2021.4.30: ソースコードを修正（PC側でシリアル接続しないと動作しない問題修正）
参考にしたもの

[https://okiraku-camera.tokyo/blog/?p=8193]
作り方

キーボード: Thinkpad keyboard with Tracpoint
ハード：参考と一緒
ソフト：参考にしたものはkeyboardだけだったのでArduinoの標準ライブラリのUSBHIDBootKbdAndMouseのサンプルをもとに実装（tracpointが使いたい！！！）
注意点：Aruduinoの標準ライブラリのKeyboardのsendReport()をprivateからpublicに変更した。
keybord_converter.ino
/** 
 * KeyCodeConvert
 * original Keyboard Library must be modified.
 * in Keyboard.h, function "sendReport()" should be change to "public" from "private".
 */

# include <hidboot.h>
# include <usbhub.h>

# include "Keyboard.h"
# include "Mouse.h"

// Satisfy IDE, which only needs to see the include statment in the ino.
# ifdef dobogusinclude
# include <spi4teensy3.h>
# endif
# include <SPI.h>

// KeyReportのmodifiersのビット構成
static const uint8_t MODIFIRE_LEFT_CONTROL = 0x01;
static const uint8_t MODIFIRE_LEFT_SHIFT = 0x02;
static const uint8_t MODIFIRE_LEFT_ALT = 0x04;
static const uint8_t MODIFIRE_LEFT_GUI= 0x08;
static const uint8_t MODIFIRE_RIGHT_CONTROL = 0x10;
static const uint8_t MODIFIRE_RIGHT_SHIFT = 0x20;
static const uint8_t MODIFIRE_RIGHT_ALT = 0x40;
static const uint8_t MODIFIRE_RIGHT_GUI= 0x80;

//#define DEBUG_PRINT

class MouseRptParser : public MouseReportParser
{
  protected:
    void OnMouseMove(MOUSEINFO *mi);
    void OnLeftButtonUp(MOUSEINFO *mi);
    void OnLeftButtonDown(MOUSEINFO *mi);
    void OnRightButtonUp(MOUSEINFO *mi);
    void OnRightButtonDown(MOUSEINFO *mi);
    void OnMiddleButtonUp(MOUSEINFO *mi);
    void OnMiddleButtonDown(MOUSEINFO *mi);
};
void MouseRptParser::OnMouseMove(MOUSEINFO *mi)
{
# ifdef DEBUG_PRINT
  Serial.print("dx=");
  Serial.print(mi->dX, DEC);
  Serial.print(" dy=");
  Serial.println(mi->dY, DEC);
# else
  Mouse.move(mi->dX, mi->dY);
# endif
};
void MouseRptParser::OnLeftButtonUp	(MOUSEINFO *mi)
{
# ifdef DEBUG_PRINT
  Serial.println("L Butt Up");
# else
  Mouse.release(MOUSE_LEFT);
# endif
};
void MouseRptParser::OnLeftButtonDown	(MOUSEINFO *mi)
{
# ifdef DEBUG_PRINT
  Serial.println("L Butt Dn");
# else
  Mouse.press(MOUSE_LEFT);
# endif
};
void MouseRptParser::OnRightButtonUp	(MOUSEINFO *mi)
{
# ifdef DEBUG_PRINT
  Serial.println("R Butt Up");
# else
  Mouse.release(MOUSE_RIGHT);
# endif
};
void MouseRptParser::OnRightButtonDown	(MOUSEINFO *mi)
{
# ifdef DEBUG_PRINT
  Serial.println("R Butt Dn");
# else
  Mouse.press(MOUSE_RIGHT);
# endif
};
void MouseRptParser::OnMiddleButtonUp	(MOUSEINFO *mi)
{
# ifdef DEBUG_PRINT
  Serial.println("M Butt Up");
# else
  Mouse.release(MOUSE_MIDDLE);
# endif
};
void MouseRptParser::OnMiddleButtonDown	(MOUSEINFO *mi)
{
# ifdef DEBUG_PRINT
  Serial.println("M Butt Dn");
# else
  Mouse.press(MOUSE_MIDDLE);
# endif
};

