nils
2d9c201a12
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.
372 lines
18 KiB
Markdown
372 lines
18 KiB
Markdown
# Hardware Plan — flexible MIDI router (v1)
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Status: draft 2026-08-21. Custom all-in-one PCB, desktop, USB-powered, **bipolar
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±10 V CV** via onboard ±12 V rails, 3.5 mm TRS MIDI, 4× stereo CV, 64×32 OLED +
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tact buttons (no encoder, low profile).
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**MCU target:** develop/test on **RP2040** (Pico W dev board — the firmware
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already runs on it), product on **RP2350**. The RM2 radio module is compatible
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with both. The Rust firmware builds for either via embassy-rp features
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(`rp2040` → `thumbv6m-none-eabi`; `rp235xa` for the RP2350A used in the Pico 2 W
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→ `thumbv8m.main-none-eabihf`). Pin assignments below are RP2040/Pico-W; the
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RP2350 product PCB keeps the Pico-2-W-compatible RM2 bus and reuses the same
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peripheral choices.
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See `midi-router-architecture` (session memory) for the software/firmware
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architecture this hardware serves.
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## 1. Block diagram
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```
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USB-C 5V ── BQ24075 (charger + power-path) ── SYS (~3.0-4.2 V)
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│ │
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LiPo cell (charge/discharge) ├── 3.3 V buck-boost ── RP2040 + flash + RM2
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│ ├── +12 V boost ──┐
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│ ├── −12 V inverter┤ CV op-amps (±12 V) ── 4× stereo TRS
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MAX17048 fuel gauge (I2C) └── 4.096 V/2.048 V refs ── DAC ── diff-amp → ±10 V
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│
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RP2040: PIO0 SPI ── RM2 | USB DP/DM ── USB-MIDI composite | UART0 ── MIDI TRS
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I2C0 ── keypad | SPI0 ── keypad LEDs | SPI1 ── DAC | I2C1 ── OLED + fuel gauge
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GPIOs ── 4× tact + charge-status LED
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```
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## 2. RP2040 pin map
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Peripherals follow the Pico W reference for the RM2/CYW43 bus (so the cyw43
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driver and our firmware work unchanged) and our existing keypad wiring.
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| Function | RP2040 pins | Notes |
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|--------------------|------------------------|-----------------------------------------|
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| USB device | GP14 (DM), GP15 (DP) | USB-C; CDC + MIDI composite |
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| RM2 (CYW43) bus | GP29 (CLK), GP24 (DAT), GP25 (CS-out), GP23 (PWR), PIO0, DMA CH0/CH1 | copy Pico W |
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| RM2 onboard LED | CYW43 WL_GPIO0 | status/heartbeat |
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| Keypad buttons | GP4 (SDA), GP5 (SCL) | I2C0, expander @0x20 |
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| Keypad LEDs | GP17 (CS), GP18 (SCK), GP19 (MOSI) | SPI0 tx-only, 4 MHz, APA102 |
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| DAC (CV outs) | GP10 (SCK), GP11 (MOSI), GP13 (CS) | SPI1 tx-only, 16-bit DAC; MISO unused |
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| DAC LDAC/RESET | GP20, GP21 | GPIO (optional, immediate-update) |
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| MIDI TRS UART | GP0 (TX), GP1 (RX) | UART0 @31250 baud |
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| OLED | GP2 (SDA), GP3 (SCL) | I2C1, SSD1306 64×32 |
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| UI buttons | GP6, GP7, GP8, GP9 | 4× low-profile tact: Up/Down/Select/Back|
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| SWD debug header | SWCLK, SWDIO | 2×3 header for Picoprobe (first-flash) |
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| Spare | GP16, GP22, GP26-28 | status LEDs / future analog in |
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No conflicts; GP14/15 (USB) and GP23-25/29 (CYW43) kept clear of everything else.
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## 3. Power tree (USB + LiPo battery with power-path)
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- USB-C 5 V → polyfuse + ESD → **VBUS_5V** → **BQ24075** charger/power-path IC.
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- VBUS present → device runs from VBUS *and* charges the cell; SYS output
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is the system rail.
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- VBUS absent → battery powers SYS seamlessly (no glitch).
