metronome/hardware/DESIGN.md

PM_K-1 Core Board ("brain") — design-of-record

VARASYS PolyMeter · heirloom pro-audio · modular brain/face architecture Status: design-of-record / pre-layout. Component selection complete; PCB routing is the remaining interactive step (see Open items).

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1. Philosophy

This is meant to be a device people hand down to their great-grandkids. The core board carries all the active electronics and is specced to pro/audiophile tier with longevity and serviceability as first-class goals. We do not value-engineer the audio path.

The user selects the face/enclosure/connector components; this document specs only the core.

2. Architecture — modular brain/face

• Core ("brain"): RP2350 + power + RTC + the full pro-audio analog chain + control logic. One core design is reused across every form factor.
• Face ("form factor"): display, touch, joystick, buttons, LED, speaker, the physical audio/MIDI connectors, panel switches, enclosure. The core never decides connector type.
• Two interconnects, deliberately separate (§7):
  1. a digital ribbon whose pinout mirrors the Raspberry Pi Pico, so a stock Pico/Pico 2 on a test adapter can drive any face board for bring-up;
  2. an analog interconnect (+ a small MIDI interconnect) kept physically away from the fast digital ribbon, because a balanced audio signal must never run parallel to the 24 MHz display SPI.

3. Block diagram

USB-C 5V ─┬─► 3V3 LDO ──────────────────────► RP2350 (core reg + ext L) + W25Q128 16MB + RV-8803 RTC(+CR2032)
          │                                            │ I²S + low-jitter MCLK
          └─► TPS65131 ±18V ─► TPS7A49/30 LDO ─► clean ±15V          ▼
                                                          PCM5102A DAC ──► [summing: click + input]
                                                                                    │
  bal IN ─[ESD/DC-block/clamp/series-R]─► [LINE/INST relay] ─┬─ THAT1240 receiver ──┘
  (analog interconnect)                                      └─ OPA1641 Hi-Z DI buffer (+gain)
                                                                                    │
                                              THAT1646 balanced driver ─[47Ω build-out]─► [MUTE relay]─► bal OUT
                                                                                    shield ─[panel SW]─[GND-LIFT relay]─ gnd
  sig/clip peak detect ─► LM393 ─► RP2350 GPIO (UI) + LED lines (interconnect)
  RP2350 UART ─► [DNP: H11L1 opto IN + 74LVC14 buffer OUT] ─► MIDI interconnect   (USB-MIDI = default)

4. Functional blocks

4.1 MCU + digital

• RP2350A (QFN-60, 30 GPIO). Chosen over RP2040 as the newest in-house part: dual M33+RISC-V, 520 KB RAM, secure boot, longest production runway. Firmware runs unchanged.
  • Erratum E9: high-impedance inputs can latch — every input we read (face switches, buttons) gets an external pulldown, never relies on the internal pad alone.
  • Core supply via the RP2350 on-chip switched-mode regulator (external inductor); 3V3 IO from an external LDO (AP2112K-3.3 / TLV75533).
• Flash: Winbond W25Q128JV (16 MB) — genuine part; the CircuitPython appliance bundles the editor + tracks on a USB drive. Firmware does wear-leveling for history.json.
• Crystal: 12 MHz ±30 ppm.
• Debug/service: SWD 2×5 Cortex-Debug header + labeled test points (rails, I²S, audio nodes).

4.2 Click source (DAC)

• TI/Burr-Brown PCM5102A I²S DAC — reliability-first, widely stocked for future repair.
• Fed by a dedicated low-jitter audio oscillator (22.5792/24.576 MHz MEMS XO), not an MCLK jittered out of the RP2350 PIO — jitter is audible as a raised noise floor.

