How to convert HDMI to 4 lane MIPI DSI for display?
Understanding the Signal Conversion Process
The conversion from HDMI to MIPI DSI involves multiple stages: first, the HDMI receiver captures the TMDS signal and extracts the video data, audio data (if present), and control signals. The HDMI receiver chip, such as the TFP401 from Texas Instruments, decodes the TMDS stream into parallel RGB data with horizontal and vertical sync signals. This parallel data is then fed into a video processor or bridge chip, which handles color space conversion (e.g., from YCbCr to RGB), scaling if needed, and packetization. For MIPI DSI, the data is formatted into packets: short packets for commands and synchronization, and long packets for pixel data. The bridge chip must generate the correct MIPI DSI clock frequency, which is typically derived from the pixel clock divided by the number of lanes. For a 4 lane configuration, the clock is pixel clock / 4, but with overhead for packet headers and footers, the actual data rate per lane is higher. For example, with a 1080p@60Hz display, the pixel clock is 148.5 MHz, so the MIPI DSI clock is 148.5 / 4 = 37.125 MHz, but each lane transmits data at 4x the clock rate (DDR), so the effective data rate per lane is 2 * 37.125 = 74.25 MHz, which is too low for 24-bit color. In reality, the MIPI DSI clock is set to a higher value, often around 500 MHz, to accommodate the 24-bit pixel data plus overhead. The exact calculation is: total data rate = (pixel clock * bits per pixel) / number of lanes. For 1080p@60Hz, total data rate = 148.5 MHz * 24 = 3.564 Gbps. With 4 lanes, each lane carries 891 Mbps, so the MIPI DSI clock is 891 / 2 = 445.5 MHz, but due to packet overhead (typically 10-15%), the actual clock is around 500 MHz. This is why the bridge chip must support high-speed MIPI DSI clocks, typically up to 1 GHz per lane.
Bridge Chip Options and Their Specifications
Several bridge chips are available for HDMI to MIPI DSI conversion, each with different capabilities. The LT8918 from Lontium is a popular choice for 1080p applications, supporting HDMI 1.4 up to 1080p@60Hz and outputting 4 lane MIPI DSI with a maximum data rate of 1.5 Gbps per lane. It also supports audio over HDMI, but audio is not typically used in MIPI DSI displays. The TC358870 from Toshiba (now Kioxia) supports up to 4K@30Hz over HDMI 1.4 and outputs 4 lane MIPI DSI with up to 1.2 Gbps per lane. It also includes a built-in scaler and supports color space conversion. The TFP401 from Texas Instruments is a simpler HDMI receiver that outputs parallel RGB, but it requires an external MIPI DSI transmitter chip, such as the SN65DSI86 from Texas Instruments, which converts parallel RGB to MIPI DSI. The SN65DSI86 supports up to 4 lanes with a maximum data rate of 1.5 Gbps per lane and can handle up to 1080p@60Hz. For higher resolutions, the LT8918B from Lontium supports up to 4K@30Hz, and the TC358870XBG supports up to 4K@60Hz with HDMI 2.0, but this requires a more complex design. The choice of chip depends on the display's resolution, refresh rate, and interface requirements. Below is a table comparing these chips:
| Chip | HDMI Version | Max Resolution | Max MIPI DSI Data Rate | Lanes | Special Features |
|---|---|---|---|---|---|
| LT8918 | HDMI 1.4 | 1080p@60Hz | 1.5 Gbps per lane | 4 | Audio support, low power |
| TC358870 | HDMI 1.4 | 4K@30Hz | 1.2 Gbps per lane | 4 | Built-in scaler, color space conversion |
| SN65DSI86 | N/A (requires external HDMI receiver) | 1080p@60Hz | 1.5 Gbps per lane | 4 | Low power, flexible configuration |
| LT8918B | HDMI 1.4 | 4K@30Hz | 1.5 Gbps per lane | 4 | Improved ESD protection |
Electrical and Timing Considerations
The MIPI DSI interface uses differential signaling with a voltage swing of 200-400 mV, which is much lower than HDMI's 400-600 mV. This reduces power consumption and electromagnetic interference, but it also requires careful PCB layout to maintain signal integrity. The MIPI DSI clock is a differential pair, and the data lanes are also differential. The maximum cable length for MIPI DSI is typically limited to a few centimeters (e.g., 10-20 cm) for high-speed signals, because the signal degrades quickly over longer distances. In contrast, HDMI cables can run up to 5-10 meters. Therefore, the adapter board must be placed close to the display, often directly attached to the display's FPC connector. The timing parameters for the