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- **SYS rail (~3.0-4.2 V)** — the battery-voltage domain, the new system input.
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- SYS → **3.3 V buck-boost** (RT6150B-33GQW — already a buck-boost, so it
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holds 3.3 V through the whole SYS range, including battery sag) → RP2040,
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RM2, flash, DAC VDD, OLED, pull-ups.
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- SYS → **+12 V boost** (TPS61088) → +12 V CV op-amp rail (input 3.0-4.2 V
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when on battery is fine; just draws more current at low voltage).
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- SYS → **−12 V inverter** (TPS63700) → −12 V CV op-amp rail.
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- **4.096 V reference** (REF3040AIDBZ) and **2.048 V mid-ref** (REF2040) —
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powered from +3V3 (post-buck, clean), NOT from SYS (sags with battery) or
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the 3.3 V rail directly (too noisy for 1 V/oct pitch).
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- **LiPo cell** (single, 3.7 V nominal, ~1500-2000 mAh, integrated protection
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PCB) on BAT. Charge rate set by BQ24075 PROG pin (~500 mA to stay within USB
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2.0; raise if using USB-C 1.5/3 A).
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- **MAX17048 fuel gauge** (I2C1, shared with OLED) → battery state-of-charge
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on the OLED + low-battery warning in firmware.
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**Power budget (BLE-only):** we use Bluetooth only, never WiFi TX. The cyw43
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driver must load the WiFi firmware to bring up BT (BT-only init isn't supported
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yet), but WiFi never transmits → power stays at BLE levels, not WiFi-TX levels.
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| Rail / consumer | Typical | Peak |
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|------------------------|-----------|----------|
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| RM2 (BLE TX, no WiFi TX)| ~30 mA @3.3 V | ~80 mA |
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| RP2040 / RP2350 | ~30 mA | ~50 mA |
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| DAC8568 | ~2 mA | ~2 mA |
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| OPA4192 ×2 (quiescent) | ~9 mA @±12 V | ~9 mA |
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| OLED SSD1306 | ~20 mA | ~40 mA |
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| ±12 V rails load | ~20 mA @±12 V → ~60 mA @SYS (×2 converters, ~80% eff) | ~75 mA @SYS |
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| MIDI out LED (brief) | — | ~15 mA @3.3 V |
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| **System draw from SYS** | **~200-260 mA** | **~340-400 mA** |
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Battery life: a 2000 mAh cell at ~250 mA avg → ~7-8 h (peak BLE + display); a
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1500 mAh cell → ~5-6 h. SYS sags to ~3.3 V at end of discharge, all converters
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stay in regulation. When USB present, SYS = charge voltage (~4.2 V max); charge
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current limited to ~500 mA by default to respect USB 2.0 (battery tops up in
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~3-4 h for a 1500 mAh cell).
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Rationale: USB *or* battery operation via one power-path IC; the existing
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converters already accept the SYS voltage range (buck-boost on 3.3 V is the key
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— a plain buck would drop out near end-of-discharge). BLE-only keeps the budget
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comfortable; WiFi TX would push ~600 mA peak and halve battery life.
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## 4. CV analog section (4 stereo = 8 channels, bipolar ±10 V)
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- **DAC:** 8-channel 16-bit SPI (DAC8568IAPW), Vref = 4.096 V → 0-4.096 V out.
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16-bit over ±10 V (20 V span) ≈ 0.3 mV/step ≈ 0.003 semitone at 1 V/oct —
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plenty for pitch.
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- **Bipolar scaling:** each channel maps 0-4.096 V → −10 V…+10 V. Difference
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amp: `Vout = 4.88·(Vdac − 2.048)` where 2.048 V is the mid-reference
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(buffered divider from the 4.096 V ref, or REF2040). At Vdac=0 → −10 V,
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Vdac=2.048 → 0 V, Vdac=4.096 → +10 V. Per-channel gain/offset trimpot for
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pitch calibration (0.2 % ref → ~±20 mV untrimmed ≈ 0.02 semitone — trimmable
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to <1 mV).
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- **Op-amps:** OPA4192IPWR (quad, RRO, 25 µV offset) ×2 on the ±12 V rails.
