feat(behaviors): add usb MIDI behavior support

adds support for sending MIDI notes across 10 octaves
adds support for sending MIDI control change codes
adds MIDI behavior documentation

app/CMakeLists.txt

@@ -33,6 +33,7 @@ target_sources(app PRIVATE src/events/activity_state_changed.c)
 target_sources(app PRIVATE src/events/position_state_changed.c)
 target_sources(app PRIVATE src/events/sensor_event.c)
 target_sources(app PRIVATE src/events/mouse_button_state_changed.c)
+target_sources(app PRIVATE src/events/midi_key_state_changed.c)
 target_sources_ifdef(CONFIG_ZMK_WPM app PRIVATE src/events/wpm_state_changed.c)
 target_sources_ifdef(CONFIG_USB_DEVICE_STACK app PRIVATE src/events/usb_conn_state_changed.c)
 target_sources(app PRIVATE src/behaviors/behavior_reset.c)
@@ -72,6 +73,14 @@ if ((NOT CONFIG_ZMK_SPLIT) OR CONFIG_ZMK_SPLIT_ROLE_CENTRAL)
   target_sources(app PRIVATE src/events/keycode_state_changed.c)
   target_sources_ifdef(CONFIG_ZMK_HID_INDICATORS app PRIVATE src/hid_indicators.c)
 
+  if (CONFIG_ZMK_MIDI)
+    target_sources(app PRIVATE src/midi.c)
+    target_sources(app PRIVATE src/midi_listener.c)
+    target_sources_ifdef(CONFIG_ZMK_BEHAVIOR_MIDI_KEY_PRESS app PRIVATE src/behaviors/behavior_midi_key_press.c)
+    target_sources_ifdef(CONFIG_ZMK_USB app PRIVATE src/usb_midi.c)
+    target_sources_ifdef(CONFIG_ZMK_USB app PRIVATE src/usb_midi_packet.c)
+  endif()
+
   if (CONFIG_ZMK_BLE)
     target_sources(app PRIVATE src/events/ble_active_profile_changed.c)
     target_sources(app PRIVATE src/behaviors/behavior_bt.c)

app/Kconfig

@@ -375,6 +375,14 @@ config ZMK_MOUSE
 #Mouse Options
 endmenu
 
+menu "USB MIDI Options"
+
+config ZMK_MIDI
+    bool "Enable ZMK midi emulation"
+
+#MIDI Options
+endmenu
+
 menu "Power Management"
 
 config ZMK_BATTERY_REPORTING

app/Kconfig.behaviors

@@ -17,6 +17,12 @@ config ZMK_BEHAVIOR_SOFT_OFF
     default y
     depends on DT_HAS_ZMK_BEHAVIOR_SOFT_OFF_ENABLED && ZMK_PM_SOFT_OFF
 
+config ZMK_BEHAVIOR_MIDI_KEY_PRESS
+    bool
+    default y
+    depends on DT_HAS_ZMK_BEHAVIOR_MIDI_KEY_PRESS_ENABLED
+    imply ZMK_MIDI
+
 config ZMK_BEHAVIOR_SENSOR_ROTATE_COMMON
     bool
 
@@ -35,4 +41,4 @@ config ZMK_BEHAVIOR_SENSOR_ROTATE_VAR
 config ZMK_BEHAVIOR_MACRO
     bool
     default y
-    depends on DT_HAS_ZMK_BEHAVIOR_MACRO_ENABLED || DT_HAS_ZMK_BEHAVIOR_MACRO_ONE_PARAM_ENABLED || DT_HAS_ZMK_BEHAVIOR_MACRO_TWO_PARAM_ENABLED
+    depends on DT_HAS_ZMK_BEHAVIOR_MACRO_ENABLED || DT_HAS_ZMK_BEHAVIOR_MACRO_ONE_PARAM_ENABLED || DT_HAS_ZMK_BEHAVIOR_MACRO_TWO_PARAM_ENABLED

app/dts/behaviors.dtsi

@@ -21,3 +21,5 @@
 #include <behaviors/macros.dtsi>
 #include <behaviors/mouse_key_press.dtsi>
 #include <behaviors/soft_off.dtsi>
+#include <behaviors/midi_key_press.dtsi>
+

