SPI Protocol: Brief Understanding and Integration


In the world of electronics and embedded systems, communication between various components is crucial. One widely used communication protocol that stands out for its simplicity and versatility is the Serial Peripheral Interface (SPI) protocol. In this blog, we will delve into the fundamentals of SPI, provide a clear diagram to illustrate its workings and present some practical examples to help you grasp this communication standard better.

What is SPI?

SPI, which stands for Serial Peripheral Interface, is a synchronous serial communication protocol that transfers data between a master device and one or more peripheral devices. It was initially developed by Motorola and has since become an industry standard due to its simplicity and speed.

SPI employs a Master-Slave architecture, where a single master device controls one or more slave devices. The master device initiates and manages data transfer, while the slave devices respond to commands and exchange data as needed.

SPI Hardware Connections: Before diving into SPI's operational details, let's establish the essential hardware connections commonly used for SPI communication:

  • Master Device (MCU): This is typically a microcontroller or a Raspberry Pi acting as the controller. It generates the clock signal (SCLK) and manages data transmission.

  • Slave Devices: These can be various peripherals, such as sensors, displays, or memory modules. Each slave device has a unique Slave Select (SS) or Chip Select (CS) pin, allowing the master to select a specific slave for communication.

  • Data Lines: SPI employs two data lines for communication:

    • MOSI (Master Out Slave In): The master sends data to the slave on this line.

    • MISO (Master In Slave Out): The slave sends data to the master on this line.

  • Clock Line (SCLK): The clock signal is generated by the master and controls the timing of data transmission.

SPI Data Transfer Sequence:

Now, let's break down the SPI data transfer sequence step by step, using a diagram to visualize the process:

  1. The Master Device (MCU) initiates communication by pulling the Slave Select (SS) pin low for the desired slave device.

  1. The Master sends a clock signal (SCLK) to synchronize data transfer.

  2. While sending clock pulses, the Master simultaneously transmits data (outgoing data) on the MOSI line. The Slave listens and captures this data.

  1. At the same time, the Slave may also send data (incoming data) on the MISO line, which the Master captures.

  2. Once the desired amount of data is transmitted, the Master releases the SS pin, indicating the end of communication with that particular slave.

  3. The process can be repeated for other slave devices if needed.

Clock Polarity and Clock Phase:

Clock Polarity (CPOL):

  • CPOL determines the idle state of the clock signal.

  • When CPOL is 0, the clock signal is idle low, meaning it rests at a low voltage level when not actively transmitting data.

  • When CPOL is 1, the clock signal is idle high, meaning it rests at a high voltage level when not actively transmitting data.

Clock Phase (CPHA):

  • CPHA defines when data should be sampled and when it should be transmitted relative to the clock signal.

  • In Mode 0 and Mode 2, data is captured on the leading edge (rising edge in Mode 0, falling edge in Mode 2) and propagated on the trailing edge (falling edge in Mode 0, rising edge in Mode 2) of the clock.

  • In Mode 1 and Mode 3, data is captured on the trailing edge (falling edge in Mode 1, rising edge in Mode 3) and propagated on the leading edge (rising edge in Mode 1, falling edge in Mode 3) of the clock.

These four modes allow flexibility in configuring SPI communication to match the requirements of different devices and ensure data is transmitted and received correctly. The choice of CPOL and CPHA settings depends on the specifications provided by the device's datasheet or communication requirements in your specific application.

Multi-Slave Configuration:

Regular SPI Mode:

  • In regular mode, each subnode requires an individual chip select (CS) signal from the main.

  • When the CS signal for a subnode is enabled (pulled low) by the main, the clock and data on the MOSI/MISO lines are available for that selected subnode.

  • Enabling multiple CS signals simultaneously can corrupt the data on the MISO line because the main can't differentiate which subnode is transmitting the data.

    Daisy-Chain Method:

  • In daisy-chain mode, all subnodes share a common chip select signal, and data propagates from one subnode to the next.

  • All subnodes receive the same SPI clock simultaneously.

  • The data from the main is directly connected to the first subnode, which provides data to the next subnode in the chain.

  • The number of clock cycles required for data to reach a specific subnode in the daisy chain is proportional to its position in the chain.

Advantages of Daisy-Chain Method:

  • Daisy-chain mode can reduce the number of required GPIOs compared to regular mode.

  • It simplifies the system-level design and minimizes the number of inputs and outputs needed from the main.

  • However, not all SPI devices support daisy chain, so it's essential to check the product datasheet for compatibility.

Practical Examples:

Let's explore two practical examples of SPI communication:

Example 1: Interfacing with an SPI Display

Imagine you want to interface with an SPI-based OLED display to show sensor data from your Arduino. You would set up the Arduino as the master and the OLED display as the slave. By sending appropriate commands and data through SPI, you can draw shapes, text, and graphics on the display.

Example 2: SPI Flash Memory

Many microcontrollers and single-board computers use SPI flash memory for program storage. To read or write data from/to the flash memory, the MCU acts as the master, sending commands and data as needed to read, write, or erase sectors of memory.