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shawn 0ae2ca6e8e feat: PCM1808+PCM5102 I2S HAT design files 
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
2026-06-07 23:52:13 -04:00

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PCM1808+PCM5102 I2S HAT — Bill of Materials + Wiring Diagram

Project: Pi Multi-FX Pedal
HAT: Custom PCM1808 ADC + PCM5102 DAC I2S audio HAT
Board: 65×56mm 2-layer PCB (1590B compatible)
Revision: 1.0 — 2026-06-07

Bill of Materials

Audio ICs

Ref Part Description Package Qty Est. Cost Source
U3 PCM1808 24-bit stereo ADC, I2S output TSSOP-14 1 $3.50 Mouser/Digikey
U4 PCM5102A 32-bit stereo DAC, I2S input TSSOP-20 1 $3.00 Mouser/Digikey
U2 TL072CN Dual JFET op-amp (preamp + buffer) DIP-8 1 $1.50 Mouser/Digikey

Power Regulation

Ref Part Description Package Qty Est. Cost Source
U1 AMS1117-3.3 3.3V LDO regulator, 1A SOT-223 1 $0.50 Mouser/Digikey
C1 10µF electrolytic Input decoupling (5V rail) 5mm radial 1 $0.15 —
C2 100nF ceramic Input decoupling (5V rail) 0805 1 $0.05 —
C3 10µF electrolytic Output decoupling (3.3V rail) 5mm radial 1 $0.15 —
C4 100nF ceramic Output decoupling (3.3V rail) 0805 1 $0.05 —

Preamp Stage (Guitar Input → PCM1808)

Ref Part Description Package Qty Est. Cost Source
J2 6.35mm TS jack Guitar input jack, switched (NC contact for bypass) Panel-mount 1 $2.00 Switchcraft/Neutrik
R1 1MΩ Input pulldown (prevents floating input noise) 1/4W axial 1 $0.05 —
R2 10kΩ Series resistor to op-amp non-inverting input 1/4W axial 1 $0.05 —
R3 100kΩ Feedback resistor (gain = 1 + R3/R4) 1/4W axial 1 $0.05 —
R4 10kΩ Ground reference for feedback divider 1/4W axial 1 $0.05 —
C5 10µF electrolytic DC-blocking cap after op-amp output 5mm radial 1 $0.15 —

ADC Decoupling + Configuration

Ref Part Description Package Qty Est. Cost Source
C6 100nF ceramic PCM1808 VCC decoupling 0805 1 $0.05 —
C7 10µF electrolytic PCM1808 VCC bulk decoupling 5mm radial 1 $0.15 —

DAC + Output Stage (+ config resistor)

Ref Part Description Package Qty Est. Cost Source
J3 6.35mm TS jack Audio output jack Panel-mount 1 $2.00 Switchcraft/Neutrik
C8 100nF ceramic PCM5102 VCC decoupling 0805 1 $0.05 —
C9 10µF electrolytic PCM5102 VCC bulk decoupling 5mm radial 1 $0.15 —
C10 10µF electrolytic DC-blocking cap on DAC output (before output jack) 5mm radial 1 $0.15 —
R5 330Ω Output series resistor (PCM5102 hiss reduction filter) 1/4W axial 1 $0.05 —
R6 10kΩ Output stage resistor (output buffer feedback) 1/4W axial 1 $0.05 —

Connectors

Ref Part Description Qty Est. Cost Source
J1 2×20 female header RPi 40-pin GPIO stacking header, 8.5mm height 1 $2.50 Adafruit/Amazon
— 2×20 male pin strip Breakaway header for HAT-to-PCB mount 1 $1.00 —

Hardware

Part Description Qty Est. Cost Source
M2.5 standoffs Brass, 12mm female-female, for RPi mounting 4 $2.00 Amazon
M2.5 screws Pan head, 6mm, for RPi + HAT sandwich 8 $1.50 —
Pedal enclosure 1590B diecast aluminum (112×60×31mm) 1 $8.00 Hammond/Amazon
Rubber feet adhesive, 10mm diameter 4 $1.00 —

Cost Summary

Section Est. Cost
Audio ICs (PCM1808 + PCM5102 + TL072) $8.00
Power regulation $0.90
Preamp passives $2.40
ADC/DAC decoupling + output $2.45
Connectors + stacking header $3.50
Hardware (standoffs, screws, enclosure) $12.50
Total HAT (board + component) ~$29.25
+ Enclosure + hardware ~$41.75

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.