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RRO needed to reach ±10 V cleanly on ±12 V (TL074 clips at ±~10.5 V — too
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close; OPA4192 swings to ±11.9 V).
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- **Per-channel model:** every CV channel is an independent
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`(midi_channel, function, polarity)` sink. The output stage is always
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bipolar-capable (±10 V); the function decides the mapping:
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- pitch → 0…+10 V (note/12, standard unipolar-positive on a bipolar output)
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- gate → 0/+10 V (or 0/+5 V)
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- velocity → 0…+10 V (vel/127·10)
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- CC (unipolar) → 0…+10 V; CC (bipolar) → −10…+10 V (cc/127·20 − 10)
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Gate = a DAC channel driven to 0/+10 V (keeps routing uniform).
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- **Output protection:** 1 kΩ series + **bidirectional TVS** per output (outputs
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swing negative) to TRS tip/ring.
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## 5. MIDI TRS (3.5 mm) — 2 jacks (in + out), TRS-A wiring
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- **In:** tip → 220 Ω → 6N138 optocoupler LED → cathode to ring. Transistor:
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collector to 3.3 V via 1 k pull-up → UART0 RX (GP1); emitter to GND; base
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speed-up (1N4148 + 1 k). Isolated; powered from the input loop.
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- **Out:** UART0 TX (GP0) → 220 Ω → tip; ring to 3.3 V. (3.3 V/220 Ω ≈ 15 mA,
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fine for MIDI-out LED.)
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- TRS-A: tip = signal, ring = return (Korg / Make Noise convention).
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## 6. Display + UI
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- **OLED:** SSD1306 64×32 (0.42"), I2C1. Small — UI is paginated (≈2 lines of
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small text); routing/CV assignments shown a few at a time.
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- **Buttons:** 4× low-profile SMD tact (Up / Down / Select / Back) on GP6-9. No
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rotary encoder (height constraint).
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## 7. RM2 radio module — risk largely eliminated
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Use the **Raspberry Pi Radio Module 2 (RM2, part RMC20452T)**: a CYW43439-based
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module (16.5 × 14.5 mm, 21 castellated pads) with integrated inverted-F PCB
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antenna and internal PA/LNA/T-R switch. Full modular radio approval in US/EU/
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UK/CA (FCC ID 2ABCB-RMC2GW4B52) — no radio design or certification needed on
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the host board. Software-compatible with the Pico W SDK, so our cyw43 firmware
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works unchanged.
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Host PCB responsibilities (from the RM2 datasheet):
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- Mount the 21 castellated pads; route the gSPI host pins to RP2040 (Pico W
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pinout: CLK=GP29, DAT=GP24, CS=GP25, PWR=GP23, on PIO0 + DMA CH0/CH1).
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- 3.3 V supply (no other rails).
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- **Antenna keepout area** must remain free of metal per the datasheet for
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the module's PCB antenna to perform.