4.3 Analog audio chain (the heart)

• Input (balanced, switchable line/instrument):
  • Line mode: THAT1240 laser-trimmed balanced receiver (~high CMRR, no hand-matched resistors).
  • Instrument mode: OPA1641 JFET Hi-Z buffer (≥1 MΩ) + ~+10–15 dB gain (active DI).
  • Selected by a gold-contact signal relay on the core (1 GPIO; touchscreen toggle, optional face panel switch on a separate GPIO input).
  • Protection (non-negotiable): series DC-blocking film cap (blocks +48 V phantom — the real input-killer), clamp diodes/TVS to the rails, series current-limit resistor. A wrong-mode plug then only sounds wrong; nothing is damaged.
• Mix: digital/firmware (touchscreen). Analog stage at unity; click level set via the DAC.
• Output driver: THAT1646 balanced line driver, 47 Ω build-out per leg for cable-capacitance stability and short-circuit tolerance (§5.1).
• No electrolytics in the signal path — film coupling caps (WIMA). 0.1 % thin-film resistors.

4.4 Output protection / conditioning

• Power-up/down mute relay — fail-safe, de-energized = muted (shorts hot+cold to gnd). A hardware rail supervisor + RC turn-on delay un-mutes only after ±15 V settles; on power loss the coil drops and mutes faster than the rails can thump. Not MCU-dependent. The MCU can also assert mute (for clean line/inst flips and DAC reconfig).
• Ground-lift — both a face panel switch and a core GPIO relay, wired in series in the shield-ground path: bonded only when both closed, either opening lifts it. Soft lift = 100 Ω ∥ 10 nF (not a hard open) so RF/safety keeps a path.

4.5 RTC

• Micro Crystal RV-8803 (integrated 32.768 kHz crystal → no second crystal near the RP2350's own) + CR2032 in a socketed holder. Shares the touch I²C bus (no extra GPIO). Drift is irrelevant for a practice-log timestamp; reliability and zero-fuss layout win.

4.6 MIDI (default USB, hardware optional)

• USB-MIDI is the default and already in firmware — IN/OUT/THRU are software routing to a computer/tablet host. Zero extra parts.
• Dedicated DIN/TRS MIDI is a DNP populate-option: the RP2350 UART lines route to the MIDI interconnect with H11L1 opto IN + 74LVC14 buffered OUT/THRU footprints left unpopulated. A "stage" face can populate them for laptop-free hardware sync. (USB-MIDI can't peer-to-peer with standalone DIN gear — the device is a USB peripheral, not a host — which is why the hardware option stays available.) Analog pulse/clock sync: not included (MIDI only).

4.7 Monitor speaker

• DNP-optional class-D amp (PAM8302) footprint on the core; speaker +/- routed on the analog interconnect. Populated only for form factors that want a built-in monitor.

5. The five remaining pro details (decided)

5.1 Output impedance & level calibration

• THAT1646 source impedance is near-zero; 47 Ω build-out resistors per leg give stable driving into long/capacitive cables and survive a shorted output.
• Level calibration: a 25-turn precision trimmer (Bourns 3296W) in the driver gain network, factory-set so DAC full-scale → +4 dBu nominal, leaving ~+24 dBu peak headroom on ±15 V. Set-and-forget on the core; not a face control.

5.2 Signal / clip indication

• A peak detector (Schottky + hold cap) on the input (signal-present, ~−40 dBu) and on the driver-input/summing node (clip, within ~3 dB of rail) feeds an LM393 dual comparator.
• Comparator outputs go to RP2350 GPIOs (clip/signal shown on the touchscreen) and are mirrored to SIG/CLIP LED drive lines on the digital interconnect so a face can fit discrete LEDs.

5.3 ESD / EMI hardening

• USB-C: USBLC6-2SC6 (or TPD4E05U06) ESD array on D±/CC/VBUS; common-mode choke on the data pair; shell bonded to chassis via RC; ferrite + TVS + bulk on VBUS.
• Interconnects: ~33 Ω series on fast SPI lines for edge-rate control; ESD clamp arrays on any line reaching a user-touchable cable; interleaved ground pins; ferrite beads where 3V3/5V cross into the analog domain.
• Board: full ground planes + stitching vias; the boost/inverter switcher lives in a guarded corner away from the analog section; analog/digital grounds meet at a single star point.
• Heirloom option: conformal coating for humidity/longevity (build-time choice).

5.4 Chassis / strain-relief (core-side)

• 4× M3 mounting holes with keep-outs; a dedicated chassis-ground pad/pin.
• Through-hole-anchored USB-C jack (SMD-only tabs shear off with cable wiggle — unacceptable for a 50-year device).
• Shrouded, keyed, latching interconnect headers (can't insert backward or vibrate loose).
• Panel strain-relief and connector mounting live on the face/enclosure.

