SPI Protocol: A Complete Guide for Embedded Engineers

What is SPI?

Serial Peripheral Interface (SPI) is a synchronous, full-duplex communication protocol used to connect microcontrollers to sensors, displays, flash memory, and other peripherals. It’s faster than I2C but uses more pins.

The Four Wires

• SCLK rial Clock (master generates)
• MOSI ster Out, Slave In
• MISO ster In, Slave Out
• CS/SS ip Select (active low, one per slave)

Clock Polarity and Phase (CPOL/CPHA)

This trips up everyone at least once:

Mode	CPOL	CPHA	Clock Idle	Data Sampled
0	0	0	Low	Rising edge
1	0	1	Low	Falling edge
2	1	0	High	Falling edge
3	1	1	High	Rising edge

The golden rule: Check the slave device’s datasheet for which mode it expects. Get this wrong and you’ll read garbage.

Basic STM32 SPI Transaction

uint8_t spi_transfer(SPI_HandleTypeDef *hspi, uint8_t data) {
    uint8_t rx;
    HAL_SPI_TransmitReceive(hspi, &data, &rx, 1, HAL_MAX_DELAY);
    return rx;
}

// Read accelerometer register
void read_accel(SPI_HandleTypeDef *hspi, uint8_t reg, uint8_t *buf, uint8_t len) {
    HAL_GPIO_WritePin(CS_PORT, CS_PIN, GPIO_PIN_RESET);  // CS low
    spi_transfer(hspi, reg | 0x80);  // Read bit set
    for (uint8_t i = 0; i < len; i++) {
        buf[i] = spi_transfer(hspi, 0x00);  // Dummy byte to clock data in
    }
    HAL_GPIO_WritePin(CS_PORT, CS_PIN, GPIO_PIN_SET);    // CS high
}

DMA Transfers

For high-throughput applications (displays, SD cards), use DMA to offload the CPU:

// Non-blocking DMA transfer
HAL_SPI_Transmit_DMA(&hspi1, framebuffer, FRAME_SIZE);

// Callback when complete
void HAL_SPI_TxCpltCallback(SPI_HandleTypeDef *hspi) {
    // Frame sent, prepare next
    frame_complete = 1;
}

Common Pitfalls

1. Wrong clock speed st slaves have a maximum SCLK frequency. Exceeding it causes intermittent errors.
2. CS timing me devices need a delay between CS assertion and first clock edge.
3. Shared bus conflicts ways deassert CS before talking to a different slave.
4. MISO floating e pull-ups or configure as alternate function properly.
5. Bit order I is MSB-first by default, but some devices use LSB-first.

SPI vs I2C: When to Use What

Factor	SPI	I2C
Speed	Up to 50+ MHz	Up to 3.4 MHz
Pins	3 + 1 per slave	2 (shared bus)
Complexity	Simple	Address-based
Full duplex	Yes	No

Use SPI when you need speed (displays, flash, ADCs). Use I2C when you have many slow devices and limited pins (sensors, EEPROMs).