How to test dual screen HDMI to MIPI DSI adapter with oscilloscope?
How to Test Dual Screen HDMI to MIPI DSI Adapter with Oscilloscope
To test a dual screen HDMI to MIPI DSI adapter with an oscilloscope, you need to verify signal integrity, timing, and voltage levels across both MIPI DSI lanes and the HDMI input. Start by connecting the adapter to a stable HDMI source (e.g., a 1080p60 signal generator) and two MIPI DSI displays. Use a 4-channel oscilloscope with at least 1 GHz bandwidth and 5 GS/s sampling rate, like a Keysight Infiniium or Tektronix MSO series, to capture differential signals. Set probes to 10x attenuation and use differential probes for MIPI DSI data lanes (D0+, D0-; D1+, D1-; clock lane). Measure the HDMI TMDS signals first: check differential voltage swing (should be 400-600 mV peak-to-peak) and jitter (less than 0.3 UI at 1.65 Gbps for 1080p). Then, probe the MIPI DSI output: each lane should have a differential swing of 200-400 mV, common-mode voltage around 200 mV, and data rate up to 1 Gbps per lane. For dual-screen operation, verify that both displays receive synchronized clock and data—trigger on the clock lane and check for consistent data packets on both DSI ports. Use eye diagram analysis to quantify signal quality: aim for a vertical eye opening of at least 150 mV and horizontal opening of 0.5 UI. If the adapter uses a bridge chip like LT8912B or IT66121, monitor its I2C configuration lines for proper initialization. Always ground the oscilloscope and adapter to a common reference to avoid ground loops. This approach ensures the adapter delivers reliable signals to both MIPI DSI panels, critical for applications like automotive clusters or industrial HMI.
When testing a dual screen HDMI to MIPI DSI adapter with an oscilloscope, start with the HDMI input side. The HDMI source typically runs at 1.65 Gbps per lane for 1080p60, with TMDS clock at 148.5 MHz. Use a differential probe (e.g., Tektronix P7350) on the HDMI D2+ and D2- pins to capture the eye diagram. The HDMI specification requires a minimum differential voltage of 150 mV and a maximum of 800 mV, but typical adapters expect 400-600 mV. Measure rise time: should be 75-150 ps for 1.65 Gbps. If the signal shows overshoot above 800 mV or undershoot below -800 mV, the adapter might fail to lock. Next, check the HDMI clock pin: it should be a clean square wave at 148.5 MHz with jitter under 0.25 UI. Use the oscilloscope’s histogram function to measure peak-to-peak jitter—anything above 0.3 UI indicates a problem with the source or cable. For a dual-screen adapter, the HDMI input is often split internally, so a single HDMI source feeds both MIPI DSI outputs. This means the HDMI signal must be robust enough to drive two decoders. If you see amplitude drop or increased jitter, the adapter’s input buffer might be marginal. For example, the LT8912B chip in some adapters requires a minimum 400 mV swing to maintain PLL lock. Record these measurements in a table:
| Parameter | HDMI Input (Measured) | Specification | Pass/Fail |
|---|---|---|---|
| Differential voltage (mV peak-to-peak) | 520 | 400-600 | Pass |
| Rise time (ps) | 110 | 75-150 | Pass |
| Jitter (UI peak-to-peak) | 0.22 | <0.3 | Pass |
| Clock frequency (MHz) | 148.5 | 148.5 ± 0.1 | Pass |
Now move to the MIPI DSI output side. A typical dual screen HDMI to MIPI DSI adapter uses two DSI ports, each with one clock lane and up to four data lanes. For a 1080p display, you might need four data lanes at 1 Gbps each. Use a differential probe on the clock lane (CLK+, CLK-). The MIPI DSI clock should be a continuous differential signal with a frequency equal to the data rate divided by 2 (e.g., 500 MHz for 1 Gbps data). The common-mode voltage should be 200 mV ± 50 mV, and the differential swing should be 200-400 mV. Measure the clock jitter: MIPI DSI allows up to 0.2 UI of jitter at 1 Gbps. If the clock has excessive jitter, the data lanes will be corrupted. For data lanes, probe each lane pair (D0+, D0-; D1+, D1-; etc.). The data is transmitted in bursts during active video periods, with low-power states between bursts. Use the oscilloscope’s persistence mode to see the full eye diagram. The data eye should be open with at least 150 mV vertical opening and 0.5 UI horizontal opening. If the eye is closed, check for impedance mismatches—MIPI DSI requires 100 ohm differential impedance. The adapter’s PCB traces should be length-matched to within 5 mm. For dual-screen operation, both DSI ports must be synchronized. Trigger the oscilloscope on the clock lane of port 1, then measure the clock phase of port 2. The phase difference should be less than 1 ns for proper frame synchronization. If the phase shift is larger, the adapter’s clock distribution might be faulty. Here’s a sample measurement table for one DSI port:
| Parameter | DSI Port 1 (Measured) | MIPI Specification | Pass/Fail |
|---|---|---|---|
| Clock differential swing (mV) | 350 | 200-400 | Pass |
| Clock common-mode voltage (mV) | 210 | 200 ± 50 | Pass |
| Clock jitter (UI peak-to-peak) | 0.15 | <0.2 | Pass |
| Data lane D0 eye opening (mV) | 180 | >150 | Pass |
| Data lane D0 eye opening (UI) | 0.6 | >0.5 | Pass |
| Phase difference between ports (ns) | 0.8 | <1 | Pass |