bool modifyKey(uint8_t* pKey, bool bDown);
bool modifyModifier(uint8_t before, uint8_t after);
void sendReport(KeyReport* report);

class KbdRptParser : public KeyboardReportParser
{
    void PrintKey(uint8_t mod, uint8_t key);

  protected:
    void OnControlKeysChanged(uint8_t before, uint8_t after);
    void OnKeyDown	(uint8_t mod, uint8_t key);
    void OnKeyUp	(uint8_t mod, uint8_t key);
//    void OnKeyPressed(uint8_t key);
};

KeyReport keyReport;

void sendReport(KeyReport* report)
{
  Serial.print("== ");
  Serial.print(report->modifiers, HEX);
  Serial.print(":");
  Serial.print(report->keys[0], HEX);
  Serial.println(" ==");

  Keyboard.sendReport(report);
}

/**
 * 修飾キー変換
 * @note Controlの場合はCapsLockに変更
 * @param [in] MODIFIERKEYS before: 前回の修飾キー状態 
 * @param [in] MODIFIERKEYS after: 今回の修飾キー状態
 * @return bool : 修飾キー変換したかどうか
 */
bool modifyModifier(MODIFIERKEYS before, MODIFIERKEYS after)
{
  // Control->CAPSLOCK(0x39)
  if (before.bmLeftCtrl != after.bmLeftCtrl)
  {
    if (after.bmLeftCtrl)
    {
      Serial.println("CAPSLOCK DOWN");
      keyReport.keys[0] = 0x39;      
    }
    else
    {
      Serial.println("CAPSLOCK UP");
      keyReport.keys[0] = 0;      
    }
    sendReport(&keyReport);
    return true;
  }

  return false;
}

/**
 * キー変換
 * @param [in/out] uint8_t* pKey: キーコード
 * @param [in] bool bDown: 押下
 * @return bool : true キー変換実施, false modifierとして処理
 */
bool modifyKey(uint8_t* pKey, bool bDown)
{
  switch(*pKey) {
    case 0x29: // ESC(0x29)->半角/全角(0x35)
      *pKey = 0x35;
      break;
    case 0x35: // 半角/全角(0x35)->ESC(0x29)
      *pKey = 0x29;
      break;
    case 0x8A: // 変換(0x8A)->半角/全角(0x35)
      *pKey = 0x35;
      break;
    case 0x8B: // 無変換(0x8B)->半角/全角(0x35)
      *pKey = 0x35;
        break;
    case 0x39: // CAPSLOCK(0x39)->Control
    {
      KBDINFO* pKbdInfo;
      pKbdInfo = (KBDINFO*)&keyReport;
      if (bDown) 
      {
        pKbdInfo->bmLeftCtrl = 1;
        Serial.println("LeftCtrl down");      
      }
      else
      {
        pKbdInfo->bmLeftCtrl = 0;
        Serial.println("LeftCtrl up");      
      }
      return false;
    }
    default:
      break;
  }

  Serial.print("changed to ");
  Serial.println(*pKey, HEX);

  return true;  
}

void KbdRptParser::PrintKey(uint8_t m, uint8_t key)
{
  MODIFIERKEYS mod;
  *((uint8_t*)&mod) = m;
  Serial.print((mod.bmLeftCtrl   == 1) ? "C" : " ");
  Serial.print((mod.bmLeftShift  == 1) ? "S" : " ");
  Serial.print((mod.bmLeftAlt    == 1) ? "A" : " ");
  Serial.print((mod.bmLeftGUI    == 1) ? "G" : " ");

  Serial.print(" >");
  PrintHex<uint8_t>(key, 0x80);
  Serial.print("< ");

