For a TRMNL PCB to accept data transfer via a USB-C cable, it needs to be in "boot mode," also sometimes called flashing mode.
Quick Links:
TRMNL OG
If you have a 7.5" native TRMNL device...
Basic method
Plug a data enabled USB-C cable into your TRMNL.
Turn off the TRMNL.
While pressing down on the circular ("boot") button below the power switch, turn the device back on.
Let go of the boot button.
Stubborn method (if Basic does not work)
Turn off the device.
Hold down on the circular ("boot") button below the power switch.
While pressing down, plug a data enabled USB-C cable into your TRMNL.
Keep pressing the boot button and turn the device back on.
Let go of the boot button.
If the above methods both fail, turn off your device for 10-15 minutes and try again. There may be a static charge or heat buildup that needs to dissipate.
TRMNL X
This model has 2x modes that bypass the firmware, "reset" and "flashing."
Flashing / Boot Mode
Rotate your charging dock so the flat (back) side is facing the back of your TRMNL. Then, "hover" the dock from the bottom left corner to the middle, and plug it back in.
A USB-C cable should also be attached if you are flashing new firmware from https://trmnl.com/flash or your own development environment.
Here's a couple angles in action. First, from above:
Next, from the side:
Complete this motion at a 2-3 second speed. If you do it too quickly, each of the 2x reset points may not recognize the magnet inside your dock.
Reminder: You will then need to connect your USB-C cable to the dock and the dock to the back of your device so that the pins connect to the TRMNL X so it can be flashed from your computer.
If the device is not recognized (USB JTAG/serial...) when you click Connect on the flashing page, it is not in Boot Mode or does not have a good connection through the dock.
We have a guide for how to get a good connection.
Soft Reset / Restart device: Perform a soft reset and then repeat the boot mode steps.
Seeed Studio
Check out our Seeed guide for steps to understand the buttons and how to enter flashing mode on your DIY Kit, E1001, E1002, and other devices.