Deep interpretation of SPI features, mechanisms and drivers and examples

SPI, short for Serial Peripheral Interface, is a communication protocol that enables data exchange between microcontrollers and peripheral devices. Initially developed by Motorola for its MC68HCXX family of processors, SPI has since become widely adopted in various applications such as EEPROMs, flash memory, real-time clocks, ADCs, and digital signal processors. The SPI interface is known for its high-speed, full-duplex, and synchronous nature, using only four pins (SCK, MOSI, MISO, and SS/CS) to facilitate communication. This design helps save pin count on the chip, making it ideal for compact PCB layouts. Due to its simplicity and efficiency, an increasing number of chips now integrate this protocol.

SPI operates in a master-slave configuration, where the master device controls the communication. A single master can communicate with multiple slave devices by providing a clock signal through the SCK pin and managing the SS/CS pin to select the active slave. The slave device relies entirely on the master's clock signal, meaning it cannot generate or control the clock itself. Without the clock, the slave will not function correctly.

In SPI, data is transferred synchronously based on the clock signal generated by the master. The clock signal determines when data is sampled and sent, ensuring both devices are synchronized. The timing is controlled by two parameters: clock polarity (CPOL) and clock phase (CPHA), which define the edge at which data is sampled and shifted.

During SPI communication, each clock cycle allows the transfer of one bit of data. Both the master and the slave send and receive data simultaneously. Before any data is exchanged, the master must select the desired slave via the SS/CS pin. If the received data is not read before the next transmission, it may be lost, causing errors in the communication process. Therefore, after sending data, it is common practice to read the received data from the SPI buffer, even if it is dummy data.

SPI supports four different modes based on CPOL and CPHA combinations. These modes determine when data is sampled relative to the clock signal. Additionally, data can be transmitted either as Most Significant Bit (MSB) first or Least Significant Bit (LSB) first.

SPI is strictly a master-slave system. Every data transfer involves both the master and the slave exchanging data. Sending a byte requires receiving a byte in return, and vice versa. This bidirectional communication ensures that no data is lost during the exchange.

Common terms used in SPI include CPOL (Clock Polarity), CPHA (Clock Phase), SCK (Serial Clock), and Edge (the point where the clock changes state). These terms help describe the timing and behavior of the SPI signal.

CPOL and CPHA can each be set to 0 or 1, resulting in four possible operating modes: Mode 0 (CPOL=0, CPHA=0), Mode 1 (CPOL=0, CPHA=1), Mode 2 (CPOL=1, CPHA=0), and Mode 3 (CPOL=1, CPHA=1). Each mode defines when the data is sampled relative to the clock edges.

CPOL determines the idle state of the SCK signal. When CPOL=0, the clock is low when idle, and becomes high during active periods. When CPOL=1, the clock is high when idle and goes low during active periods. This setting affects the timing of data sampling.

CPHA determines whether data is sampled on the first or second edge of the clock signal. For example, if CPHA=0, data is sampled on the first edge, while CPHA=1 means data is sampled on the second edge. This parameter is critical for ensuring accurate data transfer.

To ensure correct communication, the master and slave must be configured to the same mode. The slave’s mode is usually defined in its datasheet, and the master must match this configuration. Some devices allow software configuration of CPOL and CPHA, typically by writing specific values to the corresponding registers.

SPI uses a ring bus structure involving SS/CS, SCK, SDI, and SDO. Let’s say we want to transmit the 8-bit value 10101010. With rising edge transmission and falling edge reception, the data will be sent on the rising edge of the clock, and the received data will be sampled on the falling edge. After eight clock cycles, the data will have been fully exchanged between the two devices.

STM32 microcontrollers come with built-in SPI support, making implementation straightforward.

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Void SPIInit(void)

{

SPI_InitTypeDef SPI_InitStructure;

// Initialize the connection between SPI and GPIO port

FLASH_GPIO_Init();

/*!< Deselect the FLASH: Chip Select high */

CE_High();

/*!< SPI configuration */

SPI_InitStructure.SPI_Direction = SPI_Direction_2Lines_FullDuplex;

SPI_InitStructure.SPI_Mode = SPI_Mode_Master;

SPI_InitStructure.SPI_DataSize = SPI_DataSize_8b;

SPI_InitStructure.SPI_CPOL = SPI_CPOL_High;

SPI_InitStructure.SPI_CPHA = SPI_CPHA_2Edge;

SPI_InitStructure.SPI_NSS = SPI_NSS_Soft;

SPI_InitStructure.SPI_BaudRatePrescaler = SPI_BaudRatePrescaler_8;

SPI_InitStructure.SPI_FirstBit = SPI_FirstBit_MSB;

SPI_InitStructure.SPI_CRCPolynomial = 7;

SPI_Init(SPI1, &SPI_InitStructure);

/*!< Enable the sFLASH_SPI */

SPI_Cmd(SPI1, ENABLE);

}

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Uint8 Send_Byte(uint8 data)

{

// Wait until the transmit buffer is empty

While(!(SPI1->SR & SPI_I2S_FLAG_TXE));

SPI1->DR = data;

// Read the received data

While(!(SPI1->SR & SPI_I2S_FLAG_RXNE));

Return SPI1->DR;

}

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Uint8 Get_Byte(void)

{

// Wait until the transmit buffer is empty

While(!(SPI1->SR & SPI_I2S_FLAG_TXE)); // Transmit buffer is empty, exit loop

SPI1->DR = 0xFF; // Send dummy data

// Wait until the receive buffer is not empty

While(!(SPI1->SR & SPI_I2S_FLAG_RXNE)); // Receive buffer is not empty, exit loop

Return SPI1->DR;

}