feat(pcb): main control board rev3

docs/hardware/interfaces.md

@@ -41,7 +41,7 @@ Bring-up configuration in [`src/main.cpp`](../../src/main.cpp):
 
 Firmware writes identical samples to both stereo slots. MAX98357A is mono. On the main control board, gain is the solder jumper next to the amplifier ([board README](../../hardware/boards/main-control-board/README.md)). On the advanced breakout, **GAIN** and **SD** stay unwired (GAIN floating ≈ 9 dB, SD pulled up ≈ left channel) — [wiring.md](wiring.md).
 
-Amp **Vin** = USB **5V**. I2S wires are 3.3 V logic. Speaker connects to **SPK+** / **SPK−** — [main-control-board.md](main-control-board.md).
+On the main control board, amp **Vin** is **AUDIO_5V** (net-tied from **SERVO_5V** after the eFuse), not raw USB **+5V** — [power.md](power.md). On the advanced breakout, amp **Vin** is the shared USB **5V** rail — [wiring.md](wiring.md). I2S wires are 3.3 V logic. Speaker connects to **SPK+** / **SPK−** — [main-control-board.md](main-control-board.md).
 
 > [!WARNING]
 > MAX98357A SPK- is not ground.
@@ -69,9 +69,9 @@ Default on (`PCA9685_OE_WIRED = true` in [`include/pins.h`](../../include/pins.h
 
 Single USB-C on the robot: the **main control board**.
 
-| Net | Board USB-C | ESP32 |
+| Net | Board USB-C | ESP32 / loads |
 | --- | --- | --- |
-| VBUS | +5V rail | **5V** pad |
+| VBUS | through fuse → **+5V** | ESP32 **5V** pad; eFuse input → **SERVO_5V** (servos + amp) |
 | GND | common GND | **GND** |
 | D− | data | **GPIO18** |
 | D+ | data | **GPIO19** |

docs/hardware/pinout.md

@@ -20,7 +20,7 @@ Logic level: **3.3 V**.
 
 | GPIO | Status | Reason |
 | --- | --- | --- |
-| GPIO9 | **Reserved** | BOOT button / strapping. Do not pick casually for peripherals |
+| GPIO9 | **Reserved** | BOOT / strapping. On the main control board it is also on **J4** (extension only) — do not pull low at reset (download mode). Do not pick casually for peripherals |
 | GPIO10 | **Occupied** | Onboard WS2812 |
 | GPIO12–GPIO17 | **Unavailable** | Stacked flash, not brought out |
 | GPIO18 | **Reserved** | Native USB D− (main control board USB-C; advanced path: [5993](wiring.md#usb-connector-5993)) |
@@ -30,14 +30,57 @@ Logic level: **3.3 V**.
 
 | GPIO | Status | Notes |
 | --- | --- | --- |
-| GPIO6 | Free | Unused |
-| GPIO7 | Free | Unused |
-| GPIO8 | Free | Unused |
-| GPIO20 | Default UART0 RX | Silkscreen RX. Free for other use only if USB CDC remains the console (`ARDUINO_USB_CDC_ON_BOOT=1`) |
-| GPIO21 | Default UART0 TX | Silkscreen TX. Same caveat as GPIO20 |
+| GPIO6 | Free | Brought out on **J4** (main control board) |
+| GPIO7 | Free | Brought out on **J4** |
+| GPIO8 | Free | Brought out on **J4**; strapping — do not pull low at reset |
+| GPIO20 | Default UART0 RX | Silkscreen RX; also on **J4**. Free for other use only if USB CDC remains the console (`ARDUINO_USB_CDC_ON_BOOT=1`) |
+| GPIO21 | Default UART0 TX | Silkscreen TX; also on **J4**. Same caveat as GPIO20 |
 
 Power pads (not GPIO): **5V**, **GND**, **3V3**.
 
