The data rate of an HDMI to MIPI DSI bridge is not a single fixed number—it depends heavily on the specific bridge chip, the HDMI input version, the MIPI DSI output configuration (number of lanes, lane speed, and resolution), and the signal integrity of the PCB design. In practice, for a typical bridge like the LT8912B or TC358870XBG, the maximum effective data rate from HDMI 1.4 input to MIPI DSI output can reach up to 3.4 Gbps per lane on the DSI side when using 4 lanes, but this is under ideal conditions. Most common bridges support HDMI 1.4a input, which has a maximum TMDS clock of 340 MHz and a data rate of 3.4 Gbps per channel (three channels total, so 10.2 Gbps raw), but the MIPI DSI output is usually limited to around 1.0 to 1.5 Gbps per lane in real-world applications, especially for 4-lane configurations. For example, a bridge driving a 1920x1080@60Hz 24-bit RGB display requires roughly 3.2 Gbps of uncompressed video data, which fits comfortably within a 4-lane DSI link running at 1.0 Gbps per lane (4 Gbps total), but if you push to 4K@30Hz (3840x2160, ~6.4 Gbps), you need either higher lane speeds or more lanes, which many bridges cannot handle. The actual throughput is also constrained by the bridge's internal buffer size, clock recovery PLL jitter, and the EDID handshake with the HDMI source. A common chip like the hdmi to 4 lane mipi dsi adapter board based on the LT8912B chip supports up to 1920x1080@60Hz with 4-lane DSI at 1.2 Gbps per lane, giving you a total of 4.8 Gbps raw bandwidth, but after overhead (packetization, blanking, and ECC), the usable video data rate is around 3.5 Gbps. For 2K resolutions like 2560x1440@60Hz, you need roughly 5.6 Gbps, which exceeds the 4.8 Gbps raw limit, so you'd have to drop to 30Hz or use compression like DSC (Display Stream Compression) if the bridge supports it—but most low-cost bridges don't. The data rate also varies with color depth: 24-bit RGB uses 8 bits per channel, but 30-bit (10-bit) or 36-bit (12-bit) deep color can double or triple the required bandwidth. For instance, a 1080p@60Hz 30-bit display needs about 4.8 Gbps, which is right at the edge for a 4-lane 1.2 Gbps DSI link. The HDMI input side also imposes limits: HDMI 1.4a maxes out at 340 MHz pixel clock, which for 24-bit color gives 10.2 Gbps raw, but the bridge's internal processing (color space conversion, scaling, or frame buffer) can reduce the effective throughput. Some bridges include a small FIFO buffer (e.g., 128 KB to 512 KB) to handle clock domain crossing between HDMI's asynchronous TMDS clock and MIPI DSI's continuous clock, but this buffer can overflow if the HDMI input data rate exceeds the DSI output rate for more than a few lines. The MIPI DSI specification itself allows lane speeds from 80 Mbps to 2.5 Gbps per lane (for D-PHY version 2.0), but most bridges are limited to D-PHY 1.2, which caps at 1.5 Gbps per lane. In practice, many boards use 1.0 Gbps per lane to ensure signal integrity over longer traces or FPC cables. The data rate is also affected by the number of DSI lanes: a 2-lane configuration halves the total bandwidth compared to 4-lane, so for a 1080p@60Hz display, you'd need each lane to run at 2.0 Gbps, which is beyond the typical 1.5 Gbps limit, making 2-lane bridges only suitable for lower resolutions like 720p or 800x480. The table below summarizes typical data rates for common bridge configurations:
Table 1: Typical HDMI to MIPI DSI Bridge Data Rates
| Resolution | Refresh Rate | Color Depth | HDMI Input Data Rate (Gbps) | DSI Lanes | DSI Lane Speed (Gbps) | Total DSI Bandwidth (Gbps) | Usable Video Data Rate (Gbps) | Bridge Chip Example |
|------------|--------------|-------------|-----------------------------|-----------|-----------------------|----------------------------|-------------------------------|---------------------|
| 1920x1080 | 60 Hz | 24-bit | 3.2 | 4 | 1.0 | 4.0 | 3.2 | LT8912B |
| 1920x1080 | 60 Hz | 30-bit | 4.8 | 4 | 1.2 | 4.8 | 3.8 | TC358870XBG |
| 1280x720 | 60 Hz | 24-bit | 1.4 | 2 | 0.8 | 1.6 | 1.3 | SSD2828 |
| 3840x2160 | 30 Hz | 24-bit | 6.4 | 4 | 1.5 | 6.0 | 4.8 | LT8918B (limited) |
| 800x480 | 60 Hz | 18-bit | 0.3 | 2 | 0.2 | 0.4 | 0.3 | RH850 (custom) |
The data rate also depends on the bridge's ability to handle blanking intervals. HDMI includes horizontal and vertical blanking periods (e.g., for 1080p@60Hz, Hblank is 280 pixels, Vblank is 45 lines), which consume about 20% of the total bandwidth. The MIPI DSI protocol also uses blanking packets (HFP, HBP, VFP, VBP) and packet overhead (8 bytes per packet header, plus CRC). The bridge must convert HDMI's TMDS data stream into MIPI DSI packets, and this conversion introduces latency and overhead. For example, a typical bridge might have a latency of 1-2 scanlines (about 30-60 microseconds for 1080p), which is fine for most displays but can cause issues with touch synchronization or video playback. The PLL (Phase-Locked Loop) in the bridge generates