Understanding SPI Protocol, Hardware Interface Design, Timing Diagram, and Linux Kernel Driver Framework

SPI (Serial Peripheral Interface) is a high‑speed, full‑duplex, synchronous communication bus originally defined by Motorola for its MC68HCXX series. It uses only four pins (CS/SS, SCK, MOSI, MISO), which saves I/O resources and makes the interface simple and convenient.

The hardware interface consists of four signal lines: SS (chip‑select) , SCK (clock) , MOSI (master‑out‑slave‑in) , and MISO (master‑in‑slave‑out) . MOSI carries data from the master to the slave, while MISO carries data from the slave to the master. The clock is generated by the master, and the chip‑select line is active low to enable a specific slave. In many sensor modules, additional lines such as RST , AVDD , and INT are required for power, reset, and interrupt handling.

SPI supports four operating modes defined by CPOL (clock polarity) and CPHA (clock phase). CPOL determines the idle level of the clock (0 = low, 1 = high) and CPHA determines whether data is sampled on the first or second clock edge. Mode 0 (CPOL=0, CPHA=0) and Mode 3 (CPOL=1, CPHA=1) are the most commonly used.

The timing diagram (Mode 0) illustrates the following key points: • CPOL low idle, CPHA first edge for data capture. • SS is active low. • Data is transmitted on the rising edge and received on the falling edge. • MSB is sent first.

Full‑duplex communication proceeds as follows: 1) CS is pulled low, 2) the clock starts toggling, 3) the first rising edge initiates data sampling, 4) MOSI sends data to the slave while MISO simultaneously receives data from the slave, and 5) steps 3–4 repeat for each bit, completing a full data exchange cycle.

The Linux kernel provides a complete SPI driver framework. Three core data structures are used: • spi_driver – registers the driver and its probe function. • spi_transfer – describes a single data transfer (tx_buf, rx_buf, length). • spi_message – aggregates one or more spi_transfer objects and is submitted with spi_sync . Typical driver steps are: 1) Register the SPI device (probe, create class, set bus speed). 2) Configure device parameters (mode, max_speed_hz, bits_per_word). 3) Perform data transfer, e.g., using spidev_write or spidev_sync_write which internally build a spi_message and invoke spi_sync .

In summary, SPI is a fundamental communication protocol for sensors, IoT devices, smartphones, and automotive systems. Mastering the hardware interface and the Linux kernel driver stack is essential for embedded developers. Linux offers a robust platform for learning and extending SPI drivers, enabling developers to build reliable, high‑performance sensor solutions.