shawn
c38a7b0fd8
New files:
main.py - PedalApp: boots all subsystems in order,
wires MIDI/footswitch callbacks, graceful
teardown reverses boot order
src/system/config.py - YAML config loader with deep-merge
(separated to avoid hardware deps)
src/system/services.py - systemd unit generator for pedal.service
+ multi-fx-pedal.target
scripts/install_service.sh - copies project, creates venv, installs
+ enables service units
tests/test_integration.py - 41 tests: boot, routing, display sync,
teardown, systemd content, CLI, edge cases
Modified:
tests/conftest.py - add project root to sys.path
142 lines
4.2 KiB
Markdown
142 lines
4.2 KiB
Markdown
# MIDI Optoisolator Circuit — 5-pin DIN Input
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## Why Optoisolation?
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MIDI specification (MIDI 1.0, §3.1) **requires** galvanic isolation on the
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MIDI IN port to prevent ground loops between connected devices. Without it:
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- Hum/buzz from ground potential differences
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- Possible damage to the Raspberry Pi's GPIO/UART when hot-plugging
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- Electrical noise coupling into the audio path
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The 6N138 (or compatible H11L1) optoisolator breaks the ground path while
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preserving the 31.25 kbaud MIDI data stream.
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## Schematic
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```
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+5V (3.3V for RPi)
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│
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┌┤ R2 ─┐
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│ 270Ω │
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│ │
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MIDI Pin 4 ──┬── R1 ──┐├ │
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(Current) │ 220Ω ││ │
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│ ││ │
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┌┴┐ 1N4148 │
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│ │ ││
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│ │ ││
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└┬┘ ││
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│ ││
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MIDI Pin 5 ──┴──────────────┘│
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(Return) 6N138 │
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│ │
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│ ├─── Pin 3 (TX) ──> RPi UART RX (GPIO 15)
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│ │
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┌┤ │
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│R3 │
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│4k7Ω
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│ │
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└───┘
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│
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GND
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Protection:
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D1: 1N4148 reverse-protection diode
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R1: 220Ω current limit (MIDI spec 5 mA loop)
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R2: 270Ω pull-up for 6N138 output (use 470Ω for 3.3V)
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R3: 4.7kΩ pull-down to keep UART line low when no MIDI connected
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```
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## Bill of Materials
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| Component | Value | Notes |
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|-----------|-------|-------|
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| U1 | 6N138 | Optoisolator (DIP-8) — use socket |
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| Alt U1 | H11L1 | Drop-in replacement, faster switching |
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| R1 | 220Ω | 1/4W, ±5% |
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| R2 | 270Ω (5V) / 470Ω (3.3V) | Pull-up for opto output |
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| R3 | 4.7kΩ | Pull-down for UART line |
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| D1 | 1N4148 | Signal diode |
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| C1 | 100nF | Decoupling cap near 6N138 (optional) |
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| J1 | 5-pin DIN | Female chassis-mount (180°) |
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| J2 | 3-pin header | Connection to RPi GPIO |
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## SixN138 Pinout
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```
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1: NC (not connected)
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2: Anode ──┬── R1 ──> MIDI Pin 4
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│
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D1 (cathode) ──> MIDI Pin 4
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3: Cathode ──> MIDI Pin 5 (GND return)
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4: NC
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5: GND
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6: Output ──> RPi GPIO 15 (UART RX)
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7: Enable ──> GND (always enabled)
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8: Vcc ──> 3.3V (RPi) or 5V
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```
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## MIDI OUT (direct, no opto)
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MIDI OUT does not need optoisolation — the Raspberry Pi drives the
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UART TX directly. Use a 220Ω resistor in series on MIDI Pin 5
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to comply with the MIDI current loop spec:
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```
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RPi TX (GPIO 14) ──> 220Ω ──┬──> MIDI Pin 4
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│
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GND (MIDI Pin 2)
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```
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## Raspberry Pi UART Setup
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Enable UART on the RPi 4B for MIDI:
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```bash
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# Enable UART hardware
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echo "enable_uart=1" | sudo tee -a /boot/config.txt
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# Disable Bluetooth serial (frees UART from BT)
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echo "dtoverlay=disable-bt" | sudo tee -a /boot/config.txt
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# For stable 31.25 kbaud on RPi 4B:
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echo "core_freq=250" | sudo tee -a /boot/config.txt
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# Disable serial console on UART
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sudo raspi-config nonint do_serial 2
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# Remove console from cmdline.txt
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sudo sed -i 's/console=serial[^ ]* //g' /boot/cmdline.txt
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# Reboot
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sudo reboot
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```
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After reboot, the UART appears as `/dev/ttyAMA0`.
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## Verification
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After hardware is wired:
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```bash
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# Check UART device exists
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ls -l /dev/ttyAMA0
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# Connect a MIDI keyboard/controller, then test
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python scripts/midi_test.py --uart /dev/ttyAMA0 --no-usb --discover
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# Monitor raw bytes (test without pyserial)
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cat /dev/ttyAMA0 | xxd
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```
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Expected output on xxd when pressing keys — bursts of 3-byte groups
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at 31.25 kbaud. Each group is one MIDI message (status + 1-2 data bytes).
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## PCB Layout Notes
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- Keep the optoisolator close to the DIN connector (< 5 cm traces)
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- Separate MIDI ground from audio ground — single-point star ground
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- Add a 100 nF ceramic cap between Vcc and GND near the 6N138
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- Use 6N138 in a DIP-8 socket for serviceability
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- Route UART TX/RX traces away from I2S audio data lines (GPIO 18-21) |