Advantages and disadvantages of SPI interface and QSPI interface for small-size LCD screen

     Some customer engineers are not familiar with SPI interface and QSPI interface of small-size LCD screen, and they will encounter difficulties in design. Here is a brief introduction to the advantages and disadvantages of both sides. First of all, SPI is a serial peripheral interface, which usually has four lines: SCLK (clock), MOSI (master send slave receive), MISO (master receive slave send), SS (chip select); while QSPI is queue SPI, which is an extension of SPI, which may reduce the number of pins or improve efficiency. The advantages and disadvantages of both sides are as follows:

1. Physical pins and connection methods

· SPI interface:

Standard SPI uses 4 independent signal lines (excluding power/ground):

· SCLK (clock signal): synchronous clock provided by the master device;

· MOSI (Master Out Slave In): master→slave data transmission line;

· MISO (Master In Slave Out): slave→master data transmission line;

· SS (Slave Select, chip select): the master device selects the slave device (multiple SS are required for multiple slaves).

For small-sized screens, if the driver IC only supports SPI, it usually needs to occupy 4 IO ports (single slave scenario), which has certain requirements for PCB layout.

· QSPI interface:

QSPI is an extended protocol of SPI (some manufacturers call it "Quad-SPI" or "Fast SPI"), which reduces the number of physical interfaces by multiplexing data pins. Typical QSPI only retains 3 core signal lines (some scenarios can be further simplified):

· SCLK (clock);

· IO0/IO1/IO2/IO3 (four-wire data bus, which can be flexibly configured as input/output);

· SS (chip select, optional, replaced by timing in some scenarios).

    In actual applications, QSPI driver ICs for small-sized screens often merge MOSI/MISO functions into the four-wire data bus (such as controlling the data direction through instructions), and only 3~4 lines are needed to complete bidirectional communication, significantly reducing pin occupancy (for example, common small-screen QSPI only requires 3 lines: SCLK+3 IO).

2. Communication protocol and efficiency

· Communication characteristics of SPI:

· Full-duplex mode: while the master device sends data (MOSI), the slave device can return data (MISO). Theoretically, 1 bit of bidirectional transmission is completed per clock cycle;

· Instruction/data separation: each communication requires sending instructions (such as "write register" and "send display data") first, and then sending the corresponding data. The process is fixed;

· No queue mechanism: the master device needs to wait for the slave device to complete the current operation (such as data reception/processing) before initiating the next communication. The delay is limited by the response time of the slave device.

· Communication features of QSPI:

· Queue transmission (Queue): Supports the master device to preload multiple instructions/data into the FIFO queue inside QSPI, and automatically execute them in sequence without waiting for the previous instruction to complete (similar to "instruction pipeline");

· Flexible data direction: Through the configuration of "Instruction Phase" and "Data Phase", the same data bus can switch the input/output direction at different stages (for example, send write instructions first, and then send display data continuously);

· Higher effective bandwidth: Although the clock frequency of QSPI (usually 10~50MHz) is similar to SPI, the actual data transmission efficiency is higher by reducing the overhead of control signals (such as no additional SS switching); especially in scenarios where small screens need to be refreshed frequently (such as dynamic update of graphical interfaces), QSPI's queue mechanism can reduce the frequency of CPU intervention.

3. Control complexity and applicable scenarios

· Applicable scenarios for SPI:

· The driver IC only supports the SPI protocol (old or low-cost solutions);

· The screen function is simple (such as text display only, no complex instruction queue is required);

· The main control IO resources are sufficient (no need to save pins).

Disadvantages: Many pins are occupied, complex scenarios require frequent switching of instructions/data, and the CPU needs to actively manage each step of communication.

· Applicable scenarios for QSPI:

· Driver ICs for small-size screens (such as 0.96~2.8 inches) support QSPI (mainstream solutions such as ILI9341, ST7735, etc. all support QSPI mode);

· Need to simplify PCB design (save IO ports, suitable for miniaturized devices);

· Require high real-time or dynamic display (such as GUI interface, animation), and need to reduce the communication waiting time between the CPU and the screen.

Advantages: Few pins, flexible protocols, suitable for efficient interaction between resource-constrained microcontrollers (such as MCU) and small screens.

      In short, for small-sized LCD screens, QSPI is a better choice: through pin multiplexing and queue mechanism, while maintaining sufficient transmission rate, PCB design and master control resource usage are greatly simplified, which is especially suitable for scenarios that require dynamic display and limited space. SPI is only applicable to extreme cases where the driver IC does not support QSPI or has extremely simple functions. When actually selecting, it is necessary to first confirm the interface specifications of the screen driver IC (some screens support both SPI and QSPI, which can be switched by configuring pins). Shenzhen Hongjia Technology has 12 years of professional research and development, production and sales of 1.14-inch to 12.1-inch LCD screens and matching touch screens. There are various sizes of SPI interface and QSPI interface LCD screens, which can also be customized. Customers are welcome to email consultation.