diff --git a/docs/BOM.md b/docs/BOM.md
index 65779cc..2437155 100644
--- a/docs/BOM.md
+++ b/docs/BOM.md
@@ -121,10 +121,19 @@ type: reference
 | GND | 6 |
 | TX | 10 (UART1 RX) |
 | RX | 8 (UART1 TX), needed only to send config commands |
-| *Realign button (optional)* | 7 (GPIO09, `PAC.06`) through the button to ground on 9; 10 kΩ from 7 to 3.3 V (pin 1). Pin to confirm |
+| *Realign button (optional)* | 7 (GPIO09, `PAC.06`) through the button to ground on 9; 100 kΩ from 7 to 3.3 V (pin 1 or 17) |
 
 Everything sits in header pins 1–10, so one 2×5 connector can carry it. Pins 3/5 (I²C bus 7) carry the UPS monitor.
 
+Header facts (carrier board spec SP-11324-001 v1.3, Table 3-3 and 5-3; raw copy in `/mnt/wdelem/dropzone/`):
+
+- All signals are 3.3 V. Pins 7, 8 and 10 pass through **TXB0108 level translators**: their outputs are very weak (±20 µA) and can be
+  overdriven, and **any pull-up or pull-down on them must be weaker than 50 kΩ** (I²C excepted). Hence 100 kΩ for the button, not 10 kΩ.
+- 3.3 V on the header: 1 A per pin, but **0.1 A in total** for the header from `VDD_3V3_SYS`. 5 V: 0.5 A in total.
+- Pin 8 (UART1 TX) is also a boot strap (`BOOT2_STRAP`): nothing should drive it while the board powers up. The WT61's RX is an input,
+  so it does not.
+- Pin 7 is `PAC.06` (gpiochip0 line 144), pulled down on the module at power-on.
+
 ### Notes
 
 - **The header is 3.3 V logic and not 5 V tolerant.** Powering the module at 3.3 V keeps every signal at 3.3 V. The datasheet only says
@@ -145,8 +154,9 @@ Everything sits in header pins 1–10, so one 2×5 connector can carry it. Pins
 - **Output is low-pass filtered at 10 Hz** by default, even at 100 Hz output. Fine for vehicle motion, but it smooths out bumps and
   vibration. A gyro rate under 0.122 °/s reads as 0 (static threshold). Both are set only from WitMotion's PC software; there is no
   serial command for them.
-- **Realign button parts (optional, not fitted):** one momentary push button (normally open) and one 10 kΩ resistor. The resistor pulls
-  pin 7 up to 3.3 V, so the pin reads 1, and a press grounds it to 0. Realigning starts from the tablet app first; the button adds a way
+- **Realign button parts (optional, not fitted):** one momentary push button (normally open) and one 100 kΩ resistor. The resistor pulls
+  pin 7 up to 3.3 V, so the pin reads 1, and a press grounds it to 0. Weak on purpose: the pin sits behind a TXB0108 (see header facts);
+  check both states with `gpioget` once wired. Realigning starts from the tablet app first; the button adds a way
   that works without the tablet, triggering the same command.
 - **Realign:** a press levels the frame to gravity (the mounting rotation, from averaged accelerometer readings) and sends
   `FF AA 52` to zero yaw. The result is stored on disk, not redone every drive. Refuse while moving (gyro or GPS speed); record raw data