To test an HDMI to 4 lane MIPI DSI adapter with an oscilloscope, you need to verify three critical signal domains: the HDMI input side, the bridge chip output (typically a converter like LT8918 or TC358748), and the MIPI DSI differential pairs. Start by connecting a 4-channel oscilloscope with at least 1 GHz bandwidth and 5 GS/s sampling rate, because MIPI DSI lanes run at 80 Mbps to 1 Gbps per lane. Use active differential probes with <1 pF loading, like the Keysight N2750A, to avoid loading the high-speed signals. First, probe the HDMI TMDS clock channel on the adapter’s input side—expect a differential swing of 400 mV to 600 mV peak-to-peak at 25 MHz to 340 MHz depending on resolution. For 1080p at 60 Hz, the HDMI clock is 148.5 MHz; measure the jitter—it should be under 0.15 UI (unit interval) per HDMI spec. Next, move to the bridge chip’s output: the MIPI DSI clock lane (DDR clock) should show a differential voltage of 200 mV to 300 mV peak-to-peak, with a common-mode voltage around 200 mV. For a 4-lane configuration, each data lane (D0, D1, D2, D3) plus the clock lane must be probed individually. Use the oscilloscope’s eye diagram function to check the eye opening—minimum 0.18 UI for data lanes at 1 Gbps. Capture the data lanes’ differential signals; they should show a clean, symmetric eye with no overshoot exceeding 10% of the swing. Also, verify the LP (low-power) mode transitions: the MIPI DSI bus idles at LP-11 state (1.2 V on both Dp and Dn), then transitions to HS (high-speed) mode. The oscilloscope’s persistence mode helps spot glitches on these transitions. Finally, check the adapter’s power rails: the 1.8 V and 3.3 V supplies should have ripple under 50 mV peak-to-peak at 100 MHz bandwidth. If you’re using a low-cost scope, a 500 MHz bandwidth with 2.5 GS/s might work for 720p, but for 4K at 60 Hz, you’ll need the 1 GHz scope. For a specific adapter board, refer to the hdmi to 4 lane mipi dsi adapter datasheet for exact pinout and timing specs.
Understanding the Signal Chain: HDMI Input to MIPI DSI Output
The adapter board contains a bridge chip—common ones are the LT8918 from Lontium or the TC358748 from Toshiba—that converts HDMI TMDS signals to MIPI DSI. The HDMI input carries three data channels (red, green, blue) plus a clock, all differential. Each TMDS pair has a nominal impedance of 100 ohms differential, and the signal swing is 400 mV to 600 mV peak-to-peak. The oscilloscope must be set to AC coupling with 50-ohm termination to match the impedance. Probe the HDMI clock first: for 1080p at 60 Hz, the clock frequency is 148.5 MHz; for 4K at 30 Hz, it’s 297 MHz. Measure the clock jitter using the oscilloscope’s histogram function—peak-to-peak jitter should be less than 0.3 UI (2 ns for 148.5 MHz). If jitter exceeds this, the bridge chip might have poor PLL locking, causing MIPI lane errors. On the HDMI data lanes, check the skew between channels; HDMI spec allows up to 0.15 Tbit of skew, which at 148.5 MHz is about 1 ns. Use the scope’s deskew function to align the channels for accurate eye measurements.
After the bridge chip, the output is MIPI DSI with 4 data lanes and 1 clock lane. Each lane is a differential pair with 100-ohm impedance, but the signal levels are different: in HS mode, the differential swing is 200 mV to 300 mV peak-to-peak, with a common-mode voltage of 200 mV ± 40 mV. In LP mode, the voltage swings from 0 V to 1.2 V single-ended. The oscilloscope must handle both modes. For HS mode, use the scope’s differential math function: subtract channel A from channel B to get the differential signal. Set the vertical scale to 50 mV/div and timebase to 2 ns/div for 1 Gbps data. The eye diagram should show a clear opening with at least 0.18 UI of eye width at 1e-12 BER. For a 4-lane configuration, each lane must be tested individually because lane-to-lane skew can cause data corruption. The MIPI spec allows maximum skew between lanes of 0.2 UI plus 0.5 ns. For 1 Gbps, 0.2 UI is 200 ps, so total skew tolerance is 700 ps. Measure the rising edge of each lane’s differential signal relative to the clock lane; if skew exceeds 700 ps, the display will show artifacts.