Testing the adapter’s power delivery is also critical. The dual screen HDMI to MIPI DSI adapter typically requires 5V input at 1-2A, depending on the displays. Use a current probe or measure voltage drop across a shunt resistor on the power input. The oscilloscope should show a stable 5V ± 5% with ripple under 50 mV peak-to-peak. If the voltage drops below 4.75V during display switching, the adapter might reset. Also, check the MIPI DSI power pins (typically 1.8V or 1.2V for the chip). Use a single-ended probe to measure these rails. For example, the LT8912B’s core voltage should be 1.2V ± 0.06V with ripple under 30 mV. If the power is noisy, add decoupling capacitors. For dual-screen operation, the adapter might draw more current when both displays are active. Measure the current spike during initialization: it can reach 1.5A for 10 ms. If the oscilloscope shows a voltage sag, the power supply is inadequate. Document these measurements:
| Power Rail | Voltage (V) | Ripple (mV peak-to-peak) | Current (A) | Pass/Fail |
|---|---|---|---|---|
| Input 5V | 5.02 | 35 | 1.2 | Pass |
| Core 1.2V | 1.19 | 20 | 0.4 | Pass |
| I/O 1.8V | 1.81 | 25 | 0.3 | Pass |
Signal integrity testing also involves checking for reflections and crosstalk. Use time-domain reflectometry (TDR) with the oscilloscope if available, or measure the step response. For the MIPI DSI lanes, the impedance should be 100 ohms ± 10%. If the adapter has long traces (e.g., 10 cm), reflections can cause data errors. Set the oscilloscope to a 1 ns/div timebase and look for reflections after the first edge. A reflection amplitude over 10% of the signal indicates a mismatch. For dual-screen adapters, crosstalk between the two DSI ports is a concern. Probe one port’s data lane while the other port is active. The crosstalk voltage should be under 20 mV. If it’s higher, the PCB layout might have insufficient spacing. For example, a common issue is that the clock lane of port 1 couples into the data lane of port 2, causing bit errors. Use the oscilloscope’s FFT function to check for clock harmonics in the data lane—anything above -40 dBc is problematic. Also, verify the adapter’s I2C bus for configuration. The host sends commands to set the MIPI DSI format (e.g., RGB888, 24-bit) and resolution. Probe the I2C clock (SCL) and data (SDA) lines. The clock should be at 100 kHz or 400 kHz, with clean edges and no glitches. The data packets should match the expected sequence from the adapter’s datasheet. If the I2C bus has noise, the adapter might not configure correctly, leading to blank screens. Use a logic analyzer or the oscilloscope’s serial decode feature to verify the I2C transactions. For a dual-screen adapter, the I2C address might be different for each port (e.g., 0x48 for port 1, 0x49 for port 2). Check that both are addressed correctly. Here’s a sample I2C measurement:
| I2C Parameter | Measured | Expected | Pass/Fail |
|---|---|---|---|
| SCL frequency (kHz) | 398 | 400 | Pass |
| SDA rise time (ns) | 45 | <100 | Pass |
| Port 1 address | 0x48 | 0x48 | Pass |
| Port 2 address | 0x49 | 0x49 | Pass |
Temperature effects can also impact the adapter’s performance. The dual screen HDMI to MIPI DSI adapter might heat up during operation, especially if driving two high-resolution displays. Use a thermal camera or thermocouple to measure the chip temperature. The LT8912B, for example, has a maximum junction temperature of 125°C. If the oscilloscope shows signal degradation as the temperature rises, the adapter might have thermal issues. For instance, the clock jitter might increase from 0.15 UI at 25°C to 0.25 UI at 85°C. Measure the signal at 10-minute intervals during a 1-hour test. If the eye opening shrinks by more than 20%, consider adding a heatsink. Also, check the MIPI DSI cable quality. Long cables (over 30 cm) can attenuate the signal. Use the oscilloscope to measure the signal at the display end of the cable. The differential swing should be at least 150 mV. If it’s lower, the cable might be too long or have high capacitance. For dual-screen setups, both cables should be identical in length to avoid timing skew. Measure the propagation delay of each cable: a difference of more than 1 ns can cause data misalignment. Use the oscilloscope’s delay measurement function. For example, a 30 cm cable might have a delay of 1.5 ns, while a 50 cm cable has 2.5 ns. Keep both cables within 10 cm of each other. Finally, test the adapter with real-world content. Use a test pattern generator (e.g., a pattern with alternating black and white pixels) to stress the MIPI DSI lanes. The oscilloscope should show consistent data patterns without missing bits. For dual-screen operation, both displays should show the same image or different images if the adapter supports split-screen. Use the oscilloscope’s trigger on the vertical sync pulse to verify that both ports are synchronized. If one display lags by more than one frame (16.7 ms at 60 Hz), the adapter’s buffer might be mismanaged. Capture the sync pulses on both ports and measure the time difference. It should be less than 100 µs for seamless operation. This comprehensive testing ensures the dual screen hdmi to mipi dsi adapter meets the required specifications for reliable performance in demanding applications like automotive or medical displays.