Hardware: add LiPo battery + power-path charger

BQ24075 (single-cell charger + power-path SYS): USB present -> powers
device + charges cell; USB absent -> battery takes over seamlessly.
System rail becomes SYS (~3.0-4.2V battery domain); existing converters
already accept it (3.3V is buck-boost RT6150B so survives sag). MAX17048
fuel gauge on I2C1 for SoC on OLED. Power budget updated (~200-400mA
from SYS, charge limited to ~500mA on USB 2.0). ~5-8h on a 1500-2000mAh
cell. Connection table, BOM, risks, build order updated.

NOTE: hardware/netlist.net still references +5V rails; needs the SYS/
BQ24075/BAT/MAX17048 nets added — regenerate on request.

docs/hardware-plan.md

@@ -18,20 +18,16 @@ architecture this hardware serves.
 ## 1. Block diagram
 
 ```
-USB-C 5V ──┬── 3.3 V buck ── RP2040 + W25Q16 flash + RM2 module (CYW43439, BLE/WiFi)
+USB-C 5V ── BQ24075 (charger + power-path) ── SYS (~3.0-4.2 V)
-           │                   │
+                                  │              │
-           │                   ├── PIO0 SPI ── RM2 (CYW43439 gSPI, Pico-W pinout)
+                      LiPo cell (charge/discharge)  ├── 3.3 V buck-boost ── RP2040 + flash + RM2
-           │                   ├── USB DP/DM ── (USB-MIDI composite, CDC+MIDI)
+                                              │      ├── +12 V boost ──┐
-           │                   ├── UART0 ──── MIDI TRS in (6N138) / out
+                                              │      ├── −12 V inverter┤ CV op-amps (±12 V) ── 4× stereo TRS
-           │                   ├── I2C0 ───── Pimoroni RGB Keypad (buttons)
+                          MAX17048 fuel gauge (I2C)   └── 4.096 V/2.048 V refs ── DAC ── diff-amp → ±10 V
-           │                   ├── SPI0 ───── Pimoroni RGB Keypad (APA102 LEDs)
+                                  │
-           │                   ├── SPI1 ───── 8-ch 16-bit DAC (CV outs)
+   RP2040:  PIO0 SPI ── RM2  |  USB DP/DM ── USB-MIDI composite  |  UART0 ── MIDI TRS
-           │                   ├── I2C1 ───── 64×32 SSD1306 OLED
+            I2C0 ── keypad | SPI0 ── keypad LEDs | SPI1 ── DAC | I2C1 ── OLED + fuel gauge
-           │                   └── GPIOs ──── 4× tact buttons (UI)
+            GPIOs ── 4× tact + charge-status LED
-           │
-           └── ±12 V rails (boost + inverter) ── CV op-amps ── 4× stereo TRS (8 CV/gate)
-                  │
-   4.096 V ref ── DAC (0-4.096 V) ── op-amp gain ×4.88, offset −10 V ── ±10 V ── 1 k + bidir TVS ── TRS tip/ring
 ```
 
 ## 2. RP2040 pin map
@@ -56,17 +52,27 @@ driver and our firmware work unchanged) and our existing keypad wiring.
 
 No conflicts; GP14/15 (USB) and GP23-25/29 (CYW43) kept clear of everything else.
 
-## 3. Power tree (USB-only, with CV boost)
+## 3. Power tree (USB + LiPo battery with power-path)
 
-- USB-C 5 V → polyfuse + ESD → **VBUS_5V**.
+- USB-C 5 V → polyfuse + ESD → **VBUS_5V** → **BQ24075** charger/power-path IC.
-- VBUS_5V → **3.3 V buck** (RT6150B-33GQW) → RP2040, RM2, flash, DAC VDD,
+  - VBUS present → device runs from VBUS *and* charges the cell; SYS output
-  OLED, pull-ups.
+    is the system rail.
-- VBUS_5V → **+12 V boost** (TPS61088 + LC filter) → +12 V CV op-amp rail.
+  - VBUS absent → battery powers SYS seamlessly (no glitch).
-- VBUS_5V → **−12 V inverter** (TPS63700 + LC filter) → −12 V CV op-amp rail.
+- **SYS rail (~3.0-4.2 V)** — the battery-voltage domain, the new system input.
-  Both rails feed the bipolar CV op-amps. Low current (~20 mA each), LC-filtered.
+  - SYS → **3.3 V buck-boost** (RT6150B-33GQW — already a buck-boost, so it
-- **4.096 V reference** (REF3040AIDBZ) for the DAC, and a **2.048 V mid-ref**
+    holds 3.3 V through the whole SYS range, including battery sag) → RP2040,
-  (buffered divider or REF2040) for the bipolar offset — NOT the 3.3 V rail
+    RM2, flash, DAC VDD, OLED, pull-ups.
-  (too noisy for 1 V/oct pitch).
+  - SYS → **+12 V boost** (TPS61088) → +12 V CV op-amp rail (input 3.0-4.2 V
+    when on battery is fine; just draws more current at low voltage).
+  - SYS → **−12 V inverter** (TPS63700) → −12 V CV op-amp rail.
+- **4.096 V reference** (REF3040AIDBZ) and **2.048 V mid-ref** (REF2040) —
+  powered from +3V3 (post-buck, clean), NOT from SYS (sags with battery) or
+  the 3.3 V rail directly (too noisy for 1 V/oct pitch).
+- **LiPo cell** (single, 3.7 V nominal, ~1500-2000 mAh, integrated protection
+  PCB) on BAT. Charge rate set by BQ24075 PROG pin (~500 mA to stay within USB
+  2.0; raise if using USB-C 1.5/3 A).
+- **MAX17048 fuel gauge** (I2C1, shared with OLED) → battery state-of-charge
+  on the OLED + low-battery warning in firmware.
 
