SPI Protocol 3wire Vs 4wire for LCDOLED Displays

SPI, a synchronous serial communication protocol originally developed by Motorola (now Freescale), has become ubiquitous in embedded systems for connecting microcontrollers with peripherals like sensors, memory modules, and displays. Its popularity stems from its flexibility, high-speed capability, and straightforward hardware implementation.

The standard SPI protocol utilizes four primary signal lines:

• SCK (Serial Clock): The clock signal generated by the master device (typically a microcontroller) to synchronize data transmission.
• MOSI (Master Out Slave In): The data line for master-to-slave communication.
• MISO (Master In Slave Out): The data line for slave-to-master communication.
• SS (Slave Select): Also known as CS (Chip Select), this line activates a specific slave device for communication.

SPI operates in a master-slave configuration where the master initiates all communications. While theoretically supporting full-duplex communication, most LCD/OLED applications use half-duplex mode. The protocol's shift-register-like operation enables simultaneous bidirectional data transfer, though practical implementations often require sequential command and data phases.

Display controllers commonly implement two SPI variants: "3-wire SPI" and "4-wire SPI," distinguished primarily by their approach to differentiating commands from data.

The traditional 4-wire configuration includes:

• SCK (clock)
• SS/CS (chip select)
• MOSI/SDA (data)
• C/D (command/data select)

This architecture mirrors parallel interface controllers that maintained separate command and data registers, using the C/D line (sometimes labeled A0) to toggle between them.

The streamlined 3-wire version eliminates the dedicated C/D line, instead encoding this information within the data stream:

• SCK (clock)
• SS/CS (chip select)
• MOSI/SDA (data)

Here, an additional bit (typically the MSB) indicates whether the transmission contains commands (1) or data (0), effectively creating 9-bit transmissions instead of standard 8-bit transfers.

Some controllers employ 3-wire data encoding while physically using four wires (adding MISO for read operations). More complex variants may incorporate a tenth bit to encode read/write functionality, creating bidirectional communication over a single data line.

While 3-wire SPI reduces physical connections, its non-standard transmission length can complicate hardware SPI implementations. Modern processors with flexible SPI controllers may accommodate 9-bit transfers, but many traditional systems find 4-wire SPI more straightforward to implement.

SPI's shift-register-like architecture makes it particularly suitable for daisy-chained devices:

• Cascaded devices behave like interconnected shift registers
• Data propagates through the chain via MISO→MOSI connections
• The master clocks sufficient bits (device bit width × device count) before latching data

This approach simplifies multi-device systems but introduces latency proportional to chain length and requires all devices to support daisy-chaining.

Mastering SPI implementations is essential for embedded developers working with display technologies. The choice between 3-wire and 4-wire SPI depends on specific hardware capabilities and controller requirements. While 3-wire offers connection economy, 4-wire maintains compatibility with standard SPI hardware. Understanding these nuances ensures optimal display integration in embedded projects.