Probing Techniques for High-Speed MIPI DSI Signals
Probing MIPI DSI signals is tricky because the differential pairs are often on fine-pitch connectors (0.5 mm pitch FPC or 0.4 mm pitch BGA). Use a solder-in probe tip or a differential probe with a micro-miniature clamp. The probe’s input capacitance must be under 0.5 pF to avoid loading the 100-ohm differential line. A common mistake is using a passive probe with 10 pF input capacitance, which will kill the signal. Instead, use an active differential probe like the Tektronix TDP1500 (1.5 GHz bandwidth, 0.8 pF loading). Connect the probe’s positive lead to Dp and negative to Dn of the clock lane. For data lanes, probe each pair sequentially. Set the oscilloscope to trigger on the clock lane’s differential signal; for HS mode, the clock is a 50% duty cycle square wave. Use the scope’s pattern trigger to capture specific data patterns, like the DSI sync sequence (0x00 0x00 0x00 0x01). The sync sequence is a 32-bit packet that starts with three zero bytes and a 0x01 byte. Capture this with the scope’s serial decode option if available; otherwise, use the waveform to measure the bit period. For 1 Gbps, the bit period is 1 ns; the rise time (20% to 80%) should be under 150 ps. If the rise time is slower, the adapter’s output driver might be weak or the PCB trace length is too long.
Another critical test is the LP-to-HS transition. In MIPI DSI, the bus starts in LP mode (1.2 V on both lines), then the transmitter drives a HS-0 state (0 V on both lines) for 100 ns, followed by the HS clock. Use the oscilloscope’s single-shot mode to capture this transition. The LP-11 state should show 1.2 V ± 0.1 V on both Dp and Dn. The HS-0 state should show 0 V ± 50 mV on both lines. The transition time from LP to HS should be under 100 ns. If it’s slower, the display might miss the first few pixels. Also, check the termination voltage: the adapter’s receiver should have a 100-ohm differential termination to 1.2 V. Probe the voltage across the termination resistor—it should be 200 mV ± 40 mV in HS mode. If it’s outside this range, the adapter’s voltage regulator might be faulty.
Power Integrity and Noise Analysis
The adapter board’s power supply is often overlooked but critical for signal integrity. The bridge chip requires 1.8 V for core logic and 3.3 V for I/O, plus a 1.2 V supply for the MIPI PHY. Use the oscilloscope’s AC coupling mode with a 10x probe to measure ripple on these rails. Set the vertical scale to 10 mV/div and timebase to 1 µs/div. The ripple should be under 50 mV peak-to-peak at 100 MHz bandwidth. For the 1.2 V rail, ripple above 30 mV can cause MIPI jitter. Use the scope’s FFT function to identify noise frequencies: switching noise from the HDMI source (e.g., 148.5 MHz harmonics) or from the adapter’s internal DC-DC converter (typically 1 MHz to 2 MHz). If you see a 148.5 MHz spike on the 1.2 V rail, the HDMI clock is coupling through the bridge chip’s substrate. Add a ferrite bead or a 100 nF capacitor near the chip’s power pin to filter it. Also, measure the power-up sequence: the 1.8 V rail should come up before the 3.3 V rail, with a delay of at least 1 ms. Use the oscilloscope’s persistence mode to capture the ramp-up time—if the 3.3 V rail rises faster than the 1.8 V rail, the chip might latch up.
Ground bounce is another issue: the adapter’s ground plane should have low impedance. Use a ground spring on the probe tip instead of the long ground lead to reduce loop inductance. For a 4-layer board, the ground plane impedance should be under 10 mΩ at 1 GHz. Measure the ground bounce by probing between the adapter’s ground pin and the oscilloscope’s ground—any voltage above 50 mV peak-to-peak indicates a poor ground connection. This is common when the adapter is powered from a USB port with long cables. For accurate measurements, power the adapter from a bench supply with short leads and a 100 µF electrolytic capacitor at the input.