app/dts/behaviors/midi_key_press.dtsi

@@ -0,0 +1,14 @@
+/*
+ * Copyright (c) 2024 The ZMK Contributors
+ *
+ * SPDX-License-Identifier: MIT
+ */
+
+/ {
+    behaviors {
+        /omit-if-no-ref/ midi: midi_key_press {
+            compatible = "zmk,behavior-midi-key-press";
+            #binding-cells = <1>;
+        };
+    };
+};

app/dts/bindings/behaviors/zmk,behavior-midi-key-press.yaml

@@ -0,0 +1,8 @@
+# Copyright (c) 2024 The ZMK Contributors
+# SPDX-License-Identifier: MIT
+
+description: Midi key press/release behavior
+
+compatible: "zmk,behavior-midi-key-press"
+
+include: one_param.yaml

app/include/dt-bindings/zmk/midi.h

@@ -0,0 +1,223 @@
+/*
+ * Copyright (c) 2024 The ZMK Contributors
+ *
+ * SPDX-License-Identifier: MIT
+ */
+
+#pragma once
+
+#include <zephyr/dt-bindings/dt-util.h>
+
+// NOTE MIDI octave index is +2 vs "normal" octave index
+//  so standard C3 is MIDI C5 here
+
+// All NOTE_* have the identical keycode and midi code
+#define NOTE_C 0x0
+#define NOTE_Cs 0x1
+#define NOTE_Db NOTE_Cs
+#define NOTE_D 0x2
+#define NOTE_Ds 0x3
+#define NOTE_Eb NOTE_Ds
+#define NOTE_E 0x4
+#define NOTE_F 0x5
+#define NOTE_Fs 0x6
+#define NOTE_Gb NOTE_Fs
+#define NOTE_G 0x7
+#define NOTE_Gs 0x8
+#define NOTE_Ab NOTE_Gs
+#define NOTE_A 0x9
+#define NOTE_As 0xa
+#define NOTE_Bb NOTE_As
+#define NOTE_B 0xb
+
+#define NOTE_C_1 0xc
+#define NOTE_Cs_1 0xd
+#define NOTE_Db_1 NOTE_Cs_1
+#define NOTE_D_1 0xe
+#define NOTE_Ds_1 0xf
+#define NOTE_Eb_1 NOTE_Ds_1
+#define NOTE_E_1 0x10
+#define NOTE_F_1 0x11
+#define NOTE_Fs_1 0x12
+#define NOTE_Gb_1 NOTE_Fs_1
+#define NOTE_G_1 0x13
+#define NOTE_Gs_1 0x14
+#define NOTE_Ab_1 NOTE_Gs_1
+#define NOTE_A_1 0x15
+#define NOTE_As_1 0x16
+#define NOTE_Bb_1 NOTE_As_1
+#define NOTE_B_1 0x17
+
+#define NOTE_C_2 0x18
+#define NOTE_Cs_2 0x19
+#define NOTE_Db_2 NOTE_Cs_2
+#define NOTE_D_2 0x1a
+#define NOTE_Ds_2 0x1b
+#define NOTE_Eb_2 NOTE_Ds_2
+#define NOTE_E_2 0x1c
+#define NOTE_F_2 0x1d
+#define NOTE_Fs_2 0x1e
+#define NOTE_Gb_2 NOTE_Fs_2
+#define NOTE_G_2 0x1f
+#define NOTE_Gs_2 0x20
+#define NOTE_Ab_2 NOTE_Gs_2
+#define NOTE_A_2 0x21
+#define NOTE_As_2 0x22
+#define NOTE_Bb_2 NOTE_As_2
+#define NOTE_B_2 0x23
+
+#define NOTE_C_3 0x24
+#define NOTE_Cs_3 0x25
+#define NOTE_Db_3 NOTE_Cs_3
+#define NOTE_D_3 0x26
+#define NOTE_Ds_3 0x27
+#define NOTE_Eb_3 NOTE_Ds_3
+#define NOTE_E_3 0x28
+#define NOTE_F_3 0x29
+#define NOTE_Fs_3 0x2a
+#define NOTE_Gb_3 NOTE_Fs_3
+#define NOTE_G_3 0x2b
+#define NOTE_Gs_3 0x2c
+#define NOTE_Ab_3 NOTE_Gs_3
+#define NOTE_A_3 0x2d
+#define NOTE_As_3 0x2e
+#define NOTE_Bb_3 NOTE_As_3
+#define NOTE_B_3 0x2f
+
+#define NOTE_C_4 0x30
+#define NOTE_Cs_4 0x31
+#define NOTE_Db_4 NOTE_Cs_4
+#define NOTE_D_4 0x32
+#define NOTE_Ds_4 0x33
+#define NOTE_Eb_4 NOTE_Ds_4
+#define NOTE_E_4 0x34
+#define NOTE_F_4 0x35
+#define NOTE_Fs_4 0x36
+#define NOTE_Gb_4 NOTE_Fs_4
+#define NOTE_G_4 0x37
+#define NOTE_Gs_4 0x38
+#define NOTE_Ab_4 NOTE_Gs_4