the MIPI DSI clock from the HDMI pixel clock, but if the HDMI clock is unstable or has jitter (e.g., from a cheap source), the DSI lane speed can drift, causing bit errors. Most bridges include a clock recovery circuit with a bandwidth of 100-200 kHz, which can track slow jitter but not fast jitter above 1 MHz. The data rate is also limited by the PCB trace length and impedance: for a 4-lane DSI link running at 1.0 Gbps, the trace length should be under 10 cm to avoid signal degradation, and the impedance must be matched to 50 ohms (single-ended) or 100 ohms (differential). If you use an FPC cable (common in display modules), the cable's capacitance and inductance can reduce the effective lane speed by 10-20%. For example, a 15 cm FPC cable with 0.5 mm pitch might limit the lane speed to 800 Mbps instead of 1.0 Gbps. The bridge's power supply also affects data rate: a 1.8V or 3.3V supply with 10% ripple can cause the DSI transmitter to fail at high speeds. Many bridges require a clean 1.2V core voltage and separate 1.8V for the DSI PHY. The data rate is also influenced by the HDMI source's EDID: if the bridge reports a lower resolution in the EDID, the source will output a lower pixel clock, reducing the data rate. For instance, if the bridge's EDID reports 1920x1080@60Hz with a 148.5 MHz pixel clock, the HDMI source will output 3.2 Gbps, but if the bridge reports 1366x768@60Hz, the pixel clock drops to 85.5 MHz, and the data rate to 1.8 Gbps. This is why some bridges include a programmable EDID that can be customized via I2C. The actual data rate you achieve also depends on the display's panel specifications: some panels require a specific MIPI DSI clock frequency (e.g., 500 MHz for a 1080p panel), which must match the bridge's output. If the bridge's PLL cannot generate that exact frequency, you may need to use a different resolution or refresh rate. For example, a 1920x1080 panel with a 60 Hz refresh rate typically needs a pixel clock of 148.5 MHz, but the MIPI DSI clock is usually half the lane speed (for DDR mode), so at 1.0 Gbps per lane, the DSI clock is 500 MHz. The bridge must divide the HDMI pixel clock to match this, but if the HDMI clock is 148.5 MHz and the DSI clock is 500 MHz, the ratio is 3.36, which is not an integer, causing fractional clock division and potential jitter. Some bridges use a fractional-N PLL to handle this, but it adds 50-100 ps of jitter, which can reduce the effective data rate by 5-10%. The data rate also varies with the color format: HDMI can use RGB, YCbCr 4:4:4, or YCbCr 4:2:2, which have different bandwidths. For example, 1080p@60Hz YCbCr 4:2:2 uses only 2.1 Gbps, allowing the bridge to run at lower lane speeds or use fewer lanes. However, most MIPI DSI panels expect RGB 4:4:4, so the bridge must convert from YCbCr to RGB, which adds processing delay and may reduce the data rate if the conversion is done in software. In terms of real-world testing, a typical HDMI to MIPI DSI bridge board (like the one based on LT8912B) can achieve a stable 1920x1080@60Hz with 24-bit color using 4 lanes at 1.0 Gbps per lane, giving a total data rate of 4.0 Gbps raw, but after overhead, the effective video data rate is about 3.2 Gbps. If you try to push 2560x1440@60Hz, the required data rate is 5.6 Gbps, which exceeds the 4.0 Gbps raw limit, so the bridge will either fail to display or show artifacts like flickering or tearing. Some bridges support downscaling (e.g., from 4K to 1080p) using a built-in scaler, but this reduces the data rate but also adds latency (up to 2 frames, or 33 ms at 60 Hz). The scaler uses a line buffer of 2-4 scanlines, which adds 30-60 microseconds of latency per line. The data rate for the scaled output is lower, but the input data rate from HDMI remains high. For example, if you input 4K@30Hz (6.4 Gbps) and scale to 1080p@30Hz (1.6 Gbps), the bridge must buffer the entire frame (about 8 MB for 4K 24-bit) in external DDR memory, which most low-cost bridges do not have. Only bridges with integrated DDR (like the LT8918B) can handle this, but they are more expensive. The data rate also affects power consumption: a bridge running at 1.0 Gbps per lane on 4 lanes draws about 200-300 mW, while at 1.5 Gbps per lane, it draws 400-500 mW due to higher switching losses. The thermal dissipation can cause the bridge to throttle if it exceeds 85°C, reducing the data rate by 10-20%. In summary, the data rate of an HDMI to MIPI DSI bridge is a complex interplay of chip capabilities, display resolution, color depth, lane count, signal integrity, and thermal management. For most practical applications, a 4-lane bridge running at 1.0 Gbps per lane provides a usable video data rate of about 3.2 Gbps, which is sufficient for 1080p@60Hz 24-bit but not for 4K or high refresh rates. The specific bridge chip and board design (like the one linked) determine the exact limits, and you should always check the datasheet for the maximum pixel clock and lane speed.