Emulating HDMI connection

If you want to play with HDMI signals or just emulate a connected display for whatever reason, you need to fool your videocard to think something is really connected.

The display detection process starts after sensing 5V on Hot Plug Detect (pin 19). After this, the computer reads data via Display Data Channel (pins 15, 16) which is a standard I2C bus. It expects EDID data on address 0x50 and the standard way how this is achieved is by using 24C(00, 01, 02, …) I2C EEPROM connected to this bus inside your screen.

In Linux, you can access this bus via /dev/i2c-x. There is also a tool which can scan through all available i2c’s and detect EDID data. So I connected a regular display and run it:

$ sudo get-edid | hd
This is read-edid version 3.0.2. Prepare for some fun.
Attempting to use i2c interface
No EDID on bus 0
No EDID on bus 1
No EDID on bus 2
No EDID on bus 3
No EDID on bus 4
No EDID on bus 6
No EDID on bus 7
No EDID on bus 8
No EDID on bus 9
No EDID on bus 10
No EDID on bus 11
No EDID on bus 12
No EDID on bus 13
No EDID on bus 14
No EDID on bus 15
No EDID on bus 16
No EDID on bus 17
No EDID on bus 18
2 potential busses found: 5 19
Will scan through until the first EDID is found.
Pass a bus number as an option to this program to go only for that one.
128-byte EDID successfully retrieved from i2c bus 5
Looks like i2c was successful. Have a good day.
00000000 00 ff ff ff ff ff ff 00 10 ac 7a a0 53 36 37 30 |……….z.S670|
00000010 30 17 01 03 80 34 20 78 ea ee 95 a3 54 4c 99 26 |0….4 x….TL.&|
00000020 0f 50 54 a1 08 00 81 40 81 80 a9 40 b3 00 d1 c0 |.PT….@…@….|
00000030 01 01 01 01 01 01 28 3c 80 a0 70 b0 23 40 30 20 |……(<..p.#@0 |
00000040 36 00 06 44 21 00 00 1a 00 00 00 ff 00 30 46 46 |6..D!……..0FF|
00000050 58 44 33 42 53 30 37 36 53 0a 00 00 00 fc 00 44 |XD3BS076S……D|
00000060 45 4c 4c 20 55 32 34 31 32 4d 0a 20 00 00 00 fd |ELL U2412M. ….|
00000070 00 32 3d 1e 53 11 00 0a 20 20 20 20 20 20 00 34 |.2=.S… .4|
00000080

And it really has found my display on /dev/i2c-5. Then I saved the data by running this command to have some working EDID data.

$ sudo get-edid > dell.edid

Next I assembled a testing circuitry on an evaluation board.

I loaded the data with CH341a programmer and connected to my HDMI but nothing happened. So I started to investigate. I have used 24C128 EEPROM which should be just like a 24C00 but with a bigger size. But certainly not. It uses 2 bytes for addressing the memory while 24C00 to 24C16 use only one. So I quickly replaced it with 24C02 and Bingo! computer detected DELL display.

After this, I assembled a nicer board and it works perfectly:

If you want to try this, here is the schematic:

Meanwhile, I have found out that there is an utility for writing EDID directly via HDMI without an external programmer: edid-rw . So you can just attach a new uninitialized EEPROM and program it after connecting. You might see following error in dmesg but this doesn’t cause any issues:

[ 6192.520241] i915 0000:00:02.0: HDMI-A-1: EDID is invalid:
[ 6192.520248] [00] BAD 00 ff ff ff ff ff ff 00 ff ff ff ff ff ff ff ff
[ 6192.520251] [00] BAD ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
[ 6192.520253] [00] BAD ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
[ 6192.520254] [00] BAD ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
[ 6192.520256] [00] BAD ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
[ 6192.520258] [00] BAD ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
[ 6192.520260] [00] BAD ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
[ 6192.520262] [00] BAD ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff

This is how to write it:

$ sudo ./edid-rw -w <i2c_dev_number> < dell.edid 

If you are not much into soldering, you can buy ready made modules on aliexpress:

One without an output for $1.40:

And one with an output for $2.83:

But absolutely the cheapest solution is to buy a HDMI angle adapter for $0.81 and solder in 24C02 for $0.04 per piece.

That’s it, I hope I helped someone. Please share you thoughts in comments.

Bye.

Sources:
https://www.datapro.net/techinfo/hot_plug_detection.html
https://en.wikipedia.org/wiki/HDMI

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