shawn
0ae2ca6e8e
Custom I2S HAT for Pi Multi-FX Pedal — full hardware package: - KiCad project: schematic (kicad_sch), PCB layout (kicad_pcb), project file - Custom DT overlay: pcm1808-pcm5102-overlay.dts (no upstream overlay exists for PCM1808) - docs/hardware-bom.md: BOM (9.25 HAT + 1.75 with enclosure), wiring diagram, PCB design notes - docs/config-audio.md: DT overlay build, ALSA config, JACK integration, troubleshooting - scripts/install_hat.sh: One-command install + test suite (6 tests) - hardware/gerber/: JLCPCB fab instructions and assembly notes Circuit: Guitar input → TL072 preamp (gain ~20dB) → PCM1808 ADC → RPi I2S → PCM5102 DAC → RC filter → output jack I2S: BCLK/GPIO18, LRCLK/GPIO19, DIN/GPIO20, DOUT/GPIO21 Stacking header passes all 40 GPIO pins for footswitches/LEDs/display
219 lines
9.9 KiB
Markdown
219 lines
9.9 KiB
Markdown
# PCM1808+PCM5102 I2S HAT — Bill of Materials + Wiring Diagram
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> **Project:** Pi Multi-FX Pedal
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> **HAT:** Custom PCM1808 ADC + PCM5102 DAC I2S audio HAT
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> **Board:** 65×56mm 2-layer PCB (1590B compatible)
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> **Revision:** 1.0 — 2026-06-07
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---
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## Bill of Materials
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### Audio ICs
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| Ref | Part | Description | Package | Qty | Est. Cost | Source |
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|-----|------|-------------|---------|:---:|:---------:|--------|
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| U3 | **PCM1808** | 24-bit stereo ADC, I2S output | TSSOP-14 | 1 | $3.50 | Mouser/Digikey |
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| U4 | **PCM5102A** | 32-bit stereo DAC, I2S input | TSSOP-20 | 1 | $3.00 | Mouser/Digikey |
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| U2 | **TL072CN** | Dual JFET op-amp (preamp + buffer) | DIP-8 | 1 | $1.50 | Mouser/Digikey |
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### Power Regulation
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| Ref | Part | Description | Package | Qty | Est. Cost | Source |
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|-----|------|-------------|---------|:---:|:---------:|--------|
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| U1 | **AMS1117-3.3** | 3.3V LDO regulator, 1A | SOT-223 | 1 | $0.50 | Mouser/Digikey |
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| C1 | 10µF electrolytic | Input decoupling (5V rail) | 5mm radial | 1 | $0.15 | — |
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| C2 | 100nF ceramic | Input decoupling (5V rail) | 0805 | 1 | $0.05 | — |
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| C3 | 10µF electrolytic | Output decoupling (3.3V rail) | 5mm radial | 1 | $0.15 | — |
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| C4 | 100nF ceramic | Output decoupling (3.3V rail) | 0805 | 1 | $0.05 | — |
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### Preamp Stage (Guitar Input → PCM1808)
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| Ref | Part | Description | Package | Qty | Est. Cost | Source |
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|-----|------|-------------|---------|:---:|:---------:|--------|
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| J2 | 6.35mm TS jack | Guitar input jack, switched (NC contact for bypass) | Panel-mount | 1 | $2.00 | Switchcraft/Neutrik |
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| R1 | **1MΩ** | Input pulldown (prevents floating input noise) | 1/4W axial | 1 | $0.05 | — |
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| R2 | **10kΩ** | Series resistor to op-amp non-inverting input | 1/4W axial | 1 | $0.05 | — |
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| R3 | **100kΩ** | Feedback resistor (gain = 1 + R3/R4) | 1/4W axial | 1 | $0.05 | — |
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| R4 | **10kΩ** | Ground reference for feedback divider | 1/4W axial | 1 | $0.05 | — |
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| C5 | 10µF electrolytic | DC-blocking cap after op-amp output | 5mm radial | 1 | $0.15 | — |
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### ADC Decoupling + Configuration
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| Ref | Part | Description | Package | Qty | Est. Cost | Source |
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|-----|------|-------------|---------|:---:|:---------:|--------|
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| C6 | 100nF ceramic | PCM1808 VCC decoupling | 0805 | 1 | $0.05 | — |
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| C7 | 10µF electrolytic | PCM1808 VCC bulk decoupling | 5mm radial | 1 | $0.15 | — |
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### DAC + Output Stage (+ config resistor)
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| Ref | Part | Description | Package | Qty | Est. Cost | Source |
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|-----|------|-------------|---------|:---:|:---------:|--------|
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| J3 | 6.35mm TS jack | Audio output jack | Panel-mount | 1 | $2.00 | Switchcraft/Neutrik |
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| C8 | 100nF ceramic | PCM5102 VCC decoupling | 0805 | 1 | $0.05 | — |
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| C9 | 10µF electrolytic | PCM5102 VCC bulk decoupling | 5mm radial | 1 | $0.15 | — |
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| C10 | 10µF electrolytic | DC-blocking cap on DAC output (before output jack) | 5mm radial | 1 | $0.15 | — |
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| R5 | **330Ω** | Output series resistor (PCM5102 hiss reduction filter) | 1/4W axial | 1 | $0.05 | — |
