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Updating U4

1. Unscrew the case via the 4 screws on the back.
2. Desolder the positive lead of the rechargeable battery. This is necessary because the bootloader of U4 is only present immediately after power-on.
3. Configure Tigard (or other USB-TTL device) as follows:
   1. VTGT switch to 3V3
   2. TX to P1 (left side of U4, clearly marked
   3. RX to P2 (just above P1)
   4. GND to GND (just below P1, unmarked)
   5. Prep VTGT to connect to VTGT (just above P2, unmarked, trace goes to pin 2 of U4)