Getting started with SPI

Coming soon

This article explains what SPI is and how to use it through examples.

1. What is a serial peripheral interface (SPI)?

The Serial Peripheral Interface (SPI) is a synchronous serial communication interface specification used for short-distance communication, primarily in embedded systems. The interface was developed by Motorola in the mid-1980s and has become a de facto standard. SPI devices can communicate in full duplex mode using a master-slave architecture, with most often a single master. The master (controller) device originates the frame for reading and writing. Multiple slave-devices may be supported through selection with individual chip select (CS), sometimes called slave select (SS) lines.

In full duplex mode with multi-slaves, 4 buses are used:

• SCK (Serial Clock) bus, controlled by master.
• MISO (Master Input Slave Output) bus.
• MOSI (Master Output Slave Input) bus.
• SS (Slave Select) bus, useful in case of multiple slave configuration.

1.1. STM32 SPI Modes

The SPI interface is highly configurable to support many standard protocols and can operate in various modes.

• Full-Duplex
• Half-Duplex
• Receive Only
• Transmit Only

1.2. STM32 SPI Parameters

Basic parameters:

• Frame format: Motorola or TI
• Data size: 8 to 16 bits depending on STM32
• First bit: MSB or LSB

Clock parameters:

• Prescaler: from 2 to 256. Note that SPI clock speed cannot exceed half of the internal bus frequency.
• Clock polarity (CPOL): Low or high
• Clock Phase (CPHA): 1 or 2 Edge

Advanced Parameters

• CRC (Cycle Redundancy Check) calculation

1.3. Main SPI HAL functions

Based on STM32Cube HAL functions, SPI data transfer can be performed in 3 modes: Blocking Mode, Interrupt Mode or DMA Mode

• Blocking Mode:
  

The communication is performed in polling mode. The status of all data processing is returned by the same function after finishing transfer.

HAL_SPI_Transmit()

HAL_SPI_Receive()

HAL_SPI_TransmitReceive()

• Non-blocking modes

The communication is performed using Interrupts or DMA. These functions return the status of the transfer startup.

The end of the data processing will be indicated through the dedicated SPI IRQ when using Interrupt mode or the DMA IRQ when using DMA mode.

• Interrupt Mode:

HAL_SPI_Transmit_IT()

HAL_SPI_Receive_IT()

HAL_SPI_TransmitReceive_IT()

• DMA Mode:

HAL_SPI_Transmit_DMA()

HAL_SPI_Receive_DMA()

HAL_SPI_TransmitReceive_DMA()

2. Configure the SPI to communicate between 2 boards

2.1. Objective

In this example, the objective is to set up communication through SPI between two NUCLEO-L476RG.

2.2. Create project in STM32CubeIDE

• The NUCLEO-L476RG pinout is helpful to set the wiring between the boards in the project.

• File>New>STM32 Project in main panel.
  

In this example the NUCLEO-L476RG board is used to test SPI.

• Select NUCLEO-L476RG board using Board Selector as shown in the below figure.
  

In case you haven't downloaded the STM32L476 Cube library, it will be downloaded automatically, it may take some time.

• Save the project.
  

2.3. Configure SPI on master board

• Open the STM32CubeMX project from the workspace.
  

2.3.1. SPI settings

• Under Connectivity, select SPI1.
  
• Select Full-Duplex Master mode:
  

• Pin PA5 as SCK
  
• Pin PA6 as MISO
  
• Pin PA7 as MOSI
  

2.3.2. NVIC and DMA settings

• Interrupt Mode only

• In the NVIC settings tab
  
• Enable interrupt
  

2.6. Project workspace

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