The many advantages of the QSPI interface of the LCD screen

      What is QSPI? It is an extension of SPI. Its full name is Quad SPI, which is four-wire SPI. The traditional SPI is a single data line (MOSI/MISO), while QSPI uses four data lines, and may also have separate clocks and chip selects. The liquid crystal display (LCD) adopts the design of the QSPI (Quad SPI, four-wire SPI) interface, which has unique advantages in embedded systems and consumer electronics, mainly reflected in high-speed transmission, simplified wiring, low power consumption, high compatibility and other aspects. The following is a detailed analysis of its "advantages" from the perspective of technical principles and practical applications:

1. High-speed transmission: four-wire parallel, greatly improved bandwidth

The traditional SPI (Serial Peripheral Interface) uses a single data line (MOSI/MISO) + clock line (SCK) + chip select (CS) in simplex or half-duplex mode, and the data transmission rate is limited by the bandwidth of the single line (usually a few Mbps to tens of Mbps). As an extension of SPI, QSPI realizes four-wire parallel transmission through four independent data lines (IO0~IO3), and can transmit 4 bits of data in the same clock cycle (instead of 1 bit of traditional SPI).

· Theoretical bandwidth is increased by 4 times: If the main clock frequency is the same (such as 50MHz), the bandwidth of QSPI can reach 200Mbps (50MHz×4bit), while the traditional SPI is only 50Mbps (50MHz×1bit).

· Better actual efficiency: QSPI supports more compact protocol design (such as reducing control instruction overhead), and with the high-speed response of LCD driver chips, it can significantly shorten the screen refresh time, especially suitable for scenes that require dynamic display (such as animation, video) or high resolution (such as QVGA, WVGA).

2. Simplified wiring: fewer pins, low complexity

For embedded systems (such as MCU, SoC), pin resources are often limited. QSPI only needs 6 core signals (SCK, CS, IO0~IO3), plus power and ground, to complete communication with the LCD driver chip; while traditional SPI may require additional pins (such as MISO) if bidirectional transmission is required (such as sending commands and receiving status at the same time), and parallel interfaces (such as 8-bit/16-bit parallel buses) require more pins (such as 8 data lines + control lines), resulting in complex PCB layout and increased area.

· Save pin resources: QSPI only needs a small number of pins to support high-speed data transmission, which is suitable for resource-constrained microcontrollers (such as Cortex-M0/M3 series).

· Reduce PCB design difficulty: Fewer pins means shorter routing, simpler layout, reduced signal interference (such as EMI), and improved system stability.

3. Low power consumption: efficient transmission, reduced standby time

The high-speed characteristics of QSPI indirectly reduce system power consumption:

· Shorten data transmission time: Under the same amount of data, the transmission time of QSPI is only 1/4 of that of traditional SPI, reducing the working time of MCU/LCD driver chips and reducing dynamic power consumption.

· Support fast entry into low-power mode: The QSPI protocol usually supports the "deep sleep" mode (such as putting the LCD driver chip into sleep mode through specific instructions), and wakes up only when the screen needs to be updated, which is suitable for battery-powered devices (such as smart watches, IoT terminals).

4. High compatibility: flexible adaptation to multiple scenarios

QSPI is not completely independent of traditional SPI, but is backward compatible with the SPI protocol. The working mode (SPI/QSPI) can be switched by configuration:

· Compatible with traditional SPI devices: When connecting an LCD driver that only supports SPI, the QSPI controller can be downgraded to SPI mode to avoid hardware waste.

· Support extended functions: Some QSPI interfaces also support the "double data rate (DDR)" mode (that is, data is sampled on both the rising and falling edges of the clock), further improving the bandwidth (for example, at a 100MHz clock, the DDR QSPI bandwidth can reach 800Mbps), meeting the needs of higher resolution or refresh rate.

5. Special optimization for LCD driver adaptation

LCD driver chips (such as ILI9341, ST7789, etc.) generally integrate QSPI interfaces, and the protocol is optimized for display scenarios:

· Command and data multiplexing: QSPI's four lines can transmit "commands" and "data" at the same time (distinguished by chip select or control signals), without the need for additional control pins, simplifying the communication process. For example, after sending the "set display area" command, pixel data can be directly transmitted continuously on the same bus to reduce interaction delays.

· Pipeline operation: QSPI supports "chip select hold" (CS is not immediately pulled high), allowing the MCU to continuously send multiple sets of commands/data, avoiding the overhead of frequently pulling down/pulling up CS, and improving overall efficiency.

     In short, the application of QSPI interface in LCD is essentially to achieve high-speed, low-latency data transmission with minimal pin resources, which perfectly meets the needs of embedded systems for "small size, low power consumption, and high cost performance". Especially in the scenarios of small and medium-sized LCDs (such as 1.54 inches to 7 inches) and medium and low resolutions (such as 240×320, 480×800), QSPI's comprehensive advantages (speed, wiring, power consumption) far exceed traditional SPI, I²C and even some parallel interfaces, becoming the preferred solution in consumer electronics, IoT devices, industrial instruments and other fields.

      Typical application scenarios: smart watches, portable medical devices, industrial HMI panels, smart home terminals, educational electronic devices and other scenarios that require small-sized high-refresh screens. Shenzhen Hongjia Technology has QSPI interface displays of various sizes, which can also be customized. Customers are welcome to email us for consultation.