What is the SPI Protocol?

SPI? or Serial Peripheral Interface protocol is a high-speed, synchronous serial communication protocol used primarily for short-distance communication, often between microcontrollers and peripheral devices such as sensors, memory, or display modules.

Evolution of SPI Protocol

The Serial Peripheral Interface Protocol was developed in the mid-1980s by Motorola. It facilitates communication between microcontrollers and peripheral devices within electronic systems. At that time, there was a growing need for a high-speed, efficient, and simple communication protocol to handle the increasing complexity of embedded systems.

Existing communication protocols like UART (Universal Asynchronous Receiver/Transmitter) and I2C (Inter-Integrated Circuit) were either too slow or possessed limitations in data transfer rates and complexity. Designers required a protocol that was easy to implement in hardware and software, with minimal overhead and maximum throughput.

It led to the introduction of a synchronous serial communication protocol, SPI. It achieves higher data rates by using a simple master-slave architecture and a minimal set of control lines. Due to its effectiveness, the SPI bus protocol quickly gained popularity and was adopted by various manufacturers for interfacing microcontrollers with peripherals like sensors, memory chips, and display modules.

SPI Protocol Signals

As mentioned, SPI protocol uses a master-slave architecture. The master device controls the communication, and one or more slave devices get connected to it. The SPI protocol typically involves the following four key signals:

• MOSI (Master Output Slave Input): Data line through which the master sends data to the slave.
• MISO (Master Input Slave Output): Data line in which the slave sends data back to the master.
• SCK (Serial Clock): This clock signal synchronizes the data transfer between the master and the slave.
• SS/CS (Slave Select/Chip Select): The signal from the master to select the active slave device.

SPI Communication Protocol Working Principle

• The master generates the clock signal (SCLK) and controls the communication.
• Data transmission occurs bit-by-bit over MOSI and MISO.
• Slave device selection uses the SS/CS line: When SS is low, the corresponding slave is active; other slaves remain inactive.
• The master and slave exchange data in full-duplex mode, synchronized with the clock edges.

SPI Modes

The SPI communication protocol has two settings:

Clock Polarity (CPOL): This controls the idle state of the clock line (SCK). There are two possibilities:

• CPOL = 0: The clock is low when idle (0V).
• CPOL = 1: The clock is high when idle (Vcc).

Clock Phase (CPHA): It determines if the data is read on the rising or falling edge of the clock.

• CPHA = 0: Data is sampled on the leading edge of the clock (when the clock transitions from low to high).
• CPHA = 1: Data is sampled on the trailing edge of the clock (when the clock transitions from high to low).

Together, these two settings determine the timing of when the data clocks in and out, and they ensure compatibility between the master and slave.

Key Advantages of SPI Communication Protocol

The key features of SPI communication protocol are:

Synchronous Communication: It is a communication method in which data transfer between devices is synchronized with a clock signal. The sender and receiver agree to transfer data at specific intervals controlled by a shared clock.

Master-Slave Architecture: In a Master-Slave Architecture, one device, known as the master, controls the communication flow and timing, while one or more other devices, called slaves, follow the master's instructions.?

Full-Duplex Communication: SPI communication protocol allows simultaneous data transmission and reception. In full-duplex communication, data transmission and reception occur simultaneously, meaning the data is sent and received in real-time.?

Simple Hardware Interface: Requires four minimal signal lines. Unlike the I2C protocol, SPI does not use a device address, simplifying the communication.

No Handshaking or Acknowledgment: SPI bus communication does not include built-in error checking, handshaking, or acknowledgment mechanisms, reducing communication overhead.

Multiple Slave Configuration Options

• Independent Slave Select Lines: Each slave device can have its own SS line for individual selection.
• Daisy-Chaining: Some systems allow slaves to be connected in a chain, reducing the number of SS lines needed.

Daisy-Chaining vs. Independent Slave Select (SS) Lines in SPI Communication Protocol

When implementing SPI bus communication in systems with multiple devices (slaves), you have two primary methods for managing different devices on the same bus: using independent SS (Slave Select) lines or daisy-chaining.?

Independent Slave Select (SS) Lines

In a system with independent SS lines, each slave device has its own dedicated SS line (sometimes called CS for Chip Select). The master device communicates with one slave at a time by toggling the relevant SS line between the slave it wants to interact with.

