Hardware plan: RM2 module, RP2040->RP2350 target

Use the pre-certified Raspberry Pi Radio Module 2 (RM2, CYW43439 + PCB
antenna) instead of a bare CYW43439 — eliminates RF design + cert; only
the datasheet antenna keepout remains. Dev/test on RP2040 (Pico W),
product on RP2350; firmware dual-builds via embassy-rp features.

docs/hardware-plan.md

@@ -1,8 +1,16 @@
 # Hardware Plan — flexible MIDI router (v1)
 
-Status: draft 2026-08-21. Custom all-in-one PCB, RP2040 + CYW43439, desktop,
-USB-powered, 0-10 V CV via onboard boost, 3.5 mm TRS MIDI, 4× stereo CV,
-64×32 OLED + tact buttons (no encoder, low profile).
+Status: draft 2026-08-21. Custom all-in-one PCB, desktop, USB-powered, 0-10 V
+CV via onboard boost, 3.5 mm TRS MIDI, 4× stereo CV, 64×32 OLED + tact buttons
+(no encoder, low profile).
+
+**MCU target:** develop/test on **RP2040** (Pico W dev board — the firmware
+already runs on it), product on **RP2350**. The RM2 radio module is compatible
+with both. The Rust firmware builds for either via embassy-rp features
+(`rp2040` → `thumbv6m-none-eabi`; `rp235xa` for the RP2350A used in the Pico 2 W
+→ `thumbv8m.main-none-eabihf`). Pin assignments below are RP2040/Pico-W; the
+RP2350 product PCB keeps the Pico-2-W-compatible RM2 bus and reuses the same
+peripheral choices.
 