 **Power budget (BLE-only):** we use Bluetooth only, never WiFi TX. The cyw43
 driver must load the WiFi firmware to bring up BT (BT-only init isn't supported
@@ -79,17 +85,20 @@ yet), but WiFi never transmits → power stays at BLE levels, not WiFi-TX levels
 | DAC8568                | ~2 mA     | ~2 mA    |
 | OPA4192 ×2 (quiescent) | ~9 mA @±12 V | ~9 mA |
 | OLED SSD1306           | ~20 mA    | ~40 mA   |
-| ±12 V rails load (op-amps + DAC out) | ~20 mA @12 V → ~50 mA @5 V (×2 converters, ~85% eff) | ~60 mA @5 V |
+| ±12 V rails load | ~20 mA @±12 V → ~60 mA @SYS (×2 converters, ~80% eff) | ~75 mA @SYS |
 | MIDI out LED (brief)   | —         | ~15 mA @3.3 V |
-| **Total from 5 V**     | **~180-230 mA** | **~330-380 mA** |
+| **System draw from SYS** | **~200-260 mA** | **~340-400 mA** |
 
-Fits USB 2.0 (500 mA) with headroom; USB-C default (900 mA) is comfortable.
+Battery life: a 2000 mAh cell at ~250 mA avg → ~7-8 h (peak BLE + display); a
-If WiFi TX were ever enabled, RM2 spikes to ~350 mA and the total approaches
+1500 mAh cell → ~5-6 h. SYS sags to ~3.3 V at end of discharge, all converters
-~650 mA — stay BLE-only or require a USB-C 1.5 A/3 A source.
+stay in regulation. When USB present, SYS = charge voltage (~4.2 V max); charge
+current limited to ~500 mA by default to respect USB 2.0 (battery tops up in
+~3-4 h for a 1500 mAh cell).
 
-Rationale: USB-only power (single cable, no external PSU) while delivering
+Rationale: USB *or* battery operation via one power-path IC; the existing
-bipolar ±10 V CV via two small low-current rails. The LC filters + dedicated
+converters already accept the SYS voltage range (buck-boost on 3.3 V is the key
-Vref keep pitch clean.
+— a plain buck would drop out near end-of-discharge). BLE-only keeps the budget
+comfortable; WiFi TX would push ~600 mA peak and halve battery life.
 