+#define NOTE_A_4 0x39
+#define NOTE_As_4 0x3a
+#define NOTE_Bb_4 NOTE_As_4
+#define NOTE_B_4 0x3b
+
+#define NOTE_C_5 0x3c
+#define NOTE_Cs_5 0x3d
+#define NOTE_Db_5 NOTE_Cs_5
+#define NOTE_D_5 0x3e
+#define NOTE_Ds_5 0x3f
+#define NOTE_Eb_5 NOTE_Ds_5
+#define NOTE_E_5 0x40
+#define NOTE_F_5 0x41
+#define NOTE_Fs_5 0x42
+#define NOTE_Gb_5 NOTE_Fs_5
+#define NOTE_G_5 0x43
+#define NOTE_Gs_5 0x44
+#define NOTE_Ab_5 NOTE_Gs_5
+#define NOTE_A_5 0x45
+#define NOTE_As_5 0x46
+#define NOTE_Bb_5 NOTE_As_5
+#define NOTE_B_5 0x47
+
+#define NOTE_C_6 0x48
+#define NOTE_Cs_6 0x49
+#define NOTE_Db_6 NOTE_Cs_6
+#define NOTE_D_6 0x4a
+#define NOTE_Ds_6 0x4b
+#define NOTE_Eb_6 NOTE_Ds_6
+#define NOTE_E_6 0x4c
+#define NOTE_F_6 0x4d
+#define NOTE_Fs_6 0x4e
+#define NOTE_Gb_6 NOTE_Fs_6
+#define NOTE_G_6 0x4f
+#define NOTE_Gs_6 0x50
+#define NOTE_Ab_6 NOTE_Gs_6
+#define NOTE_A_6 0x51
+#define NOTE_As_6 0x52
+#define NOTE_Bb_6 NOTE_As_6
+#define NOTE_B_6 0x53
+
+#define NOTE_C_7 0x54
+#define NOTE_Cs_7 0x55
+#define NOTE_Db_7 NOTE_Cs_7
+#define NOTE_D_7 0x56
+#define NOTE_Ds_7 0x57
+#define NOTE_Eb_7 NOTE_Ds_7
+#define NOTE_E_7 0x58
+#define NOTE_F_7 0x59
+#define NOTE_Fs_7 0x5a
+#define NOTE_Gb_7 NOTE_Fs_7
+#define NOTE_G_7 0x5b
+#define NOTE_Gs_7 0x5c
+#define NOTE_Ab_7 NOTE_Gs_7
+#define NOTE_A_7 0x5d
+#define NOTE_As_7 0x5e
+#define NOTE_Bb_7 NOTE_As_7
+#define NOTE_B_7 0x5f
+
+#define NOTE_C_8 0x60
+#define NOTE_Cs_8 0x61
+#define NOTE_Db_8 NOTE_Cs_8
+#define NOTE_D_8 0x62
+#define NOTE_Ds_8 0x63
+#define NOTE_Eb_8 NOTE_Ds_8
+#define NOTE_E_8 0x64
+#define NOTE_F_8 0x65
+#define NOTE_Fs_8 0x66
+#define NOTE_Gb_8 NOTE_Fs_8
+#define NOTE_G_8 0x67
+#define NOTE_Gs_8 0x68
+#define NOTE_Ab_8 NOTE_Gs_8
+#define NOTE_A_8 0x69
+#define NOTE_As_8 0x6a
+#define NOTE_Bb_8 NOTE_As_8
+#define NOTE_B_8 0x6b
+
+#define NOTE_C_9 0x6c
+#define NOTE_Cs_9 0x6d
+#define NOTE_Db_9 NOTE_Cs_9
+#define NOTE_D_9 0x6e
+#define NOTE_Ds_9 0x6f
+#define NOTE_Eb_9 NOTE_Ds_9
+#define NOTE_E_9 0x70
+#define NOTE_F_9 0x71
+#define NOTE_Fs_9 0x72
+#define NOTE_Gb_9 NOTE_Fs_9
+#define NOTE_G_9 0x73
+#define NOTE_Gs_9 0x74
+#define NOTE_Ab_9 NOTE_Gs_9
+#define NOTE_A_9 0x75
+#define NOTE_As_9 0x76
+#define NOTE_Bb_9 NOTE_As_9
+#define NOTE_B_9 0x77
+
+#define NOTE_C_10 0x78
+#define NOTE_Cs_10 0x79
+#define NOTE_Db_10 NOTE_Cs_10
+#define NOTE_D_10 0x7a
+#define NOTE_Ds_10 0x7b
+#define NOTE_Eb_10 NOTE_Ds_10
+#define NOTE_E_10 0x7c
+#define NOTE_F_10 0x7d
+#define NOTE_Fs_10 0x7e
+#define NOTE_Gb_10 NOTE_Fs_10
+#define NOTE_G_10 0x7f
+// 0x7f aka 127 is the max value
+
+// NOTE sentinals
+#define MIDI_MIN_NOTE NOTE_C
+#define MIDI_MAX_NOTE NOTE_G_10
+#define MIDI_INVALID 0xFF
+
+// Midi control change keycodes
+// appended with 0xB0
+#define SUSTAIN 0xB040
+#define PORTAMENTO 0xB041
+#define SOSTENUTO 0xB042
+#define OCT_UP 0xB081
+#define OCT_DOWN 0xB082
+
+// midi control sentinals
+#define MIDI_MIN_CONTROL SUSTAIN
+#define MIDI_MAX_CONTROL OCT_DOWN