MIPI DSI interface are defined by the video mode, which includes horizontal and vertical blanking intervals, sync pulses, and pixel clock. For a typical 1080p display, the horizontal timing might include a front porch of 88 pixels, sync width of 44 pixels, and back porch of 148 pixels, while vertical timing includes a front porch of 4 lines, sync width of 5 lines, and back porch of 36 lines. These values are specific to the display and must be programmed into the bridge chip via I2C or SPI. The bridge chip reads the HDMI EDID (Extended Display Identification Data) from the source to determine the source's capabilities, but it must also generate its own EDID to tell the source what resolutions are supported. The EDID is typically stored in an EEPROM on the adapter board, and it must match the display's native resolution. For example, if the display is 1920x1080, the EDID should report that as the preferred resolution, and the bridge chip will scale or pass through the video accordingly.
Power Supply Requirements
The conversion process requires multiple power rails: the HDMI receiver typically needs 3.3V and 1.8V, the bridge chip needs 1.2V (core) and 1.8V or 3.3V (I/O), and the MIPI DSI interface needs 1.8V or 1.2V for the PHY. The display itself may require additional voltages, such as 3.3V for the logic and 5V or 12V for the backlight. The total power consumption depends on the resolution and frame rate. For a 1080p@60Hz display, the bridge chip typically consumes 200-400 mW, while the HDMI receiver consumes 100-200 mW. The display's backlight can consume several watts, depending on the size and brightness. A typical 5-inch 1080p display might have a backlight power of 2-3 watts. The adapter board should include a voltage regulator to generate the required voltages from a single input, such as 5V or 12V. For example, the LT8918 datasheet recommends a 5V input, with internal regulators generating 3.3V, 1.8V, and 1.2V. The board must also handle inrush current and provide proper decoupling capacitors to ensure stable operation. The power sequencing is critical: the core voltage must come up before the I/O voltage, and the MIPI DSI PHY must be powered after the logic is stable. Many bridge chips have built-in power sequencing, but external circuitry may be needed for the display.
Common Challenges and Solutions
One common challenge is signal integrity, especially when using long cables or poor PCB layout. The MIPI DSI signals are high-speed, with rise times in the picosecond range, so any impedance mismatch or reflection can cause data errors. The differential impedance for MIPI DSI is typically 100 ohms, and the PCB traces must be matched in length and routed with controlled impedance. The HDMI signals also require 100 ohm differential impedance, but the connector and cable are more robust. Another challenge is timing synchronization: the HDMI source expects a stable video stream, but the MIPI DSI display may have different timing requirements. The bridge chip must buffer the video data and adjust the timing to match the display's requirements. This is done using a FIFO (First-In, First-Out) buffer, which can store several lines of video data. The buffer size depends on the resolution and the timing difference. For example, if the HDMI source sends 1080p at 60 Hz with a pixel clock of 148.5 MHz, and the display requires a slightly different pixel clock (e.g., 150 MHz), the buffer must be large enough to handle the difference. A typical FIFO size is 128-256 lines, which is sufficient for most applications. The bridge chip also needs to handle audio if the HDMI source sends it, but most MIPI DSI displays do not support audio, so the audio data is typically discarded or converted to a separate I2S output. The adapter board may include an audio jack for this purpose. Another issue is the EDID programming: if the EDID is not correctly programmed, the HDMI source may not output the correct resolution, or it may output a resolution that the bridge chip cannot handle. The EDID must be carefully written to match the display's capabilities, including the supported resolutions, refresh rates, and color depths. For example, a 1080p display with 24-bit color should report that as the only supported resolution, with a pixel clock of 148.5 MHz. The bridge chip's I2C interface is used to read and write the EDID, and some chips allow you to override the EDID with custom settings.
