Reverse Engineering The Apple Touch Bar Screen

The Apple Touch Bar was an oddity on a fairly small number of Apple laptops which replaced the function key row with a touch display. Yet what is special about this display other than its odd form factor when you consider it as a generic touch display? As [Wenting Zhang] describes in a recent reverse-engineering video, this 2,170 x 60 pixel display is somewhat limited in that it doesn’t support the MIPI DSI video mode, only command mode, along with a special instruction (0x3C) for automatic address offsets. The results of this project can be found on the GitLab account.

In a way these limitations make sense when you consider Apple’s use case for these special MIPI-DSI displays. As a touch screen with dynamic controls being displayed on it, features such as video playback never were a goal, and thus Apple likely decided to save a few bucks, possibly also due to MIPI licensing costs. What this means is that if you had dreamed of snapping up an extremely long and narrow OLED display for a video project you’re in for somewhat of a bad time. Although animated content is possible – as [Wenting] demonstrates – this comes with all the limitations of command mode, meaning slower updates, higher power usage and a lot more overhead.

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How To Drive Smartphone Screens Over HDMI

Compared to most small LCDs sold to makers, smartphone screens boast excellent color, brightness, and insanely high resolution. Unfortunately, driving them is rarely straightforward. In an attempt to make it easier, [peng-zhihui] set about developing tools to allow such screens to be driven from a simple HDMI feed. For those whose Chinese is a little rusty, the Google Translate link might prove useful.

The first attempt was using Toshiba’s TC358870XBG ASIC, capable of driving screens over MIPI DSI 1.1 from an HDMI input. [peng-zhihui] designed a simple test module for the chip based on the company’s evaluation board design, with [ylj2000] providing software to help get that solution off the ground.

However, for now that solution is imperfect, so [peng-zhihui] also experimented with the Longxun LT6911 HDMI to MIPI driver. While cheap, information on the part is scarce, and the company’s own source code for using the hardware is only accessible by signing an NDA. However, [peng-zhihui] made pre-compiled firmware available for those that wish to work with the hardware.

[peng-zhihui] has put these learnings to good use, building a power bank with a MIPI screen using what appears to be the Longxun chip. The device can supply power over USB and also act as an HDMI display.

While it’s early days yet, and driving these screens remain difficult, it’s great to see hackers getting out there and finding a way to make new parts work for them. We’ve seen similar work before, using an FPGA rather than an off-the-shelf ASIC. If you’ve found your own way to get these high-end displays working, be sure to drop us a line!

[Thanks to peterburk for the tip!]