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## 8. BOM (concrete parts)
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| Ref | Part | Package | Notes |
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|-----|------|---------|-------|
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| MCU | RP2040 (dev) / RP2350A (product) | QFN-56 / QFN-60 | dual target via embassy-rp features |
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| Flash | W25Q32JVSSIQ | SOIC-8 | 32 Mb (4 MB) — Rust firmware + blobs ~870 KB UF2, headroom for router code |
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| Radio | RM2 module RMC20452T | 16.5×14.5 mm, 21-pad | CYW43439 + PCB antenna, pre-certified |
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| DAC | DAC8568IAPW | TSSOP-16 | 8-ch 16-bit SPI, external Vref mode (AD5668BRUZ-1 alt) |
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| Op-amp | OPA4192IPWR ×2 | TSSOP-14 | quad RRO, 25 µV offset, on ±12 V rails — pitch-accurate |
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| Vref | REF3040AIDBZ | SOT-23 | 4.096 V DAC ref, 0.2%, 30 ppm/°C |
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| Mid-ref | REF2040AIDBZ (or buffered divider) | SOT-23 | 2.048 V bipolar offset reference |
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| Calib | 8× trimpot (e.g. 3296W) | THT/SMD | per-channel gain/offset trim |
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| MIDI opto | 6N138 | DIP-8 / SMD | MIDI in isolation |
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| Charger | BQ24075DRCR | MSOP-10 | single-cell LiPo charger + power-path (SYS pin) |
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| Fuel gauge | MAX17048G+T10 | TDFN-8 (or bare module) | LiPo SoC over I2C1 (optional) |
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| Battery | LiPo 3.7 V, 1500-2000 mAh | cell w/ protection PCB | integrated DW01/FS8205A protection |
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| 3.3 V reg | RT6150B-33GQW | QFN | buck-boost, 800 mA, Pico-proven (TPS62A01 alt) |
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| +12 V boost | TPS61088 | QFN | 5→12 V, low-noise + LC filter |
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| −12 V inverter | TPS63700 | SOT-23-6 / QFN | 5→−12 V, low-noise + LC filter (LM2611 alt) |
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| Display | SSD1306 64×32 OLED | 0.42" I2C | — |
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| Jacks | 6× 3.5 mm TRS | SMD | 2 MIDI + 4 CV stereo |
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| USB | USB-C receptacle | SMD | 5 V data + power |
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| UI | 4× low-profile tact | SMD | Up/Down/Select/Back (no encoder) |
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| Debug | 2×3 SWD header | THT | Picoprobe first-flash |
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| — | polyfuse, bidirectional TVS array, passives | — | bipolar output protection + rail protection |
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## 9. Risks / open items
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1. **Bipolar ±10 V offset/gain calibration** — the 2.048 V mid-ref and ×4.88
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gain set the 0 V center and ±10 V span; 0.2 % ref → ~±20 mV untrimmed
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(≈0.02 semitone). Per-channel trimpots trim gain + offset to <1 mV; or
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calibrate in firmware (measure + store correction) to avoid trimpots.
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2. **±12 V rail noise on pitch** — two switching converters (+12 V boost,
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−12 V inverter); LC filter both rails + 4.096 V/2.048 V refs are the pitch
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reference, not the rails, so op-amp PSRR + filtering keeps it clean.
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3. **Custom RP2040 bring-up** — needs a SWD header + Picoprobe; USB bootloader
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only works after first flash.
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4. **64×32 OLED UI** — tight; menu must be paginated and minimal.
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5. **Boost current budget** — RM2 TX spikes + 8 CV op-amps + two converters;
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BLE-only keeps the system draw ~340-400 mA peak from SYS, plus up to ~500 mA
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battery charge from USB → USB-C 5 V/900 mA-1.5 A recommended (USB 2.0 500 mA
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only if charging paused or BLE-only/low charge rate).
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6. **Battery sag at end-of-discharge** — SYS can sag to ~3.3 V; the 3.3 V path
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must be a buck-boost (RT6150B is) or it drops out. Fuel gauge drives a
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low-battery warning in firmware; cut off at ~3.3 V to protect the cell.
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7. **Charge thermal** — BQ24075 thermal-regulates; keep it away from the CV
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references / DAC and away from the LiPo for thermal separation.
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8. **LiPo shipping / safety** — LiPo cells have shipping (UN3481) and safety
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considerations; use a protected cell, add a charge LED, follow charge-IC
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layout guidance.
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9. **RM2 antenna keepout** — follow the datasheet keepout; the one layout
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constraint the radio imposes.
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## 10. Build order (hardware)
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1. Schematic (this doc is the spec).
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2. RM2 module footprint + antenna keepout.
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3. RP2040 + flash + USB + SWD.
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4. Power (BQ24075 power-path + LiPo, 3.3 V buck-boost, +12 V boost,
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−12 V inverter, 4.096 V + 2.048 V refs, MAX17048 fuel gauge).
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5. CV analog (DAC + difference-amp + offset + trim + bidir protection + TRS).
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6. MIDI TRS circuit.
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7. OLED + buttons.
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8. Keypad header (the Pimoroni keypad connects via a header or is integrated).
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9. PCB layout, DRC, review, fab.
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## 11. Connection table (netlist source of truth)
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Reference designators and net membership. RP2040 pins are logical GPIO names
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(map to physical QFN pins in KiCad). A machine-importable KiCad netlist lives
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at `hardware/netlist.net`.