  Serial.print((mod.bmRightCtrl   == 1) ? "C" : " ");
  Serial.print((mod.bmRightShift  == 1) ? "S" : " ");
  Serial.print((mod.bmRightAlt    == 1) ? "A" : " ");
  Serial.println((mod.bmRightGUI    == 1) ? "G" : " ");
};

void KbdRptParser::OnKeyDown(uint8_t mod, uint8_t key)
{
  // キー変換
  if (modifyKey(&key, true))
  {
    Serial.print("DN ");
    keyReport.keys[0] = key;
    PrintKey(keyReport.modifiers, key);
    sendReport(&keyReport);

//  uint8_t c = OemToAscii(mod, key);
//  if (c)
//    OnKeyPressed(c);    
  }
}

void KbdRptParser::OnKeyUp(uint8_t mod, uint8_t key)
{
  if (modifyKey(&key, false))
  {
    Serial.print("UP ");
    keyReport.keys[0] = 0;
    PrintKey(keyReport.modifiers, key);
    sendReport(&keyReport);  
  }
}

void KbdRptParser::OnControlKeysChanged(uint8_t before, uint8_t after) {

  MODIFIERKEYS beforeMod;
  *((uint8_t*)&beforeMod) = before;

  MODIFIERKEYS afterMod;
  *((uint8_t*)&afterMod) = after;

  if (before != after) {
    if (modifyModifier(beforeMod, afterMod))
    {
      return;
    }
    Serial.print("Modifer changed:");
    Serial.println(after);
  }
  keyReport.modifiers = after;

  if (beforeMod.bmLeftCtrl != afterMod.bmLeftCtrl) {
    Serial.println("LeftCtrl changed");
  }
  if (beforeMod.bmLeftShift != afterMod.bmLeftShift) {
    Serial.println("LeftShift changed");
  }
  if (beforeMod.bmLeftAlt != afterMod.bmLeftAlt) {
    Serial.println("LeftAlt changed");
  }
  if (beforeMod.bmLeftGUI != afterMod.bmLeftGUI) {
    Serial.println("LeftGUI changed");
  }

  if (beforeMod.bmRightCtrl != afterMod.bmRightCtrl) {
    Serial.println("RightCtrl changed");
  }
  if (beforeMod.bmRightShift != afterMod.bmRightShift) {
    Serial.println("RightShift changed");
  }
  if (beforeMod.bmRightAlt != afterMod.bmRightAlt) {
    Serial.println("RightAlt changed");
  }
  if (beforeMod.bmRightGUI != afterMod.bmRightGUI) {
    Serial.println("RightGUI changed");
  }

}

//void KbdRptParser::OnKeyPressed(uint8_t key)
//{
//  Serial.print("ASCII: ");
//  Serial.println((char)key);
//  Keyboard.write(key);
//};

USB     Usb;
USBHub     Hub(&Usb);

HIDBoot < USB_HID_PROTOCOL_KEYBOARD | USB_HID_PROTOCOL_MOUSE > HidComposite(&Usb);
HIDBoot<USB_HID_PROTOCOL_KEYBOARD>    HidKeyboard(&Usb);
HIDBoot<USB_HID_PROTOCOL_MOUSE>    HidMouse(&Usb);

KbdRptParser KbdPrs;
MouseRptParser MousePrs;

void setup()
{
  Serial.begin( 9600 );
# if 0 // シリアル接続待ちするので無効化 #if !defined(__MIPSEL__)
  while (!Serial); // Wait for serial port to connect - used on Leonardo, Teensy and other boards with built-in USB CDC serial connection
# endif
  Serial.println("Start");

  if (Usb.Init() == -1)
    Serial.println("OSC did not start.");

  delay( 200 );

  HidComposite.SetReportParser(0, &KbdPrs);
  HidComposite.SetReportParser(1, &MousePrs);
  HidKeyboard.SetReportParser(0, &KbdPrs);
  HidMouse.SetReportParser(0, &MousePrs);

  Mouse.begin();
  Keyboard.begin();
}

void loop()
{
  Usb.Task();
}