company news about Mastering Interface Selection: Optimizing Performance and Cost with MCU Parallel vs. SPI for Mid-Sized Displays

When integrating a mid-resolution display like a 2.8-inch 240x320 TFT into an embedded system, one of the first and most impactful decisions an engineer makes is selecting the communication interface. This choice dictates processor load, refresh rate, system cost, PCB complexity, and ultimately, the user experience. With modules offering multiple options, how do you choose the right one for your project?

This article tackles the critical design challenge of selecting and optimally implementing the display interface to balance performance, cost, and power consumption. We will conduct a comparative analysis of MCU Parallel (8080-series) and Serial SPI interfaces, using the versatile SFTO280PY-7422AN Transflective TFT Module from Saef Technology Limited as a practical case study, complete with timing analysis from its datasheet.

The Core Challenge: The Bandwidth Bottleneck in Embedded Graphics

The SFTO280PY-7422AN has a resolution of 240 x 320 x 18-bit color (262K). A full-screen image requires transferring 240 * 320 * 18 bits = 1,382,400 bits (≈172.8 KB) of data. The challenge is moving this data from the host microcontroller to the display's frame memory quickly and efficiently enough to achieve a responsive UI.

The datasheet reveals the module supports three primary modes via IM[2:0] pin configuration:

1. 8/16-bit MCU Parallel Interface (8080-series): A traditional, high-bandwidth bus.

2. 4-line SPI: A serial interface with a separate Command/Data line.

3. 3-line SPI: A further pin-optimized serial interface.

The trade-offs are significant and often misunderstood.

Comparative Analysis: MCU Parallel vs. SPI

Let's break down the decision using quantitative data from the SFTO280PY-7422AN datasheet (AC Characteristics, Section 7).

Scenario 1: Maximizing Frame Rate & UI Fluidity (Choose MCU Parallel)

Use Case: Industrial HMIs with complex graphics, animated gauges, or fast-updating data dashboards.

• Bandwidth Calculation:

  • 16-bit Parallel (Write): Minimum write cycle time (T_WC) = 66 ns. For 16-bit (2-byte) transfers, one pixel takes one cycle. Time per full frame = 240*320 * 66ns = 5.07 ms. This implies a theoretical maximum frame rate > 190 Hz (though limited by driver IC internal write speed).

  • 4-line SPI (Write): Minimum serial clock cycle (T_SCYCW) = 16 ns. Transferring 18 bits per pixel (requires 3 bytes) at 16 ns per clock edge (2 edges per clock cycle) gives a pixel time of ~24 * 16ns = 384 ns. Time per full frame = 240*320 * 384ns = 29.5 ms. Theoretical max frame rate ≈ 34 Hz.

• Performance Verdict: The parallel interface is ~5.8x faster for full-frame updates. This allows for smoother animations and a more responsive feel. It offloads the host MCU significantly, as data transfer is a simple memory-mapped write.

• Implementation Notes: Use the 16-bit data bus (DB0-DB15) for best performance. You must manage control pins (CS, RS, WR, RD) and ensure your MCU's external memory controller (FSMC/FMC) or GPIO bit-banging routine can meet the stringent timing (T_AS, T_AH, T_WRL). The pin count is high (up to 21 signals), increasing PCB layer count and connector size/cost.

Scenario 2: Minimizing Pin Count, Cost, and Complexity (Choose SPI)

Use Case: Portable instruments, wearable devices, space-constrained PCB designs, or systems where the MCU has limited I/O or lacks a parallel interface.

• Pin Count Advantage:

  • 4-line SPI: Requires only 4-6 pins: CS, SCL, SDA, RS/D/C, (plus optional RESET and backlight control). This is a massive saving over parallel.

  • 3-line SPI: Further reduces to ~4 pins by eliminating the separate RS/D/C line, embedding command/data in the serial stream.

• Optimization Strategy for SPI: You cannot achieve 60Hz full-screen updates, but you don't need to.

  • Partial Updates are Key: Only redraw the portion of the screen that changes. Changing a number in a data field might require updating just a few hundred pixels, making SPI latency negligible.

  • Use a Frame Buffer in MCU RAM: For more complex graphics, maintain a full-screen buffer in your MCU's internal RAM. Use DMA to stream data to the display via SPI in the background. This frees the CPU for other tasks after composing the frame.

  • Leverage Display IC Features: The ST7789T3 driver has a built-in frame memory. Use its built-in drawing commands (e.g., to draw lines, fill rectangles) sent via short SPI commands to let the driver IC handle graphic operations, minimizing data transfer.

Practical Implementation Guide

1. Hardware Configuration: Set the IM0, IM1, IM2 pins on the module according to your chosen mode. For SPI, ensure your MCU's SPI peripheral can master the required clock rates (up to 1 / T_SCYCW ≈ 62.5 MHz for write). The datasheet specifies VDDI (IOVCC) can be 1.8V, allowing level matching with low-voltage core MCUs.

2. Power Sequencing: Follow the sequence: Stabilize VCC (2.8V) and IOVCC (1.8V/3.3V), then pulse the RESET pin low for >10us (see Reset Timing, T_RW). Wait >120ms (T_RT for Sleep Out mode) before sending initialization commands.

3. Signal Integrity: For parallel interfaces, keep data bus traces matched in length and as short as possible. For high-speed SPI (>20 MHz), treat the SCL and SDA lines as controlled impedance traces, especially if the FPC cable is long.

Enhancing with Touch: A Consideration

The included 4-wire resistive touch (RTP) uses separate pins and does not interfere with the display interface choice. For a capacitive touch (CTP) add-on, an additional I2C bus would be needed. Saef Technology Limited can provide fully integrated CTP solutions where the touch controller is pre-connected and requires only power and I2C lines from your host.

Conclusion: Aligning Interface with Application Philosophy

There is no universally "best" interface. The optimal choice emerges from your system priorities:

• Choose MCU Parallel for performance-critical, graphics-rich applications where fluidity is paramount and PCB real estate is available.

• Choose SPI for cost-sensitive, miniaturized, or low-I/O-count designs where update rates are moderate and can be optimized through smart software.

The SFTO280PY-7422AN excels by offering this critical choice, future-proofing your design. Its detailed AC timing specifications empower engineers to make informed decisions and validate their designs upfront.

Struggling to choose the right display interface for your embedded project? Download the comprehensive SFTO280PY-7422AN Datasheet.pdf here to analyze the timing specs for your specific MCU. Contact the technical team at Saef Technology Limited to discuss your application requirements—we can help guide the selection and provide the optimally configured module with or without your preferred touch technology.