TMDS181: How to Force 1/40 Mode for HDMI2.0

Hi Kato,

To force 1/40 mode for HDMI2.0 you have to write 0x0B[1:0]=0x03, you disbale DDC training and manually set 1/40

SCL_SRC/SDA_SRC can be tied to GND

For SCL_SNK/SDA_SNK keep the 47k pull-ups to 5V to avoid incoming noise.

Regards

Hi Moises-san,

Thank you for your quick response.

Is my understanding correct as below ?

- HDMI1.4b
  to force 1/10 mode ---> 0x0B[1:0] = 0x01

- HDMI2.0a
  to force 1/40 mode ---> 0x0B[1:0] = 0x03

Best regards,
Kato

Hi Kato-san,
Your understanding is correct and diagram looks good.
Follow the recommendations of HDMI scalar vendor to select the right value of the pull-ups close to HDMI scalar.
Regards

Hi Kato-san,

I almost forgot about Tx termination for HDMI2.0
- HDMI1.4b
to force 1/10 mode ---> 0x0B[1:0] = 0x01

- HDMI2.0a
to force 1/40 mode ---> 0x0B[1:0] = 0x1B
This is to select Tx termination 75 to 150 ohms, as required by HDMI2.0 specification.

Regards

Hi Moises-san,

Thank you for your response.

As mentioned above, is it reasonable that Tx termination for HDMI1.4b is "No termination(0x0B[7:0] = 0x01)" ?
In addition, could you please tell me your recommended setting regarding 0x0A[1:0] DEV_FUNC_MODE ?

Best regards,
Kato

Hi Kato-san,

For HDMI1.4b there was no Tx termination defined, Tx termination was introduced for HDMI2.0(data rates > 3.4Gbps)

TMDS181 has two functional modes, redriver and retimer, by default TMDS181 monitors data rate and select between redriver or retimer

0x0A[1:0]

00:    Redriver mode 250Mbps - 3.4Gbps (no support for HDMI2.0)

01:    Automatic redriver to retimer crossover at 1Gbps

10:    Automatic retimer when HDMI2.0, based on TMDS_CLK_RATIO

11:    retimer mode 250Mbps - 6Gbps, (extra configuration is needed)

Default mode works for most cases if the customer is using HDMI2.0 only, then, default mode should work fine.

Regards

Hi Moises-san,

Thank you for your support.

As mentioned above, TMDS181 doesn't work based on TMDS_CLK_RATIO since SCL_SNK and SDA_SNK aren't connect to HDMI RX/Scalar.
So, could you please tell me each recommended configuration to force HDMI1.4b mode and HDMI2.0a mode for 0x0A[1:0] register ?

Best regards,
Kato

Hi Kato-san,
Default configuration is the right one
01: Automatic redriver to retimer crossover at 1Gbps
In this mode TMDS181 detects the data rate, it doesn't monitor TMDS_CLOCK_RATIO

This configuration can work too
10: Automatic retimer when HDMI2.0, based on TMDS_CLK_RATIO
Although SCL_SNK/SDA_SNK is not connected, the customer will assert TMDS_CLK_RATIO_STATUS by I2C, togglin in PD_EN is needed.

Another option is to swtich between fully redriver mode and fully retimer mode when asserting TMDS_CLOCK_RATIO_STATUS but default mode should work fine

Regards

Hi Moises-san,

Thank you for your information.

I understood "DEV_FUNC_MODE".
By the way, does TMDS181 automatically change between HDMI1.4b mode and HDMI2.0a mode even if SCL_SNK and SDA_SNK aren't connect to HDMI RX/Scalar ?
Could you please tell me the recommended register configuration if yes ?

Best regards,
Kato

Hi Kato-san,

In DEV_FUNC_MODE = 01: Automatic redriver to retimer crossover at 1Gbps, TMDS181 detects the data rate in high-speed lanes, it doesn't monitor SCL_SNK/SDA_SNK(register TMDS_CLOCK_RATIO)

In DEV_FUNC_MODE = 10: Automatic retimer when HDMI2.0, based on TMDS_CLK_RATIO_STATUS, if DDC train is disabled, TMDS181 doesn't monitor SCL_SNK/SDA_SNK but configures itself according to TMDS_CLK_RATIO_STATUS. Write the right value in TMDS_CLK_RATIO_STATUS, then, toggle PD_EN and TMDS181 will switch between HDM1.4 and HDMI2.0

However, DDC (SCL/SDA GPU-connector, connector-Scaler) is used to read EDID registers, in this way the GPU discovers the display's capabilities.

I assume there should be a DDC connection between GPU and Scaler, I don't know if the GPU is getting the configuration data in another way and the Scalar can set its TMDS_CLOCK_RATIO bit in another way too.

Regards

Hi Moises-san,

Thank you for your quick response.

Which should the 0x0A[1:0] register be set to 0x01 or 0x10 if SCL_SNK and SDA_SNK are connected to HDMI RX/Scalar and automatically changing between HDMI1.4b mode and HDMI2.0a mode ?
So, could you please give me your advice about it ?

Best regards,
Kato