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feat: PCM1808+PCM5102 I2S HAT design files

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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
This commit is contained in:
Shawn
2026-06-07 23:52:13 -04:00
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commit 0ae2ca6e8e
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# Audio Configuration — PCM1808+PCM5102 I2S HAT
> **HAT:** Custom PCM1808 ADC + PCM5102 DAC I2S audio HAT
> **RPi:** 4B, Raspberry Pi OS (Bookworm, 64-bit)
> **Kernel:** rpi-6.6.y (upstream Raspberry Pi kernel)
> **Last updated:** 2026-06-07
---
## Device Tree Overlay
The PCM1808 ADC has **no upstream overlay** in the rpi-6.6.y kernel tree. A custom overlay is provided at:
```
hardware/pcm1808-pcm5102-overlay.dts
```
### Build & Install
```bash
# Install device-tree-compiler if not present
sudo apt-get install device-tree-compiler
# Compile overlay
dtc -@ -I dts -O dtb -o pcm1808-pcm5102-overlay.dtbo \
hardware/pcm1808-pcm5102-overlay.dts
# Copy to boot overlays directory
sudo cp pcm1808-pcm5102-overlay.dtbo /boot/firmware/overlays/
# On older Pi OS: /boot/overlays/
```
This compiles to `/boot/firmware/overlays/pcm1808-pcm5102-overlay.dtbo`.
### Enable Overlay
Edit `/boot/firmware/config.txt` (or `/boot/config.txt` on older installs):
```ini
# ── Custom I2S HAT: PCM1808 ADC + PCM5102 DAC ─────────
dtoverlay=pcm1808-pcm5102-overlay
# Disable onboard audio to free I2S pins
dtparam=audio=off
# Optional: force I2S pins if overlay doesn't set them
# dtparam=i2s=on
```
### Verify Overlay Loads
```bash
# Check overlay is loaded
dmesg | grep -i "pcm1808\|pcm5102\|i2s"
# Check ALSA sees both devices
aplay -l
arecord -l
# Expected output for playback:
# card 0: pcm1808pcm5102hat [pcm1808-pcm5102-hat], device 0: ... []
# Subdevices: 1/1
# Expected output for capture:
# card 0: pcm1808pcm5102hat [pcm1808-pcm5102-hat], device 0: ... []
# Subdevices: 1/1
# Check /proc device tree
cat /proc/device-tree/sound/name # should show "pcm1808-pcm5102-hat"
```
---
## ALSA Configuration
### Default Card Setup
Create `/etc/asound.conf` (system-wide) or `~/.asoundrc` (per-user):
```conf
# ── PCM1808+PCM5102 I2S HAT ─────────────────────────
# Default playback device
pcm.!default {
type asym
playback.pcm {
type hw
card 0
device 0
}
capture.pcm {
type hw
card 0
device 0
}
}
# Hardware device aliases
pcm.i2s_playback {
type hw
card 0
device 0
}
pcm.i2s_capture {
type hw
card 0
device 0
}
# Software volume control (PCM1808/PCM5102 have no HW volume)
pcm.softvol_playback {
type softvol
slave.pcm "i2s_playback"
control {
name "Master Playback Volume"
card 0
}
min_dB -50.0
max_dB 0.0
}
# 48kHz upsampler for 44.1kHz sources (like YouTube/Spotify)
pcm.rate_convert {
type plug
slave {
pcm "softvol_playback"
rate 48000
format S24_3LE
channels 2
}
}
# Default: use rate conversion + softvol
pcm.!default {
type plug
slave.pcm "rate_convert"
}
# Control device
ctl.!default {
type hw
card 0
}
```
### Test Audio
```bash
# --- Playback test ---
# Sine wave at 440Hz (A4)
speaker-test -t sine -f 440 -l 1 -c 2
# Play a WAV file
aplay /usr/share/sounds/alsa/Front_Center.wav
# --- Capture test ---
# Record 3 seconds from ADC
arecord -d 3 -f cd -t wav test_record.wav
# Play back the recording
aplay test_record.wav
# --- Full loopback test ---
# Play tone while recording (connect DAC output to ADC input)
# Check latency: <10ms at 48kHz/128 frames
arecord -d 5 -f S24_3LE -r 48000 -c 2 loopback_test.wav &
speaker-test -t sine -f 440 -l 2