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### Reference designators
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| Ref | Part | Role |
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|-----|------|------|
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| U1 | RP2040 | MCU |
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| U2 | W25Q32JV | flash |
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| MOD1 | RM2 (RMC20452T) | BLE/WiFi radio |
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| U3 | RT6150B-33 | 3.3 V buck |
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| U4 | TPS61088 | +12 V boost |
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| U5 | TPS63700 | −12 V inverter |
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| U6 | REF3040 | 4.096 V DAC reference |
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| U7 | REF2040 | 2.048 V bipolar mid-reference |
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| U8 | DAC8568 | 8-ch 16-bit SPI DAC |
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| U9, U10 | OPA4192 | quad RRO op-amp ×2 (8 CV channels) |
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| U11 | 6N138 | MIDI-in optocoupler |
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| U12 | BQ24075 | single-cell LiPo charger + power-path |
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| U13 | MAX17048 | LiPo fuel gauge (I2C) |
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| BAT1 | LiPo cell | ~1500-2000 mAh, integrated protection |
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| DISP1 | SSD1306 | 64×32 OLED (I2C) |
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| J1 | USB-C | 5 V + USB data |
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| J2 | 2×3 header | SWD (Picoprobe) |
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| J3, J4 | 3.5 mm TRS | MIDI in / out |
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| J5–J8 | 3.5 mm TRS | 4× stereo CV out |
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| J9 | 2×5 header | Pimoroni keypad |
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| SW1–SW4 | tact | UI (Up/Down/Select/Back) |
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| FB1 | polyfuse | 5 V input protection |
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| L1, L2 | inductors | boost/inverter LC filters |
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| D1–D8 | bidirectional TVS | CV output protection |
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| R_cal1–R_cal8 | — | per-channel calibration (see §4) |
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### Power nets (with battery power-path)
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```
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J1.VBUS ── FB1 ── VBUS_5V
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VBUS_5V ── U12 (BQ24075 VBUS) ; USB input to charger/power-path
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BAT1 ── U12 (BAT) ; LiPo cell
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U12 (SYS) ── SYS rail (~3.0-4.2 V) ; system supply (USB or battery)
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SYS ── U3 (RT6150B-33) ── +3V3 ; buck-boost: holds 3.3 V across SYS range
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SYS ── U4 (TPS61088) + L1 LC ── +12V
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SYS ── U5 (TPS63700) + L2 LC ── −12V
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+3V3 ── U6 (REF3040) ── VREF4V096 ; refs from clean +3V3, not saggy SYS
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+3V3 ── U7 (REF2040) ── VMID2V048
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U13 (MAX17048) SDA/SCL ── I2C1 (shared with OLED) ; fuel gauge
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U12 (STAT) ── U1.GPIO16 ; charge status (→ LED + firmware)
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GND = J1.GND, BAT1.GND, all return paths
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```
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### RM2 radio (Pico-W-compatible gSPI)
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```
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MOD1.CLK ── U1.GPIO29 (PIO0 SPI CLK)
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MOD1.DAT ── U1.GPIO24 (PIO0 SPI DOUT/MOSI)
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MOD1.CS ── U1.GPIO25 (output, CS)
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MOD1.PWR ── U1.GPIO23 (output, power enable)
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MOD1.IO0 ── (onboard LED, CYW43 WL_GPIO0 — status)
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MOD1.VDD ── +3V3
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MOD1.GND ── GND
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MOD1 bus driven by U1 PIO0 + DMA CH0/CH1 (firmware copies Pico W)
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```
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### USB + flash + SWD
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```
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J1.DP ── U1.GPIO15 (USB_DP)
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J1.DM ── U1.GPIO14 (USB_DM)
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U2.CS ── U1.QSPI_CS
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U2.CLK ── U1.QSPI_SCK
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U2.DQ0 ── U1.QSPI_SD0 ... DQ3 ── SD3 (QSPI flash)
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U2.VDD ── +3V3
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J2.SWDIO ── U1.SWDIO J2.SWCLK ── U1.SWCLK J2.GND ── GND
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```
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### CV DAC (SPI1)
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```
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U8.SCLK ── U1.GPIO10
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U8.DIN ── U1.GPIO11
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U8.CS ── U1.GPIO13 (output)
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U8.LDAC ── U1.GPIO20 (output, immediate-update / strobe)
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U8.RSTB ── U1.GPIO21 (output, reset)
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U8.VREF ── VREF4V096
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U8.VDD ── +3V3
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U8.VSS ── GND
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U8.VOUTA..VOUTH ── 8 CV difference-amp inputs (see below)
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```
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### CV analog (8 channels, bipolar ±10 V)
|
||
|
||
Per channel n (n=1..8): subtractor `Vout = (Rfb/Rin)*(Vdac − VMID2V048)`,
|
||
Rin=10.0 k, Rfb=48.7 k (ratio 4.87 → ±9.98 V span; firmware calibrates exact).