 See `midi-router-architecture` (session memory) for the software/firmware
 architecture this hardware serves.
@@ -10,9 +18,9 @@ architecture this hardware serves.
 ## 1. Block diagram
 
 ```
-USB-C 5V ──┬── 3.3 V buck ── RP2040 + W25Q16 flash + CYW43439 (BLE/WiFi)
+USB-C 5V ──┬── 3.3 V buck ── RP2040 + W25Q16 flash + RM2 module (CYW43439, BLE/WiFi)
            │                   │
-           │                   ├── PIO0 SPI ── CYW43439 (Pico-W pinout)
+           │                   ├── PIO0 SPI ── RM2 (CYW43439 gSPI, Pico-W pinout)
            │                   ├── USB DP/DM ── (USB-MIDI composite, CDC+MIDI)
            │                   ├── UART0 ──── MIDI TRS in (6N138) / out
            │                   ├── I2C0 ───── Pimoroni RGB Keypad (buttons)
@@ -28,14 +36,14 @@ USB-C 5V ──┬── 3.3 V buck ── RP2040 + W25Q16 flash + CYW43439 (BLE
 
 ## 2. RP2040 pin map
 
-Peripherals follow the Pico W reference for the CYW43 bus (so the cyw43 driver
-and our firmware work unchanged) and our existing keypad wiring.
+Peripherals follow the Pico W reference for the RM2/CYW43 bus (so the cyw43
+driver and our firmware work unchanged) and our existing keypad wiring.
 
 | Function           | RP2040 pins            | Notes                                   |
 |--------------------|------------------------|-----------------------------------------|
 | USB device         | GP14 (DM), GP15 (DP)   | USB-C; CDC + MIDI composite             |
-| CYW43 PIO-SPI      | GP29 (CLK), GP24 (DAT), GP25 (CS-out), GP23 (PWR), PIO0, DMA CH0/CH1 | copy Pico W |
-| CYW43 onboard LED  | CYW43 WL_GPIO0         | status/heartbeat                        |
+| RM2 (CYW43) bus     | GP29 (CLK), GP24 (DAT), GP25 (CS-out), GP23 (PWR), PIO0, DMA CH0/CH1 | copy Pico W |
+| RM2 onboard LED    | CYW43 WL_GPIO0         | status/heartbeat                        |
 | Keypad buttons     | GP4 (SDA), GP5 (SCL)   | I2C0, expander @0x20                    |
 | Keypad LEDs        | GP17 (CS), GP18 (SCK), GP19 (MOSI) | SPI0 tx-only, 4 MHz, APA102    |
 | DAC (CV outs)      | GP10 (SCK), GP11 (MOSI), GP13 (CS) | SPI1 tx-only, 16-bit DAC; MISO unused |
@@ -92,20 +100,26 @@ on 5 V (simpler, fewer parts, but half the CV range).
 - **Buttons:** 4× low-profile SMD tact (Up / Down / Select / Back) on GP6-9. No
   rotary encoder (height constraint).
 
-## 7. CYW43439 RF section — the risk
+## 7. RM2 radio module — risk largely eliminated
 
-Bare CYW43439 on a custom PCB is the single hardest part:
-- QFN routing (0.4 mm pitch), 32.768 kHz + 37.4 MHz crystals, 50 Ω controlled-
-  impedance RF trace, chip-antenna keepout (no copper under antenna).
-- BLE transmits → FCC/CE intentional-radiator cert for commercial sale (less
-  critical for personal use, but the radio must still work).
-- **Mitigation:** Raspberry Pi open-sourced the Pico W schematic + PCB — lift the
-  CYW43439 RF section verbatim (chip, crystals, RF match, antenna, keepout).
+Use the **Raspberry Pi Radio Module 2 (RM2, part RMC20452T)**: a CYW43439-based
+module (16.5 × 14.5 mm, 21 castellated pads) with integrated inverted-F PCB
+antenna and internal PA/LNA/T-R switch. Full modular radio approval in US/EU/
+UK/CA (FCC ID 2ABCB-RMC2GW4B52) — no radio design or certification needed on
+the host board. Software-compatible with the Pico W SDK, so our cyw43 firmware
+works unchanged.
+
+Host PCB responsibilities (from the RM2 datasheet):
+- Mount the 21 castellated pads; route the gSPI host pins to RP2040 (Pico W
+  pinout: CLK=GP29, DAT=GP24, CS=GP25, PWR=GP23, on PIO0 + DMA CH0/CH1).
+- 3.3 V supply (no other rails).
+- **Antenna keepout area** must remain free of metal per the datasheet for
+  the module's PCB antenna to perform.
 
 ## 8. BOM sketch
 
-- RP2040 (QFN-56), W25Q16JV flash (16 Mb), CYW43439, 32.768 kHz + 37.4 MHz
-  crystals, chip antenna (Pico W part or equivalent).
+- RP2040 (dev) / **RP2350A** (product) MCU, W25Q16JV-equivalent flash,
+  **RM2 radio module (RMC20452T, CYW43439 + PCB antenna, pre-certified)**.
 - DAC8568IAPW (8-ch 16-bit SPI, TSSOP-16) — or AD5668.
 - OPA4192IPW ×2 (quad RRO op-amp, TSSOP-14).
 - REF3040AIDBZ (4.096 V ref, SOT-23).
@@ -118,19 +132,20 @@ Bare CYW43439 on a custom PCB is the single hardest part:
 
 ## 9. Risks / open items
 
-1. **CYW43439 RF + cert** — biggest risk; copy the open Pico W RF section.
-2. **0-10 V CV noise from the boost** — LC filter + dedicated 4.096 V Vref.
-3. **Custom RP2040 bring-up** — needs a SWD header + Picoprobe; USB bootloader
+1. **0-10 V CV noise from the boost** — LC filter + dedicated 4.096 V Vref.
+2. **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** — CYW43 TX spikes + 8 CV op-amps on a single USB
+3. **64×32 OLED UI** — tight; menu must be paginated and minimal.
+4. **Boost current budget** — RM2 TX spikes + 8 CV op-amps on a single USB
    5 V/500 mA (USB 2.0) port is tight; specify USB-C 5 V/900 mA-3 A, or limit
    CV op-amp current.
+5. **RM2 antenna keepout** — follow the datasheet keepout; the one layout
+   constraint the radio imposes.
 
 ## 10. Build order (hardware)
 
 1. Schematic (this doc is the spec).
-2. CYW43439 RF section — copied from Pico W reference.
+2. RM2 module footprint + antenna keepout.
 3. RP2040 + flash + USB + SWD.
 4. Power (3.3 V buck, 12 V boost, Vref).
 5. CV analog (DAC + op-amps + protection + TRS).