In which I pried open the glass LCD lens at the front of my Nest 2nd Gen thermostat and found the LCD display module, packaged with a flexible ribbon cable and a round pixel array, and took a months-long journey of reverse engineering, detective work, and problem-solving that taught me more about display interfaces than any tutorial ever could have.
Discovery: Finding the Details of the Display
It all started with a disassembled Nest thermostat sitting on my workbench. Following the teardown guide from iFixit, I had carefully disassembled the thermostat until I reached Step 12, where the LCD module was exposed. This is the only step in disassembly that requires prying off glue instead of a screw or a connector.
[Image: iFixit guide showing step 12; Nest thermostat partially disassembled, showing the LCD sandwiched between the front glass and plastic frame]
This revealed a sealed LCD module, with a flexible FPC connector sticking out the side. The display itself is round, although the module is a square with trapezoidal corners.
To get more detail about the module itself, I had to resort to separating the metal frame and peeling back the adhesive layers, and printed in tiny text on the back of the display module: "TM025ZDZ02 FPC1-00 2012-03". A part number! This was my first real clue to start the reverse-engineering adventure.
[Image: Close-up of the back of the LCD module showing the printed part number "TM025ZDZ02"]
The Hunt for Documentation
Armed with a part number, I did what any engineer does: I started hunting for a datasheet. The TM025ZDZ02 proved elusive—it seemed to be a custom or limited-production display. However, I discovered that Shanghai Tianma Micro-Electronics made a very similar module: the TM025ZDZ01. The specifications were nearly identical: a 2.48" round display with 320×320 resolution and a 3-wire SPI interface. You can find them at https://youritech-online.com/products/tm025zdz01-2-48-inch-320x320-tft-lcd-color-screen-circular-display-for-industrial-control-vehicle-handheld-terminal
I managed to obtain the datasheet for the TM025ZDZ01, which had the pinout, the electrical specifications but also code samples for the interface requirements and initialization sequences.
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