Data Rate and Resolution Testing
Test the adapter at multiple resolutions to verify the PLL bandwidth. For 720p at 60 Hz (74.25 MHz HDMI clock), the MIPI DSI clock is 371.25 MHz (5x multiplier). For 1080p at 60 Hz (148.5 MHz HDMI clock), the MIPI clock is 742.5 MHz. For 4K at 30 Hz (297 MHz HDMI clock), the MIPI clock is 1.485 GHz. Use the oscilloscope’s frequency counter to measure the MIPI clock frequency; it should be within ±100 ppm of the expected value. If the frequency is off, the bridge chip’s PLL might have a bad reference. The data rate per lane is 2x the clock frequency because MIPI DSI uses DDR (double data rate). For 4K at 30 Hz, each lane runs at 2.97 Gbps, which requires a 1.5 GHz bandwidth scope. At this rate, the eye diagram should show a vertical opening of at least 100 mV and a horizontal opening of 0.2 UI. If the eye is closed, the adapter’s PCB trace length might be too long—for 4 lanes, each trace should be length-matched within 5 mm. Use the scope’s built-in TDR (time domain reflectometer) function to measure the trace impedance. The impedance should be 100 ohms ± 10% for each differential pair. If it’s off, the adapter will have reflections that cause data errors.
Test the adapter with a known-good display panel, like a 5.5-inch 1080p MIPI DSI panel. Connect the adapter to the panel and capture the DSI packets with the oscilloscope. Look for the video stream packets: the DSI specification defines a packet structure with a 4-byte header (data type, virtual channel, word count, ECC), followed by the pixel data, and a 2-byte checksum. For a 1080p panel, each line has 1920 pixels, and each pixel is 24 bits (RGB888). The data rate is 1920 x 24 x 60 = 2.76 Gbps, spread across 4 lanes at 0.69 Gbps per lane. Use the scope’s serial decode to verify the packet structure. If the decoder shows CRC errors, the adapter is introducing bit errors. Measure the bit error rate (BER) by capturing 1 million bits and counting errors—the BER should be below 1e-12. If it’s higher, check the HDMI source’s signal quality first.
Common Pitfalls and Troubleshooting Data Table
Below is a table of common issues found when testing HDMI to 4-lane MIPI DSI adapters, with oscilloscope measurements and fixes:
| Issue | Oscilloscope Observation | Typical Cause | Fix |
|---|---|---|---|
| No display output | No MIPI clock signal; HDMI clock present but no PLL lock | Bridge chip not powered or HDMI source not detected | Check 1.8 V and 3.3 V rails; verify HDMI cable is connected |
| Flickering image | MIPI clock jitter > 0.3 UI; eye diagram closed | HDMI source jitter or poor PLL bandwidth | Use a shorter HDMI cable; add ferrite bead on HDMI clock |
| Color banding | Data lane skew > 700 ps between lanes | PCB trace length mismatch | Redesign PCB with length-matched traces |
| No image but backlight on | LP mode stuck at LP-11; no HS transition | Bridge chip firmware issue or wrong DSI configuration | Update firmware; check I2C configuration registers |
| Intermittent black screen | Power rail droop during HS mode; 1.2 V ripple > 50 mV | Insufficient decoupling capacitors | Add 10 µF ceramic cap near MIPI PHY |
When you see a closed eye diagram, zoom into the rising edge of the differential signal. The overshoot should be less than 10% of the swing (30 mV for 300 mV swing). If overshoot exceeds this, the adapter’s output driver has poor impedance matching. Use the scope’s TDR to measure the trace impedance—if it’s not 100 ohms, add a series resistor (typically 10 ohms to 33 ohms) on each lane near the connector. For the HDMI input side, check the TMDS signal’s common-mode voltage—it should be 3.3 V ± 0.1 V. If it’s lower, the HDMI source might be using a different standard. Also, measure the HDMI 5 V power pin—the adapter should draw under 500 mA. Use a current probe to measure the adapter’s power consumption; for a 1080p display, it should be around 200 mA at 5 V. If it’s higher, the bridge chip might be overheating—check the chip’s temperature with a thermal camera; it should be under 85°C.
Advanced Measurements: Jitter Decomposition and BER Estimation
For a thorough test, use the oscilloscope’s jitter analysis software to decompose the total jitter (TJ) into random jitter (RJ) and deterministic jitter (DJ). The MIPI DSI spec requires TJ to be under 0.3 UI at 1e-12 BER. For a 1 Gbps lane, 0.3 UI is 300 ps. Capture 100,000 bits and run the jitter decomposition. The RJ component should be under 5 ps RMS, and the DJ component should be under 200 ps. If