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| R6 | **10kΩ** | Output stage resistor (output buffer feedback) | 1/4W axial | 1 | $0.05 | — |
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### Connectors
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| Ref | Part | Description | Qty | Est. Cost | Source |
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|-----|------|-------------|:---:|:---------:|--------|
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| J1 | 2×20 female header | RPi 40-pin GPIO stacking header, 8.5mm height | 1 | $2.50 | Adafruit/Amazon |
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| — | 2×20 male pin strip | Breakaway header for HAT-to-PCB mount | 1 | $1.00 | — |
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### Hardware
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| Part | Description | Qty | Est. Cost | Source |
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|------|-------------|:---:|:---------:|--------|
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| M2.5 standoffs | Brass, 12mm female-female, for RPi mounting | 4 | $2.00 | Amazon |
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| M2.5 screws | Pan head, 6mm, for RPi + HAT sandwich | 8 | $1.50 | — |
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| Pedal enclosure | 1590B diecast aluminum (112×60×31mm) | 1 | $8.00 | Hammond/Amazon |
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| Rubber feet | adhesive, 10mm diameter | 4 | $1.00 | — |
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---
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## Cost Summary
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| Section | Est. Cost |
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|---------|:---------:|
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| Audio ICs (PCM1808 + PCM5102 + TL072) | $8.00 |
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| Power regulation | $0.90 |
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| Preamp passives | $2.40 |
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| ADC/DAC decoupling + output | $2.45 |
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| Connectors + stacking header | $3.50 |
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| Hardware (standoffs, screws, enclosure) | $12.50 |
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| **Total HAT (board + component)** | **~$29.25** |
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| + Enclosure + hardware | **~$41.75** |
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Compare to AudioInjector Stereo HAT at $35-40 (no preamp, no enclosure) — the custom build costs about the same but is purpose-designed for a stompbox.
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---
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## Wiring Diagram
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```
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RPi 4B 40-pin Header (J1)
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┌─────────────────────────────────────────┐
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│ ● ● ● ● ● ● ● ● ● ● ● ● ● │ Pins 1-20
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│ ● ● ● ● ● ● ● ● ● ● ● ● ● │ Pins 21-40
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└─────────────────────────────────────────┘
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│ │ │ │ │ │ │ │ │ │
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│ │ │ │ │ │ │ │ │ │
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▼ ▼ ▼ ▼ ▼ ▼ ▼ ▼ ▼ ▼
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┌─────────────────────────────────────────┐
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│ P1 P9 P17 P25 P33 | P2 P10 ... │ HAT PCB (top view)
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│ GND 3V3 5V BCLK LRCLK │
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│ │
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│ ┌─────┐ ┌─────┐ ┌──────────────┐ │
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│ │ U3 │ │ U4 │ │ U2 │ │
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│ │PCM1808│ │PCM5102│ │ TL072 │ │
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│ │ ADC │ │ DAC │ │ dual op-amp │ │
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│ └──┬──┘ └──┬──┘ └──────┬───────┘ │
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│ │ │ │ │
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│ └────┬────┘ │ │
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│ │ (I2S bus) │ │
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│ │ ├── J2 │
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│ ┌────┴────┐ │ Guitar In│
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│ │ U1 │ │ │
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│ │AMS1117 │ └──────────│
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│ │ 3.3V │ │
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│ └────────┘ │
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│ 5V → 3.3V │
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│ │
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│ ┌───────┐ │
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│ J3 ──│ C10 │ │
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│ Output ───────│ 10µF │ │
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│ └───────┘ │
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└─────────────────────────────────────────┘
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```
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### Detailed I2S Connections
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```
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GPIO18 (BCLK) ───── PCM1808 pin 8 (BCK) ───── PCM5102 pin 14 (BCK)
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GPIO19 (LRCLK) ───── PCM1808 pin 7 (LRCK) ───── PCM5102 pin 13 (LRCK)