+## Main control board headers
+
+Pin order matches the KiCad board and [`expected-nets.yml`](../../hardware/boards/main-control-board/expected-nets.yml). Board overview: [board README](../../hardware/boards/main-control-board/README.md).
+
+### J2 — OLED (`Conn_OLED`)
+
+| Pin | Net |
+| --- | --- |
+| 1 | **+3.3V** |
+| 2 | **GND** |
+| 3 | **I2C SDA** |
+| 4 | **I2C SCL** |
+
+### J3 — I2C extension (`Conn_extension`, JST)
+
+Same bus as J2 / PCA9685. **Pin order is not the same as J2** (GND and 3.3 V are swapped).
+
+| Pin | Net |
+| --- | --- |
+| 1 | **GND** |
+| 2 | **+3.3V** |
+| 3 | **I2C SDA** |
+| 4 | **I2C SCL** |
+
+### J4 — 12-pin extension (`Conn_02x06_Odd_Even`)
+
+| Pin | Net |
+| --- | --- |
+| 1 | **SERVO_5V** |
+| 2 | **GP21** (default UART0 TX) |
+| 3 | **GND** |
+| 4 | **GP20** (default UART0 RX) |
+| 5 | **GND** |
+| 6 | **GP9** (strapping / BOOT — do not pull low at reset) |
+| 7 | **+3.3V** |
+| 8 | **GP8** (strapping — do not pull low at reset) |
+| 9 | **I2S BCLK** |
+| 10 | **GP7** |
+| 11 | **I2S LRC** |
+| 12 | **GP6** |
+
+I2S **DIN** is not on J4 (ESP32 → amp only). **SERVO_5V** is the eFuse-protected rail — [power.md](power.md).
+
 ## PCA9685 channels (not ESP32 GPIO)
 
 | PCA9685 channel | Firmware | Mechanism |

docs/hardware/power.md

@@ -2,21 +2,25 @@
 
 ## Architecture
 
-One common **nominal 5 V** rail feeds the robot.
+USB-C brings in a **nominal 5 V**. On the [main control board](../../hardware/boards/main-control-board/README.md) that splits into a logic/host rail and a protected servo (and amp) rail.
 
 | Net | Source | Loads |
 | --- | --- | --- |
-| **+5V** | Main control board USB-C **VBUS** | ESP32 **5V**, PCA9685 **5V**, MAX98357A **Vin**; servos from PCA9685 **V+** |
-| **3V3** | ESP32 onboard LDO, from the 5 V input | ESP32 core/GPIO, PCA9685 **VCC**, OLED **VCC** |
+| **+5V** | USB-C **VBUS** → 3 A PPTC fuse | ESP32 **5V**, 5 V indicator, eFuse input |
+| **SERVO_5V** | TPS259571 eFuse out of **+5V** (~2 A via `R14`) | Servo header 5 V pins, bulk capacitor; amp supply via net tie |
+| **AUDIO_5V** | Net-tied from **SERVO_5V** (not raw **+5V**) | MAX98357A **Vin** |
+| **3V3** | ESP32 onboard LDO, from the 5 V input | ESP32 core/GPIO, PCA9685 **VCC** / **VDD**, OLED **VCC**, I2C pull-ups |
 | **GND** | Board USB-C GND + ESP32 GND | Everything |
 
-ESP32 internally runs at 3.3 V logic. Servo power **does not** pass through the ESP32 3.3 V regulator. USB **5V** → PCA9685 **5V**; PCA9685 **V+** → servo **5V**; PCA9685 **VCC** ← ESP32 **3V3**. The advanced breakout harness draws the same nets — [wiring.md](wiring.md).
+ESP32 internally runs at 3.3 V logic. Servo power **does not** pass through the ESP32 3.3 V regulator. On the main control board: **+5V** feeds the module **5V** pad; **SERVO_5V** feeds the servo plugs; PCA9685 **VDD** ← **3V3**. The chip has no package “V+” pin — servo 5 V is a board distribution net to the connector (same idea as Adafruit breakout **V+**). The advanced breakout harness draws equivalent nets without the board eFuse — [wiring.md](wiring.md).
 
 > [!WARNING]
 > Never power the servos from the ESP32 3.3 V regulator.
 
 Waveshare documents the C3-Zero LDO as ME6217C33M5G (hundreds of mA class). That is enough for logic + OLED + PCA9685 digital. It is **not** a servo supply.
 
+Multi-servo stall can hit the eFuse’s ~2 A limit before the 3 A resettable fuse trips. Use a **5 V / ≥ 2 A** supply; expect occasional **SERVO_5V** (and amp) cutouts under heavy load, then auto-retry.
+
 ## Servo current (worst case)
 
 Datasheet stall currents for PowerHD HD-1370A:
@@ -40,7 +44,7 @@ This **excludes**:
 
 ## USB supply
 
-Power and data enter on the **main control board USB-C** (one cable). Nets: VBUS → +5V, GND, D− → GPIO18, D+ → GPIO19. Details: [interfaces.md](interfaces.md).
+Power and data enter on the **main control board USB-C** (one cable). Nets: VBUS → fuse → **+5V**, GND, D− → GPIO18, D+ → GPIO19. Servos and the amp sit behind the eFuse on **SERVO_5V** / **AUDIO_5V**. Details: [interfaces.md](interfaces.md).
 
