About IMX8MP TMDS voltage level

[Yoshiharaaa]

I used a device with IMX8MP and took the certification test for HDMI 1.4b Source device.
However, the TMDS-VL item resulted in a FAIL determination.

>The FAIL items are as follows
ID7-2: TMDS-VL
　VL (L side voltage) of TMDS (D0+/-,D1+/-,D2+/-,DCLK+/-) is lower than the specified value.
　　Measured value VL=2.575～2.618V
　　Acceptable range VL=2.700V to 2.900V (max. 0.125V short)

Since low L-side voltage = large amplitude, we are considering measures to reduce the amplitude itself.

<Questions
1) Is it possible to change the drive strength of the TMDS line?
　
2) The TMDS line is directly connected to a connector from the CPU, with a common mode coil and ESD suppressor in between.
The EVK sample board has almost the same configuration. Can we expect any effect on the voltage values (amplitude) by changing the DCR value, etc.?

Please let me know how to solve this problem.

Thank you.
Yoshihara

[joanxie]

please try below tuning which has been implemented in latest BSP:

/unit_tests/memtool -32 0x32fdff5c=0x32

/unit_tests/memtool -32 0x32fdff60=0x60

[Yoshiharaaa]

Thank you for your reply.
When I tried it right away, I was able to see changes in the voltage level of the waveform.

If you know where this setting is located in the source code, could you please let me know?

Thank you.

Yoshihara

[joanxie]

this isn't from source code, they are registers, you can find them from reference manual,

you can search 13.10.3.1.1 HDMI_TX_PHY memory map from reference manual

the base address is 32FD_0000h, then you can find these registers

if you need to know where define them you can refer to the dtis file

"https://github.com/nxp-imx/linux-imx/blob/lf-6.1.y/arch/arm64/boot/dts/freescale/imx8mp.dtsi"

this dtsi file define the phy base address, and phy driver is

"https://github.com/nxp-imx/linux-imx/blob/lf-6.1.y/drivers/phy/freescale/phy-fsl-samsung-hdmi.c":

[Yoshiharaaa]

Thank you for your support in this matter.
I will try using the information you provided.

Thank you.
yoshihara