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GPIO20 (DIN) ────────────────────────────────→ PCM5102 pin 12 (DIN)
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GPIO21 (DOUT) ←──── PCM1808 pin 9 (DOUT)
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```
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### PCM1808 Configuration Pins
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| Pin | Name | Connection | Function |
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|-----|------|-----------|----------|
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| 12 | FMT | GND | I2S format (24-bit left-justified) |
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| 13 | MD1 | 3.3V | 48kHz sampling rate |
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| 14 | MD0 | GND | Standby disabled (always active) |
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### PCM5102 Configuration Pins
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| Pin | Name | Connection | Function |
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|-----|------|-----------|----------|
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| 15 | FLT | GND | Normal operation, low-latency filter |
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| 16 | DEMP | GND | No de-emphasis |
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| 17 | XSMT | 3.3V | Unmuted (always active) |
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### Preamp Stage Circuit
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```
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Guitar → J2 (TS jack)
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│
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├── R1 (1MΩ) → GND (input pulldown)
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│
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└── R2 (10kΩ) ──┬── TL072 (non-inverting input, pin 3)
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│
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│ TL072 (pin 6, output)
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│ │
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└── R3 (100kΩ) ──┐
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│
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TL072 (inverting input, pin 2)
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│
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R4 (10kΩ) → GND
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```
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Non-inverting gain = 1 + R3/R4 = 1 + 100k/10k = **11 (~20.8dB)**
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For high-output humbuckers, this is sufficient to bring ~200mV pickup output to ~2.2V (near line level). Single-coils may need a second gain stage; omit R4 and R3 becomes a 470kΩ feedback resistor for gain of 2 (~6dB).
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### Power Distribution
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```
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RPi 5V (pin 2/4)
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│
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├── U1 (AMS1117-3.3) → 3.3V
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│ ├── C1 (10µF) + C2 (100nF) ← input decoupling
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│ ├── C3 (10µF) + C4 (100nF) ← output decoupling
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│ │
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│ ├── U3 (PCM1808) — VCC pin 11
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│ │ └── C6 (100nF) + C7 (10µF) ← local decoupling
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│ │
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│ ├── U4 (PCM5102) — Vin pin 1
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│ │ └── C8 (100nF) + C9 (10µF) ← local decoupling
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│ │
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│ ├── U2 (TL072) — VCC+ pin 8
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│ └── PCM1808 MD1 (pin 13) — pull to 3.3V for 48kHz
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│
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└── PCM5102 XSMT (pin 17) — pull to 3.3V for unmuted
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```
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### Ground Strategy
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- **Star ground** at the RPi GND pin: all audio circuit grounds return to a single point
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- Separate analog and digital ground traces on PCB
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- Tie analog/digital ground planes together at the star point only
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- Keep I2S digital lines separate from analog audio traces
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- Input jack ground: connect to star via 10Ω + 100nF for ground loop isolation
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---
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## PCB Design Notes
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- **2-layer board**, 65×56mm, 1.6mm FR4, HASL finish
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- **Trace width:** 0.254mm signal, 0.5mm power
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- **Clearance:** 0.2mm min, 0.3mm for audio traces
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- **Via:** 0.6mm/0.3mm (outer/inner diameter)
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- **Copper pour:** GND flood fill on both layers
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- Keep I2S traces (BCLK, LRCLK, DIN, DOUT) equal length ±2mm
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- Place decoupling caps within 5mm of IC power pins
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- Separate analog audio section (left side of board) from digital I2S (right side) |