineers can Access Robust Testing Devices for SPI Protocol Devices

SPI is one of the most commonly used communication protocols for embedded systems, and as a result, some of the best diagnostic tools for embedded systems design have been geared towards the SPI protocol in particular. One such tool is the Beagle I2C/SPI Protocol Analyzer, an ideal and cost-effective bus monitoring solution for any engineer developing an I2C or SPI-based product.

The Beagle I2C/SPI analyzer was designed to non-intrusively monitor SPI traffic up to 24 MHz without affecting the system under test.  The captured SPI data can be streamed via High-speed USB directly to your computer. The Beagle I2C/SPI analyzer provides bit-level timing down to a 20nm resolution, enabling engineers to quickly diagnose and resolve timing issues on the chip, diagnose bugs, and ensure that their systems are perfectly designed and functional on release.

Summary

With the SPI protocol, 4 wires are used to connect a single master device to a slave device. Each wire plays a unique role: one for setting the clock, one for choosing which slave device the master will communicate with, and two wires that facilitate simultaneous communication from master to slave and from slave to master.

The design of the SPI protocol supports fast data transmission speeds, full duplex communication, and versatile applications in a variety of embedded systems. In addition, embedded systems engineers can access powerful diagnostic tools that help automate the testing and debugging process and reduce time-to-market and debugging costs for newly designed systems. Total Phase is an industry-leading provider of hardware solutions for embedded systems engineers.

If you have questions about our analyzers or other Total Phase products, feel free to email us at sales@totalphase.com. You can also request a demo that is specific for your application.

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