Wiring Diagram

RPi 4B 40-pin Header (J1)
┌─────────────────────────────────────────┐
│  ●  ●  ●  ●  ●  ●  ●  ●  ●  ●  ●  ● ● │  Pins 1-20
│  ●  ●  ●  ●  ●  ●  ●  ●  ●  ●  ●  ● ● │  Pins 21-40
└─────────────────────────────────────────┘
    │  │  │  │  │  │  │  │  │  │
    │  │  │  │  │  │  │  │  │  │
    ▼  ▼  ▼  ▼  ▼  ▼  ▼  ▼  ▼  ▼
┌─────────────────────────────────────────┐
│  P1 P9 P17 P25 P33  |  P2 P10 ...      │  HAT PCB (top view)
│  GND 3V3 5V  BCLK LRCLK                │
│                                         │
│  ┌─────┐   ┌─────┐   ┌──────────────┐  │
│  │ U3  │   │ U4  │   │    U2        │  │
│  │PCM1808│ │PCM5102│ │    TL072     │  │
│  │ ADC  │  │ DAC  │  │ dual op-amp │  │
│  └──┬──┘   └──┬──┘   └──────┬───────┘  │
│     │         │              │          │
│     └────┬────┘              │          │
│          │ (I2S bus)        │          │
│          │                   ├── J2     │
│     ┌────┴────┐              │ Guitar In│
│     │   U1    │              │          │
│     │AMS1117  │              └──────────│
│     │  3.3V   │                         │
│     └────────┘                          │
│    5V → 3.3V                            │
│                                         │
│                              ┌───────┐  │
│                         J3 ──│ C10   │  │
│                Output ───────│ 10µF  │  │
│                              └───────┘  │
└─────────────────────────────────────────┘

Detailed I2S Connections

GPIO18 (BCLK)  ───── PCM1808 pin 8 (BCK)  ───── PCM5102 pin 14 (BCK)
GPIO19 (LRCLK) ───── PCM1808 pin 7 (LRCK) ───── PCM5102 pin 13 (LRCK)
GPIO20 (DIN)   ────────────────────────────────→ PCM5102 pin 12 (DIN)
GPIO21 (DOUT)  ←──── PCM1808 pin 9 (DOUT)

PCM1808 Configuration Pins

Pin Name Connection Function
12 FMT GND I2S format (24-bit left-justified)
13 MD1 3.3V 48kHz sampling rate
14 MD0 GND Standby disabled (always active)

PCM5102 Configuration Pins

Pin Name Connection Function
15 FLT GND Normal operation, low-latency filter
16 DEMP GND No de-emphasis
17 XSMT 3.3V Unmuted (always active)

Preamp Stage Circuit

Guitar → J2 (TS jack)
  │
  ├── R1 (1MΩ) → GND       (input pulldown)
  │
  └── R2 (10kΩ) ──┬── TL072 (non-inverting input, pin 3)
                  │
                  │     TL072 (pin 6, output)
                  │       │
                  └── R3 (100kΩ) ──┐
                                   │
                                  TL072 (inverting input, pin 2)
                                   │
                                  R4 (10kΩ) → GND

Non-inverting gain = 1 + R3/R4 = 1 + 100k/10k = 11 (~20.8dB)

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).

Power Distribution

RPi 5V (pin 2/4)
  │
  ├── U1 (AMS1117-3.3) → 3.3V
  │   ├── C1 (10µF) + C2 (100nF)  ← input decoupling
  │   ├── C3 (10µF) + C4 (100nF)  ← output decoupling
  │   │
  │   ├── U3 (PCM1808) — VCC pin 11
  │   │   └── C6 (100nF) + C7 (10µF)  ← local decoupling
  │   │
  │   ├── U4 (PCM5102) — Vin pin 1
  │   │   └── C8 (100nF) + C9 (10µF)  ← local decoupling
  │   │
  │   ├── U2 (TL072) — VCC+ pin 8
  │   └── PCM1808 MD1 (pin 13) — pull to 3.3V for 48kHz
  │
  └── PCM5102 XSMT (pin 17) — pull to 3.3V for unmuted

Ground Strategy

  • Star ground at the RPi GND pin: all audio circuit grounds return to a single point
  • Separate analog and digital ground traces on PCB
  • Tie analog/digital ground planes together at the star point only
  • Keep I2S digital lines separate from analog audio traces
  • Input jack ground: connect to star via 10Ω + 100nF for ground loop isolation

PCB Design Notes

  • 2-layer board, 65×56mm, 1.6mm FR4, HASL finish
  • Trace width: 0.254mm signal, 0.5mm power
  • Clearance: 0.2mm min, 0.3mm for audio traces
  • Via: 0.6mm/0.3mm (outer/inner diameter)
  • Copper pour: GND flood fill on both layers
  • Keep I2S traces (BCLK, LRCLK, DIN, DOUT) equal length ±2mm
  • Place decoupling caps within 5mm of IC power pins
  • Separate analog audio section (left side of board) from digital I2S (right side)
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