TMDS1204EVM: Optimal AC-Coupled Mode Setup & I2C Register Programming for Basic Sanity Testing

Mahdi Osman

Part Number: TMDS1204EVM
Other Parts Discussed in Thread: TMDS1204, TDP1204,

Tool/software:

Hi everyone,

I'm working with the TMDS1204 HDMI redriver EVM and need guidance on setting it up in AC-coupled mode with I2C programming for basic sanity and stability testing. I want to pass a signal from a PC video source through to an HDMI sink/display for validation and oscilloscope probing.

Hardware Setup:

PC HDMI output → TMDS1204 EVM input
TMDS1204 EVM output → HDMI display (or breakout adapter for scope measurements)
Using Aardvark I2CSPI host for register programming

Current Configuration Questions:

AC-Coupled Mode Setup: For sink-side applications, I believe I need:

J27 jumper: Short 1 to 2 (AC_EN enabled)

Ensure 100nF capacitors are populated on OUT_D[2:0] and OUT_CLK lanes

499Ω pull-down resistors to ground after coupling caps
Essential I2C Register Programming:
What's the minimal register configuration needed for reliable signal pass-through? Based on the datasheet, I'm thinking:

0x09, 0x00 // Exit power-down (PDEN = 0)
0x0A, 0x00 // Enable rate and TXFFE snooping
0x0D, 0xA3 // Linear mode, DC-coupled TX, 0dB DCG, 100Ω term
0x0E, 0x97 // CTLE selection for HDMI 1.4/2.0/2.1
0x11, 0x0F // Enable all four lanes
DDC/HPD Configuration:
Should I enable the DDC level shifter (J31: short 2-3) and ensure proper HPD routing (populate R131, depopulate R129) for standard operation?

Specific Goals:

Achieve basic video pass-through from PC to display
Enable oscilloscope probing of TMDS lanes at output
Maintain link stability for various HDMI modes (1.4/2.0/2.1) but primarily for HDMI 2.1
Minimize register complexity while ensuring reliable operation

Questions:

Are there any critical registers I'm missing for AC-coupled operation?
What's the recommended approach for EQ settings in a sink application?
Should I disable certain features (like AEQ) for initial testing?
Any specific considerations for different HDMI data rates?

Any insights from those who've successfully configured the TMDS1204 in similar setups would be greatly appreciated!

Thanks in advance!

3 months ago

0 David (ASIC) Liu 3 months ago

TI__Guru**** 181321 points

Hi,

Are you trying to AC couple the TMDS1204 EVM input as well?

If you are using I2C to configure the TMDS1204, then please write to bit 6 of register 0x0D to disable/enable the AC_EN, and not using J27. I would also write 0x06 to register 0x09 to force TMDS1204 into active mode.

499-ohm resistor also no longer needed to be populated.

You do need to enable the external DDC level shifter (J31: short 2-3)

HPD routing (populate R131, depopulate R129) for standard operation is correct.

Thanks

David

0 Mahdi Osman 3 months ago in reply to David (ASIC) Liu

Prodigy 20 points

Hey David,

AC coupling is only needed on the output side. I'll give your suggestions a try and see how that pans out.
Thanks for the quick reply and suggestion!

0 David (ASIC) Liu 3 months ago in reply to Mahdi Osman

TI__Guru**** 181321 points

Hi,

Sounds good, please let me know if any additional support is needed.

Thanks

David

0 Mahdi Osman 3 months ago in reply to David (ASIC) Liu

Prodigy 20 points

Hey David,

I gave your suggestions a shot and seems like I was able to get some signal out of the display sink but the image on the monitor wasn't stable. So I'm assuming I will need to play around with the EQ/CTLE, Swing, etc. I was also wondering if you had any sample register programming scripts by chance?

0 David (ASIC) Liu 3 months ago in reply to Mahdi Osman

TI__Guru**** 181321 points

Hi,

There are script examples in section 8.4 Programming of the TDP1204 datasheet, you can use them as a starting point.

Thanks

David

0 Mahdi Osman 3 months ago in reply to David (ASIC) Liu

Prodigy 20 points

Thanks David, I was able to get it all sorted for getting a basic signal through but had another request now.

We've been having a bit of an issue getting signal flow through with the following setup:
BERT → Wilder HDMI 2.1 → HDMI Cable → TMDS1204 EVM (AC-coupled) → Wilder HDMI 2.1 → Scope

Have you had any experience with this sort of setup and are there any crucial steps to be mindful of?

0 David (ASIC) Liu 3 months ago in reply to Mahdi Osman

TI__Guru**** 181321 points

Hi,

Is the TMDS1204EVM input and output AC coupled, or is input AC coupled and output DC coupled?

Thanks

David

0 Mahdi Osman 3 months ago in reply to David (ASIC) Liu

Prodigy 20 points

Currently only the output is AC-Coupled since we installed 100nF AC-Coupling capacitors on that side. The input side has remained the same so I believe the default is DC-Coupled. Is that possibly a cause for my issues?

0 David (ASIC) Liu 3 months ago in reply to Mahdi Osman

TI__Guru**** 181321 points

Hi,

The TMDS1204 input DC common mode voltage is typical of 3.3V. So you would want to put AC coupling capacitor on the Bert output if the Bert output can not handle the 3.3V common mode voltage.

If you are using TMDS1204 output as AC-coupled, please make sure AC_EN is being pulled high.

Please also make sure TMDS1204 HDP_SNK is driven high to 5V so TMDS1204 will be in normal operation mode.

Thanks

David

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TMDS1204EVM: Optimal AC-Coupled Mode Setup & I2C Register Programming for Basic Sanity Testing