 ## 4. CV analog section (4 stereo = 8 channels, bipolar ±10 V)
 
@@ -161,6 +170,9 @@ Host PCB responsibilities (from the RM2 datasheet):
 | Mid-ref | REF2040AIDBZ (or buffered divider) | SOT-23 | 2.048 V bipolar offset reference |
 | Calib | 8× trimpot (e.g. 3296W) | THT/SMD | per-channel gain/offset trim |
 | MIDI opto | 6N138 | DIP-8 / SMD | MIDI in isolation |
+| Charger | BQ24075DRCR | MSOP-10 | single-cell LiPo charger + power-path (SYS pin) |
+| Fuel gauge | MAX17048G+T10 | TDFN-8 (or bare module) | LiPo SoC over I2C1 (optional) |
+| Battery | LiPo 3.7 V, 1500-2000 mAh | cell w/ protection PCB | integrated DW01/FS8205A protection |
 | 3.3 V reg | RT6150B-33GQW | QFN | buck-boost, 800 mA, Pico-proven (TPS62A01 alt) |
 | +12 V boost | TPS61088 | QFN | 5→12 V, low-noise + LC filter |
 | −12 V inverter | TPS63700 | SOT-23-6 / QFN | 5→−12 V, low-noise + LC filter (LM2611 alt) |
@@ -183,10 +195,19 @@ Host PCB responsibilities (from the RM2 datasheet):
 3. **Custom RP2040 bring-up** — needs a SWD header + Picoprobe; USB bootloader
    only works after first flash.
 4. **64×32 OLED UI** — tight; menu must be paginated and minimal.
-5. **Boost current budget** — RM2 TX spikes + 8 CV op-amps + two converters on
+5. **Boost current budget** — RM2 TX spikes + 8 CV op-amps + two converters;
-   a single USB 5 V/500 mA port is tight; BLE-only keeps it ~330-380 mA peak
+   BLE-only keeps the system draw ~340-400 mA peak from SYS, plus up to ~500 mA
-   (fits), but specify USB-C 5 V/900 mA for headroom.
+   battery charge from USB → USB-C 5 V/900 mA-1.5 A recommended (USB 2.0 500 mA
-6. **RM2 antenna keepout** — follow the datasheet keepout; the one layout
+   only if charging paused or BLE-only/low charge rate).
+6. **Battery sag at end-of-discharge** — SYS can sag to ~3.3 V; the 3.3 V path
+   must be a buck-boost (RT6150B is) or it drops out. Fuel gauge drives a
+   low-battery warning in firmware; cut off at ~3.3 V to protect the cell.
+7. **Charge thermal** — BQ24075 thermal-regulates; keep it away from the CV
+   references / DAC and away from the LiPo for thermal separation.
+8. **LiPo shipping / safety** — LiPo cells have shipping (UN3481) and safety
+   considerations; use a protected cell, add a charge LED, follow charge-IC
+   layout guidance.
+9. **RM2 antenna keepout** — follow the datasheet keepout; the one layout
    constraint the radio imposes.
 
 ## 10. Build order (hardware)
@@ -194,7 +215,8 @@ Host PCB responsibilities (from the RM2 datasheet):
 1. Schematic (this doc is the spec).
 2. RM2 module footprint + antenna keepout.
 3. RP2040 + flash + USB + SWD.
-4. Power (3.3 V buck, +12 V boost, −12 V inverter, 4.096 V + 2.048 V refs).
+4. Power (BQ24075 power-path + LiPo, 3.3 V buck-boost, +12 V boost,
+   −12 V inverter, 4.096 V + 2.048 V refs, MAX17048 fuel gauge).
 5. CV analog (DAC + difference-amp + offset + trim + bidir protection + TRS).
 6. MIDI TRS circuit.
 7. OLED + buttons.
@@ -222,6 +244,9 @@ at `hardware/netlist.net`.
 | U8 | DAC8568 | 8-ch 16-bit SPI DAC |
 | U9, U10 | OPA4192 | quad RRO op-amp ×2 (8 CV channels) |
 | U11 | 6N138 | MIDI-in optocoupler |
+| U12 | BQ24075 | single-cell LiPo charger + power-path |
+| U13 | MAX17048 | LiPo fuel gauge (I2C) |
+| BAT1 | LiPo cell | ~1500-2000 mAh, integrated protection |
 | DISP1 | SSD1306 | 64×32 OLED (I2C) |
 | J1 | USB-C | 5 V + USB data |
 | J2 | 2×3 header | SWD (Picoprobe) |
@@ -234,16 +259,21 @@ at `hardware/netlist.net`.
 | D1–D8 | bidirectional TVS | CV output protection |
 | R_cal1–R_cal8 | — | per-channel calibration (see §4) |
 
-### Power nets
+### Power nets (with battery power-path)
 
 ```
-J1.VBUS  ── FB1 ── +5V
+J1.VBUS  ── FB1 ── VBUS_5V
-+5V ── U3 (RT6150B-33) ── +3V3
+VBUS_5V ── U12 (BQ24075 VBUS)        ; USB input to charger/power-path
-+5V ── U4 (TPS61088) + L1 LC ── +12V
+BAT1 ── U12 (BAT)                    ; LiPo cell
-+5V ── U5 (TPS63700) + L2 LC ── −12V
+U12 (SYS) ── SYS rail (~3.0-4.2 V)  ; system supply (USB or battery)
-+3V3 ── U6 (REF3040) ── VREF4V096
+SYS ── U3 (RT6150B-33) ── +3V3      ; buck-boost: holds 3.3 V across SYS range
+SYS ── U4 (TPS61088) + L1 LC ── +12V
+SYS ── U5 (TPS63700) + L2 LC ── −12V
++3V3 ── U6 (REF3040) ── VREF4V096   ; refs from clean +3V3, not saggy SYS
 +3V3 ── U7 (REF2040) ── VMID2V048
-GND = J1.GND, all return paths
+U13 (MAX17048) SDA/SCL ── I2C1 (shared with OLED)  ; fuel gauge
+U12 (STAT) ── U1.GPIO16             ; charge status (→ LED + firmware)
+GND = J1.GND, BAT1.GND, all return paths
 ```
 
 ### RM2 radio (Pico-W-compatible gSPI)