Real-World Implementation Steps
To implement the conversion, you need to select the appropriate bridge chip based on the display's resolution and interface. For a 1080p display, the LT8918 or SN65DSI86 are good choices. For a 4K display, the TC358870 or LT8918B are better. Then, you need to design a PCB that includes the HDMI connector, the bridge chip, an EEPROM for EDID, voltage regulators, and the MIPI DSI connector. The PCB layout must follow high-speed design rules: keep trace lengths matched within 5 mm for differential pairs, use ground planes, and place decoupling capacitors close to the chip pins. The power supply should be designed to handle the peak current, which can be up to 500 mA for the bridge chip and 1-2 A for the display backlight. Once the board is assembled, you need to program the bridge chip's registers via I2C to set the video mode, lane count, data rate, and timing parameters. This is typically done using a microcontroller or a USB-to-I2C adapter. The programming sequence is usually described in the chip's datasheet. For example, for the LT8918, you need to set registers for the HDMI input (e.g., input resolution, color depth) and the MIPI DSI output (e.g., lane count, clock frequency, video mode). The chip also has registers for power management and error handling. After programming, you can test the board with a known HDMI source and a MIPI DSI display. If the display shows a stable image, the conversion is working. If not, you may need to adjust the timing parameters or check for signal integrity issues. A common issue is the MIPI DSI clock frequency being too high or too low, which causes the display to show a blank screen or flicker. You can use an oscilloscope to measure the MIPI DSI clock and data signals, and compare them to the expected values. The MIPI DSI clock should be a clean square wave with a frequency within 1% of the target. The data lanes should show the packet structure, with short packets for sync and long packets for pixel data. If the signals are distorted, you may need to add series resistors or adjust the drive strength of the bridge chip.
Performance Metrics and Trade-offs
The conversion introduces latency, which is typically 1-2 frame times for the FIFO buffer, plus the processing delay of the bridge chip. For a 1080p@60Hz display, this is about 16-32 ms, which is acceptable for most applications, but may be noticeable in gaming or real-time video. The latency can be reduced by using a smaller buffer or a faster chip, but this increases the risk of timing errors. The color depth is another factor: most MIPI DSI displays support 24-bit RGB, but some support 18-bit or 30-bit. The bridge chip must be configured to match the display's color depth. If the HDMI source outputs 30-bit color, the bridge chip may need to dither or truncate the data to 24-bit, which can cause color banding. The data rate of the MIPI DSI interface is limited by the chip's capabilities and the display's specifications. For a 4K@30Hz display with 24-bit color, the total data rate is 3840*2160*24*30 = 5.97 Gbps, which requires 4 lanes at 1.5 Gbps each, or 2 lanes at 3 Gbps each. Some chips support up to 4 lanes at 1.5 Gbps, so 4K@30Hz is achievable, but 4K@60Hz requires 12 Gbps, which is beyond the capabilities of most bridge chips. The TC358870 supports up to 4K@30Hz, while the LT8918B also supports 4K@30Hz. For 4K@60Hz, you would need a chip like the TC358775 or a dedicated FPGA-based solution. The power consumption also scales with resolution: a 4K bridge chip may consume 500-800 mW, compared to 200-400 mW for a 1080p chip. The thermal management is important, as the chip can get hot, especially in a small enclosure. A heatsink or thermal pad may be needed for high-power applications. The cost of the adapter board varies from $10 for a simple 1080p board to $50 for a 4K board, depending on the chip and components.
Alternative Approaches and Future Trends
Another approach is to use an FPGA (Field-Programmable Gate Array) to implement the conversion, which offers more flexibility but requires more development effort. For example, a Xilinx Spartan-6 FPGA can be programmed to decode HDMI using a