5.5 Interconnect pinout — see §7.

6. Power tree

Rail	Source	Part	Notes
+5 V	USB-C VBUS	—	ferrite + TVS + bulk
+3V3 (IO)	LDO from 5 V	AP2112K-3.3 / TLV75533	digital domain
+1.1 V core	RP2350 internal SMPS	(external inductor)	per RP2350 ref
±18 V (raw)	dual boost/inverter from 5 V	TPS65131	switcher, guarded corner
±15 V (clean)	ultra-low-noise LDO	TPS7A4901 (+) / TPS7A3001 (−)	feeds all audio op-amps

7. Interconnect pinouts

7.1 Digital ribbon — 2×13 (26-pin) IDC, Pico-pinout-compatible

Grounds interleaved around SPI. A Pico/Pico 2 test adapter maps these to the listed GP.

Pin	Signal	GP		Pin	Signal	GP
1	+5V	—		2	GND	—
3	+3V3	—		4	GND	—
5	SPI_SCK	GP2		6	GND	—
7	SPI_MOSI	GP3		8	LCD_CS	GP5
9	LCD_DC	GP6		10	LCD_RST	GP7
11	GND	—		12	I2C_SDA	GP8
13	I2C_SCL	GP9		14	GND	—
15	JOY_X (ADC0)	GP26		16	JOY_Y (ADC1)	GP27
17	BTN_A	GP15		18	BTN_B	GP14
19	WS2812	GP12		20	GND	—
21	GNDLIFT_SW (in)	GP21		22	LINEINST_SW (in)	GP22
23	SIG_LED	GP19		24	CLIP_LED	GP20
25	GND	—		26	GND	—

I²S (BCK/LRCK/DOUT), the relays (line/inst route GP16, mute GP18, gnd-lift GP17), and MCLK stay on-core — they are not on the ribbon. A Pico test brain drives the digital/face I/O above but cannot drive the analog chain (DAC/op-amps are core-only).

7.2 Analog audio interconnect — 2×5 (10-pin), twisted/shielded, away from digital

Pin	Signal		Pin	Signal
1	AOUT_HOT		2	AGND
3	AOUT_COLD		4	CHASSIS/SHIELD (face side of ground-lift)
5	AIN_HOT		6	AGND
7	AIN_COLD		8	SPK+ (DNP)
9	AGND		10	SPK− (DNP)

7.3 MIDI interconnect — 1×6, only if DNP MIDI populated

Pin	Signal
1	MIDI_OUT_A (TRS-A tip/ring leg)
2	MIDI_OUT_B
3	MIDI_IN_A (to opto)
4	MIDI_IN_B
5	+5V (OUT drive)
6	GND / shield

8. BOM

Full part list with manufacturer numbers and rough costs in hardware/BOM.csv. Headline parts: RP2350A · W25Q128JV · PCM5102A · THAT1240 + THAT1646 · OPA1641 · OPA1612 · TPS65131 + TPS7A4901/3001 · RV-8803 · USBLC6-2SC6 · 3× Panasonic TQ2SA gold-contact relays · H11L1 (DNP).

9. Manufacturing

• PCB: ENIG (gold) finish — non-negotiable for decades of reliable contacts/solderability.
• Assembly: JLCPCB/PCBWay PCBA, ~5-board prototype minimum; ~$80–200 first run. Core parts cost ~$25–40/board one-off (pro op-amps + relays dominate), trending toward ~$15–20 at qty 100.
• Most expensive items are the THAT audio ICs and the relays — that's where "heirloom" lives.

10. Open items

• PCB layout/routing is the interactive next step — placement, controlled-impedance USB pair, star ground, switcher isolation, copper pours, DRC. The KiCad project under hardware/kicad/ is a documented schematic canvas + design-of-record; symbol placement and wiring happen in Eeschema. kicad-cli is used here for ERC and PDF export verification.
• Confirm the exact 3.5" ST7796/GT911 panel connector (FPC vs header) before finalizing the face.
• Decide MIDI connector (TRS-MIDI Type-A vs DIN-5) per form factor — face decision.