|
||
One op-amp = 1/4 of an OPA4192 (U9 = ch1-4, U10 = ch5-8).
|
||
|
||
```
|
||
Channel 1 (J5.tip = pitch, J5.ring = gate):
|
||
U8.VOUTA ── Rin1a ── U9.1IN- ; Vdac into subtractor V-
|
||
VMID2V048 ── Rin1b ── U9.1IN+ ; mid-ref into V+
|
||
U9.1OUT ── Rfb1 ── U9.1IN- ; feedback
|
||
Rgnd1 (48.7k) ── U9.1IN+ ── GND ; balance resistor
|
||
U9.1OUT ── Rser1 (1k) ── D1 ── J5.tip
|
||
U9.1OUT ── (rail) +12V / −12V ; OPA4192 supply
|
||
Channel 2 (J5.ring): U8.VOUTB → U9.2... → J5.ring
|
||
...channels 3,4 → U9.3/4 → J6.tip/ring
|
||
...channels 5-8 → U10.1-4 → J7.tip/ring, J8.tip/ring
|
||
```
|
||
Pattern repeats: VOUTx → Rin → subtractor → Rfb feedback, Rgnd on V+, output
|
||
→ 1 k series → TVS → TRS tip/ring. U9/U10 V+ = +12 V, V− = −12 V.
|
||
|
||
### MIDI TRS (3.5 mm, TRS-A)
|
||
|
||
```
|
||
MIDI in (J3):
|
||
J3.tip ── R_lim (220Ω) ── U11.LED-A (6N138 anode)
|
||
U11.LED-K ── J3.ring
|
||
U11.OC-collector ── +3V3 via R_pull (1k) ── U1.GPIO1 (UART0 RX)
|
||
U11.OC-emitter ── GND
|
||
U11.base ── 1N4148 + 1k speed-up ── GND/+3V3 (per 6N138 app note)
|
||
MIDI out (J4):
|
||
U1.GPIO0 (UART0 TX) ── R_lim (220Ω) ── J4.tip
|
||
J4.ring ── +3V3
|
||
```
|
||
|
||
### Display + UI + keypad
|
||
|
||
```
|
||
DISP1.SDA ── U1.GPIO2 (I2C1 SDA) DISP1.SCL ── U1.GPIO3 (I2C1 SCL)
|
||
DISP1.VDD ── +3V3 DISP1.GND ── GND
|
||
SW1 ── U1.GPIO6 SW2 ── U1.GPIO7 SW3 ── U1.GPIO8 SW4 ── U1.GPIO9
|
||
(each SW: other side to GND; internal pull-up)
|
||
Keypad header J9 (Pimoroni RGB Keypad):
|
||
J9.SDA ── U1.GPIO4 (I2C0 SDA) J9.SCL ── U1.GPIO5 (I2C0 SCL)
|
||
J9.CS ── U1.GPIO17 J9.SCK ── U1.GPIO18 J9.MOSI ── U1.GPIO19
|
||
J9.VDD ── +3V3 J9.GND ── GND
|
||
```
|
||
|
||
### Spare GPIOs
|
||
|
||
```
|
||
U1.GPIO16, GPIO22, GPIO26, GPIO27, GPIO28 ── unassigned (status LEDs / analog in / future)
|
||
```
|
||
|