SPI-based e-paper displays cannot use HDMI for communication due to fundamental differences at the physical layer, protocol layer, and application scenarios. A detailed technical analysis follows:

1. Physical Layer Differences

Key Incompatibilities:
HDMI’s differential signals and SPI’s single-ended signals are physically incompatible. Direct connection risks voltage mismatch (potential device damage) and signal misinterpretation.

2. Protocol Layer Differences

Key Incompatibilities:
SPI controllers for e-paper displays only accept pixel data or simple commands, while HDMI outputs contain encapsulated multimedia streams (e.g., audio, timestamps, encrypted data) that e-paper cannot decode.

3. Application Scenario Differences

Key Incompatibilities:
E-paper relies on SPI’s low-speed, low-power design, while HDMI’s high bandwidth and real-time requirements exceed e-paper’s capabilities.

4. Direct Interconnection Challenges

Hardware: SPI controllers cannot process HDMI’s TMDS signals. Dedicated HDMI receiver chips are required, which e-paper lacks.

Software: E-paper drivers only support SPI protocols, not HDMI’s EDID negotiation, color mapping, or encryption.

Cost: Adding HDMI interface chips increases cost and complexity, contradicting e-paper’s low-cost, low-power design philosophy.

Alternative Solutions

To connect HDMI to e-paper, additional hardware is needed:

Bridge Chips: Use FPGAs/ASICs to decode HDMI and convert data to SPI (e.g., extracting frames via an HDMI receiver).

Middleware Controllers: Employ microcontrollers (e.g., Raspberry Pi) with HDMI capture cards to process and forward data via SPI.

However, these solutions sacrifice real-time performance and increase costs significantly.

SPI and HDMI are fundamentally incompatible due to differences in physical layers, protocols, and use cases. E-paper displays are optimized for low-speed, low-power static content, while HDMI is designed for high-speed multimedia. Bridging them requires complex, costly workarounds, making direct communication impractical.