wait
aplay loopback_test.wav
```
---
## JACK Audio Configuration
The pedal DSP runs under JACK. The existing JACK config in `main.py` uses `hw:1,0` (USB). Swap to `hw:0,0` for I2S:
### JACK Start
```bash
# Kill PulseAudio (conflicts with JACK on RPi)
pulseaudio --kill
# Start JACK at 48kHz/128 frames (recommended target: ~5.3ms buffer)
jackd -R -d alsa \
-d hw:0,0 \
-r 48000 \
-p 128 \
-n 3
# For low-latency mode (risky, check xruns):
jackd -R -d alsa -d hw:0,0 -r 48000 -p 64 -n 2
```
### Update main.py Config
In `/home/oplabs/projects/pi-multifx-pedal/main.py`, change:
```python
# FROM (USB Audio):
# config = {
# "input_device": "hw:1,0",
# "output_device": "hw:1,0",
# }
# TO (I2S HAT):
config = {
"input_device": "hw:0,0", # PCM1808 ADC
"output_device": "hw:0,0", # PCM5102 DAC
"sample_rate": 48000,
"period_size": 128, # 2.6ms buffer
"num_periods": 3, # triple-buffered
}
```
---
## Integration with NAM DSP
The I2S HAT uses the **same block size** as the NAM DSP engine (256 samples):
| Parameter | Value | Note |
|-----------|-------|------|
| Sample rate | 48,000 Hz | Both JACK and NAM |
| Block size | 256 frames | NAM internal block |
| JACK period | 128 frames | 2 × periods per block |
| Round-trip (target) | <8ms | 128fr × 2 + codec delay + DSP |
At 48kHz/128 frames × 3 periods = ~8ms round-trip, well within the 10ms target for NAM DSP. The 256-sample NAM block fits exactly into 2 JACK periods.
---
## Troubleshooting
### No card 0 detected
```bash
# Check if I2S is enabled in pin controller
dmesg | grep -i i2s
# Check loaded overlays
vcgencmd get_config dtov
# Should show: pcm1808-pcm5102-overlay
# Rebuild overlay if kernel was updated
sudo dtc -@ -I dts -O dtb -o /boot/firmware/overlays/pcm1808-pcm5102-overlay.dtbo \
/home/oplabs/projects/pi-multifx-pedal/hardware/pcm1808-pcm5102-overlay.dts
```
### No capture device
PCM1808 has no upstream overlay — the custom overlay provides the `pcm1808` codec binding. If capture isn't showing:
```bash
# Verify the overlay node compiled
dtc -I dtb -O dts /boot/firmware/overlays/pcm1808-pcm5102-overlay.dtbo | grep -i pcm1808
# Check I2S pin muxing is correct
raspi-gpio get 18 19 20 21
# Expect: GPIO 18: level=0 func=ALT5 (I2S BCLK)
# GPIO 19: level=0 func=ALT5 (I2S LRCLK)
# GPIO 20: level=0 func=ALT5 (I2S DIN)
# GPIO 21: level=0 func=ALT5 (I2S DOUT)
```
### PCM5102 hiss
- Add 330Ω + 10µF RC filter on DAC output (R5, C10 in BOM)
- Ensure XSMT (pin 17) is pulled to 3.3V, not floating
- Check 3.3V rail noise with oscilloscope (<50mV ripple at audio frequencies)
### JACK xruns
- Increase period size: `-p 256` (5.3ms, safer)
- Increase num periods: `-n 3` (triple buffer)
- Isolate RPi 4B from USB power noise
- Check `cat /proc/asound/card0/pcm0p/sub0/status` for xrun count
---
## Pinout Reference
| Signal | BCM GPIO | Physical Pin | ALT Function | Direction |
|--------|----------|:------------:|:------------:|:---------:|
| BCLK | GPIO18 | Pin 12 | ALT5 (I2S) | RPi → HAT |
| LRCLK | GPIO19 | Pin 35 | ALT5 (I2S) | RPi → HAT |
| DIN | GPIO20 | Pin 38 | ALT5 (I2S) | HAT → RPi |
| DOUT | GPIO21 | Pin 40 | ALT5 (I2S) | RPi → HAT |
| 3.3V | — | Pins 1, 17 | Power | RPi → HAT |
| 5V | — | Pins 2, 4 | Power | RPi → HAT |
| GND | — | Pins 6, 9, 14, 25, 30, 34, 39 | Ground | — |
The I2S HAT uses a **stacking header** (2×20 female socket) to pass through all 40 GPIO pins so footswitches, LEDs, and display are accessible from the top of the HAT.
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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)