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Original question:

TMDS442: Register read/write works but no switch occurs

TMDS442: Unable to Transmit TMDS Signals Through Two Devices in Series

Andrew Douglass
Prodigy 10 points
Part Number: TMDS442

Tool/software:

I’m designing a board with two TMDS442 devices connected in series, but I’m encountering an issue where I’m unable to successfully transmit TMDS signals through the devices. According to the datasheet (Figure 39), this configuration should be possible.

Here are the steps I’ve taken and the status information I’ve obtained from both TMDS442 devices while testing with Source 4 to Sink 2 on both devices:

  • Register 0x01: 0b100 – Confirmed that Sink 1 is disabled.
  • Register 0x02: 0b1011 – Confirmed that Sink 2 is enabled, and the source is set to Source 4.
  • Register 0x03: 0b11000 – Verified that I can see 5V_EN and 5V_Power from Source 4.

However, I noticed that the SP bit is not being set to H. This suggests that the device is currently prioritizing Sink 1 over Sink 2 when both have the same source. Since I’ve explicitly disabled Sink 1, I don’t believe this should affect the operation, but I’m mentioning it in case it’s relevant.

Here’s the issue I’m observing:

  • When I connect my computer to Source 4 and a monitor to Sink 2 of the second TMDS442 device, the computer recognizes some form of connection (either HPD or 5V detected) and attempts to establish communication with the monitor.
  • Despite this, no TMDS signals are being transmitted to the monitor.
  • I confirmed this by testing with an HDMI test device, which also shows that no TMDS signals are going through.

I’ve double-checked the I2C commands and hardware setup, and everything seems to align with the datasheet. Monitor works normally when TMDS442 board isn't installed. I’ve ensured that the I2C lines between devices have their own dedicated pull-up resistors, separate from the pull-ups used for the main lines controlling the TMDS442 chips. Additionally, I’ve experimented with different resistance values for these pull-ups to ensure proper communication, but the issue persists. 

Here are some multi-meter measurements:

  • Sink and source 3V HPD
  • Source VDD is 5V, sink VDD is 3V
  • I see 3V on the monitor TMDS lines and the CLK line
  • All of my I2C lines are pulled up to VDD, but no communication is noted on the lines

If anyone has experience troubleshooting the TMDS442 in a similar setup, I’d appreciate your insights! I’m happy to share schematics and board layout via DM.

Thanks in advance for your help!