app/include/zmk/endpoints.h

@@ -75,3 +75,7 @@ int zmk_endpoints_send_mouse_report();
 #endif // IS_ENABLE(CONFIG_ZMK_MOUSE)
 
 void zmk_endpoints_clear_current(void);
+
+#if IS_ENABLED(CONFIG_ZMK_MIDI)
+int zmk_endpoints_send_midi_report();
+#endif // IS_ENABLE(CONFIG_ZMK_MIDI)

app/include/zmk/events/midi_key_state_changed.h

@@ -0,0 +1,34 @@
+/*
+ * Copyright (c) 2024 The ZMK Contributors
+ *
+ * SPDX-License-Identifier: MIT
+ */
+
+#pragma once
+
+#include <zephyr/kernel.h>
+#include <zmk/event_manager.h>
+#include <zmk/midi.h>
+
+struct zmk_midi_key_state_changed {
+    zmk_midi_key_t key;
+    bool state;
+    int64_t timestamp;
+};
+
+ZMK_EVENT_DECLARE(zmk_midi_key_state_changed);
+
+static inline struct zmk_midi_key_state_changed
+zmk_midi_key_state_changed_from_encoded(uint32_t encoded, bool pressed, int64_t timestamp) {
+
+    // no decoding necessary
+    zmk_midi_key_t id = encoded;
+
+    return (struct zmk_midi_key_state_changed){.key = id, .state = pressed, .timestamp = timestamp};
+}
+
+static inline int raise_zmk_midi_key_state_changed_from_encoded(uint32_t encoded, bool pressed,
+                                                                int64_t timestamp) {
+    return raise_zmk_midi_key_state_changed(
+        zmk_midi_key_state_changed_from_encoded(encoded, pressed, timestamp));
+}