• MOSI (Master Out Slave In), MISO (Master In Slave Out), and SCK (Serial Clock) are shared among all slaves.
• Each slave gets activated by bringing its SS line low (active low), which tells the slave to listen to the SPI bus protocol.

Advantages

• Simple and Direct: Each slave device is independently addressed via its SS line. It allows easy communication with individual devices without worrying about collisions or sharing data paths.
• Flexible: Each device can operate at different SPI clock speeds, modes (CPOL/CPHA), and configurations.
• No Need for Special Firmware: The SPI controller can select the slave by toggling the SS line, and there's no need for the slaves to be aware of each other.

Disadvantages

• More Pins Required: A trade-off is the need for more GPIO pins on the master. For example, if you have five slave devices, you will need five independent SS pins on the master, which could limit pin availability.
• Scalability: As the number of slaves increases, the number of SS lines required also increases, which may not be feasible in space-constrained designs.

When to choose Independent SS Lines:

• Best for systems where speed and low latency are critical, and each device needs to operate with different configurations or speeds.
• It is useful in systems with fewer slaves or systems where the master has enough GPIO pins to handle all the slaves.
• Ideal for real-time applications where data from each slave needs to be processed separately and with minimal delay.

Daisy-Chaining (SPI Bus with Serial-to-Parallel or Parallel-to-Serial Shifting):

In a daisy-chaining configuration, slaves are in a series connection, and data passes from one slave to the next in a chain. Instead of having individual SS lines for each slave, one SS line is shared across all devices.

• Each slave device receives data from the MOSI line and passes the data to the next slave in the chain.
• Slaves transmit data back to the master over the MISO line, where each slave's data is forwarded along the chain until the master receives it.

How it works:

• The SS line activates the first device in the chain. Once the first slave is activated, the master sends data through the SPI bus communication to the first slave via MOSI.
• The first slave receives data, processes it, and passes the remaining bits down the line to the next slave.
• Each slave passes data to the next in line until the final slave sends its data back to the master via MISO.
• The master may need to use the SS line to control the first device in the chain or communicate with individual devices, depending on the implementation.

Advantages:

• Fewer Pins Required: There is only one SS line, and the number of GPIO pins required on the master does not increase with the number of slaves. Hence, it is scalable for large systems with many devices.
• Reduces Wiring Complexity: With fewer lines needed, the physical complexity of the system is reduced, making it easier to route and manage.
• Simplifies Large-Scale Systems: For IoT devices with many peripherals, a daisy-chaining approach helps keep the system clean and simple, with fewer constraints on pin resources.

Disadvantages:

• Potential for Increased Latency: Each slave in the chain must receive and forward data, which introduces latency. If you have many slaves, the total data transfer time will increase because each slave must process and pass the data sequentially.
• Fixed Communication Speed: All slaves must communicate at the same SPI speed and clock mode. Different communication speeds or configurations for each device in a daisy-chained setup are difficult.
• Complexity in Data Handling: Depending on the specific implementation, the master requires special logic to manage data transmission and understand what data belongs to which slave.

When to Choose Daisy-Chaining:

• It is best for systems with many slaves and limited GPIO pins or when the master pin count is constrained (e.g., in small or cost-sensitive designs).
• Suitable for non-time-critical systems where communication speed and latency are not as important.
• It is common in sensor networks or sensor arrays where devices with similar data rates are controlled altogether.

Comparison of SPI protocol with I2C and UART

Closing Notes

The Serial Peripheral Interface protocol is a robust and flexible solution for high-speed, full-duplex communication between a master device and one or more slave peripherals. Its ability to support high data rates makes it ideal for applications requiring fast data transfer, such as sensor interfacing, memory devices, displays, and SD cards. SPI bus protocol offers simplicity in its design and is highly efficient for communication between devices, especially when compared to protocols like I2C or UART.

In summary, SPI Protocol is a high-performing protocol, but IoT engineers should weigh its pin usage, signal integrity concerns, and complexity in multi-device systems to choose it for specific IoT applications. When implemented correctly, it remains a go-to protocol for embedded systems requiring fast, reliable data transfer with low overhead.