 Two 5.1 kΩ CC resistors on the board mark it as a USB device. The charger decides how much current arrives. Use a **5 V / ≥ 2 A** source, with extra margin if several servos move **while audio plays**. The connector is **not** USB-PD voltage conversion. Feed it 5 V USB.
 

hardware/boards/main-control-board/README.md

@@ -7,7 +7,7 @@ This board drives the robot's servos and speaker. Power and USB data come in on
 - **Built:** not manufactured
 - **Tested:** not tested
 
-![Main Control Board](./main-control-board-rev-2-preview.jpg)
+![Main Control Board](./main-control-board-rev-3-preview.jpg)
 
 ## Soldered on the board
 
@@ -21,7 +21,9 @@ The servo driver, the speaker amplifier, and the USB-C connector are parts on th
 - A **TPS259571** eFuse (`U4`) sits between that 5 V rail and servo 5 V. Current limit is set by `R14` (1 kΩ ≈ 2 A). On fault the device auto-retries.
 - **OLED header** is for the SSD1306 display. Pin order, from pin 1: 3.3 V, ground, SDA, SCL.
 - A second header shares that I2C bus. Its pin order is different: ground, 3.3 V, SDA, SCL.
-- A spare 4-pin header is on the board for later use.
+- A **12-pin extension port** (`J4`) breaks out `SERVO_5V`, two grounds, `+3.3V`, I2S BCLK/LRC, and GPIOs GP6–GP9 / GP20 / GP21.
+  - GP8 and GP9 are ESP32-C3 strapping pins. Do not pull them low at reset; GP9 low enters download mode.
+  - GP20 and GP21 are the default UART0 RX/TX. Boot log output appears on GP21 at power-up.
 
 A solder jumper next to the amplifier is marked as a 6 dB gain limit.
 

hardware/boards/main-control-board/expected-nets.yml

@@ -3,6 +3,8 @@
 # Net classes = KiCad netlist `class` list, effective class first.
 # Compare with: kicad-cli sch export netlist
 # See docs/pcb.md#expected-nets-yml
+#
+# J4 = 12-pin extension port (SERVO_5V, GND×2, +3.3V, I2S BCLK/LRC, GP6–GP9, GP20, GP21).
 
 netclasses:
   "+5V":
@@ -31,6 +33,16 @@ netclasses:
     - Default
   "/ESP32/GP6":
     - Default
+  "/ESP32/GP7":
+    - Default
+  "/ESP32/GP8":
+    - Default
+  "/ESP32/GP9":
+    - Default
+  "/ESP32/GP20":
+    - Default
+  "/ESP32/GP21":
+    - Default
   "/I2S_BCLK":
     - Default
   "/I2S_LRC":
@@ -77,9 +89,10 @@ nets:
     - U4.5:OUT
   "+3.3V":
     - C1.2:2
+    - C7.1:1
     - J2.1:Pin_1
     - J3.2:Pin_2
-    - J4.2:Pin_2
+    - J4.7:Pin_7
     - J7.3:Pin_3
     - R3.1:1
     - R4.1:1
@@ -92,11 +105,14 @@ nets:
     - C3.1:1
     - C4.1:1
     - C5.1:1
+    - C6.2:2
+    - C7.2:2
     - D1.1:K
     - D2.1:K
     - J2.2:Pin_2
     - J3.1:Pin_1
     - J4.3:Pin_3
+    - J4.5:Pin_5
     - J7.2:Pin_2
     - R1.1:1
     - R2.1:1
@@ -141,12 +157,29 @@ nets:
     - R3.2:2
     - U1.23:~{OE}
   "/ESP32/GP6":
-    - J4.4:Pin_4
+    - J4.12:Pin_12
     - J8.1:Pin_1
+  "/ESP32/GP7":
+    - J4.10:Pin_10
+    - J8.2:Pin_2
+  "/ESP32/GP8":
+    - J4.8:Pin_8
+    - J8.3:Pin_3
+  "/ESP32/GP9":
+    - J4.6:Pin_6
+    - J8.4:Pin_4
+  "/ESP32/GP20":
+    - J4.4:Pin_4
+    - J8.8:Pin_8
+  "/ESP32/GP21":
+    - J4.2:Pin_2
+    - J8.9:Pin_9
   "/I2S_BCLK":
+    - J4.9:Pin_9
     - J7.6:Pin_6
     - U3.16:BCLK
   "/I2S_LRC":
+    - J4.11:Pin_11
     - J7.7:Pin_7
     - U3.14:LRCLK
   "/I2S_DIN":