Towards a capturing MIDI signals via USB using Flipper Zero. The goal is to allow real-time monitoring of MIDI events including Note On/Off, Control Changes, Program Changes, Pitch Bend, and more.
In order to get it all working, we would need to act F0 to act as a host, and this brings a load of problems. One of them is that USB devices can draw up to 500mA which the Flipper's battery can't sustain. Besides, Fipper's USB host mode isn't well documented.
Maybe we have to try a different approach using the traditional UART (Universal Asynchronous Receiver/Transmitter) MIDI via GPIO?!
- Pin 4: Source (+5V through 220Ω)
- Pin 5: Sink (Ground through opto-isolator)
- USB MIDI Packet Handling: Parses 4-byte USB MIDI packets with CIN (Code Index Number)
- Message Display: Human-readable formatting including musical note names (C4, A#5, etc.)
- OK Button: Clear message history
- Back Button: Exits
The app formats MIDI as follows:
NoteOn Ch01 C4 Vel127 // Note On, channel 1, note C4, velocity 127
NoteOff Ch01 C4 // Note Off, channel 1, note C4
CC Ch01 #007=064 // Control Change, channel 1, CC#7, value 64
ProgChg Ch01 Prg042 // Program Change, channel 1, program 42
PitchBd Ch01 +2048 // Pitch Bend, channel 1, +2048 semitones
ChPress Ch01 Val080 // Channel Pressure, channel 1, value 80
PolyAT Ch01 C4 P080 // Poly Aftertouch, channel 1, note C4, pressure 80
System 0xF8 // System message (e.g., Clock)
A pitfall is that MIDI messages have variable lengths:
- Program Change: 2 bytes
- Note On/Off: 3 bytes
- SysEx: any length (can be hundreds of bytes)
USB MIDI uses fixed 4-byte packets. How do we know how many bytes are valid? Answer: The Code Index Number (CIN) is the answer to that issue. CIN is metadata about the packet, not part of the MIDI message itself.
| CIN | Description |
|---|---|
| 0x2 | Two-byte system common messages |
| 0x3 | Three-byte system common messages |
| 0x4 | SysEx starts or continues |
| 0x5 | SysEx ends with 1 byte |
| 0x6 | SysEx ends with 2 bytes |
| 0x7 | SysEx ends with 3 bytes |
| 0x8 | Note Off |
| 0x9 | Note On |
| 0xA | Poly-KeyPress |
| 0xB | Control Change |
| 0xC | Program Change |
| 0xD | Channel Pressure |
| 0xE | Pitch Bend |
| 0xF | Single Byte |
Minimal Set of Examples
- Note On (C4, velocity 100, channel 1)
0x09 0x90 0x3C 0x64 │ │ │ └─ Velocity: 100 │ │ └────── Note: 60 (C4) │ └─────────── Status: Note On, Channel 1 └──────────────── Cable=0, CIN=9 (Note On, 3 bytes valid) - Program Change (program 42, channel 1)
0x0C 0xC0 0x2A 0x00 │ │ │ └─ Padding (not used) │ │ └────── Program: 42 │ └─────────── Status: Program Change, Channel 1 └──────────────── Cable=0, CIN=C (Program Change, 2 bytes valid) - Longer messages, e.g. a 10-byte SysEx message:
F0 41 10 42 12 00 01 02 03 F7gets split into four USB MIDI packets:Packet 1: 0x04 0xF0 0x41 0x10 (CIN=4: Start, 3 bytes valid) Packet 2: 0x04 0x42 0x12 0x00 (CIN=4: Continue, 3 bytes valid) Packet 3: 0x04 0x01 0x02 0x03 (CIN=4: Continue, 3 bytes valid) Packet 4: 0x05 0xF7 0x00 0x00 (CIN=5: End with 1 byte)
Standard MIDI play notes, change volume, bend pitch. On top of these universal commands, there are device-specific, custom commands.
Most MIDI devices respond to a Universal Identity Request (UIR): F0 7E 7F 06 01 F7.
The answer would be something like F0 7E 7F 06 02 [Manufacturer ID] [Device Family] [Device Member] [Version] F7. For instance, a Roland JV-1080 says
F0 7E 7F 06 02 41 19 00 01 00 00 01 F7
│ │ │ │ │ │ └──┬──┘ └────┬────┘
│ │ │ │ │ │ │ └────── Software version (4 bytes)
│ │ │ │ │ │ └──────────────── Device family & member (2 bytes)
│ │ │ │ │ └────────────────────── Manufacturer ID: Roland (0x41)
│ │ │ │ └───────────────────────── Sub-ID: Identity Reply
│ │ │ └──────────────────────────── ID: General Information
│ │ └─────────────────────────────── Device ID: All devices (0x7F)
│ └────────────────────────────────── Universal Non-Real Time (0x7E)
└───────────────────────────────────── SysEx Start
Note. Not all devices support SysEx-messages
- The Swissonic EasyKey 49 will not even respond to a Universal Identity Request, since it is a simple, transmit-only MIDI controller. According to the documentation of Swisssonic EasyKey it only transmits Note On/Off, Pitch Bend, Control Change, and Program Change messages. This makes it an ideal testing device for this F0-app.
- To my knowledge, Roland hasn't published specific device family/member ID codes for the famous Roland Aira Compact series, at least I don't find them in the manuals.
- USB HAL Not Integrated: Requires Flipper firmware USB MIDI support
- SysEx Not Fully Supported: Large SysEx messages may need special handling
- No MIDI Output: Currently receive-only
- Limited History: Only 8 messages stored (adjustable via MAX_MIDI_MESSAGES)
See changelog.md