Arduino UNO R4が発売されて半年近くがたちました。
みなさん、HID(Human Interface Device)を使ってますか？
Arduinoには以前からLeonardやProMicroなどでHIDが使用可能でした。UNOではR4から対応しました。
今回はR4で同時押しの制限を緩和したのでメモがてら残します。

同時に6キーしか押せない？

HIDでキーボードとして同時にキーをいっぱい押しても、6つまでしか認識してくれません。これはどうも規格としてそう決まってるみたいでどうしようもないんでしょうか？そんなんではコントローラーを作るときにとても困ってしまいます...

回避方法

R4以前までのボードではHID-Projectというものを使って上限を回避してきたようです。しかしこれは数年前から更新が止まっているようで、当然R4では使えません。はてどうしたものか...

2021年の記事 ¹ ではKeyboard.cppを変更して回避する方法が紹介されていました。しかしなぜかこれではR4では動いてくれず。もう少し手間が必要なようです。

改造方法

警告
以下に示す変更によって思わぬ故障等が発生する可能性があります。
実施は自己責任で行ってください。

Keyboard.cppとKeyboard.hの両方を変更する必要があります。
元に戻すときはバックアップを取っておくかgithubのリポジトリ ² から復元して下さい。

Keyboard.h

/*
  Keyboard.h

  Copyright (c) 2015, Arduino LLC
  Original code (pre-library): Copyright (c) 2011, Peter Barrett

  This library is free software; you can redistribute it and/or
  modify it under the terms of the GNU Lesser General Public
  License as published by the Free Software Foundation; either
  version 2.1 of the License, or (at your option) any later version.

  This library is distributed in the hope that it will be useful,
  but WITHOUT ANY WARRANTY; without even the implied warranty of
  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  Lesser General Public License for more details.

  You should have received a copy of the GNU Lesser General Public
  License along with this library; if not, write to the Free Software
  Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA
*/

#ifndef KEYBOARD_h
#define KEYBOARD_h

#include "HID.h"

#if !defined(_USING_HID)

#warning "Using legacy HID core (non pluggable)"

#else

//================================================================================
//================================================================================
//  Keyboard

// Modifiers
#define KEY_LEFT_CTRL     0x80
#define KEY_LEFT_SHIFT    0x81
#define KEY_LEFT_ALT      0x82
#define KEY_LEFT_GUI      0x83
#define KEY_RIGHT_CTRL    0x84
#define KEY_RIGHT_SHIFT   0x85
#define KEY_RIGHT_ALT     0x86
#define KEY_RIGHT_GUI     0x87

// Misc keys
#define KEY_UP_ARROW      0xDA
#define KEY_DOWN_ARROW    0xD9
#define KEY_LEFT_ARROW    0xD8
#define KEY_RIGHT_ARROW   0xD7
#define KEY_BACKSPACE     0xB2
#define KEY_TAB           0xB3
#define KEY_RETURN        0xB0
#define KEY_MENU          0xED // "Keyboard Application" in USB standard
#define KEY_ESC           0xB1
#define KEY_INSERT        0xD1
#define KEY_DELETE        0xD4
#define KEY_PAGE_UP       0xD3
#define KEY_PAGE_DOWN     0xD6
#define KEY_HOME          0xD2
#define KEY_END           0xD5
#define KEY_CAPS_LOCK     0xC1
#define KEY_PRINT_SCREEN  0xCE // Print Screen / SysRq
#define KEY_SCROLL_LOCK   0xCF
#define KEY_PAUSE         0xD0 // Pause / Break

// Numeric keypad
#define KEY_NUM_LOCK      0xDB
#define KEY_KP_SLASH      0xDC
#define KEY_KP_ASTERISK   0xDD
#define KEY_KP_MINUS      0xDE
#define KEY_KP_PLUS       0xDF
#define KEY_KP_ENTER      0xE0
#define KEY_KP_1          0xE1
#define KEY_KP_2          0xE2
#define KEY_KP_3          0xE3
#define KEY_KP_4          0xE4
#define KEY_KP_5          0xE5
#define KEY_KP_6          0xE6
#define KEY_KP_7          0xE7
#define KEY_KP_8          0xE8
#define KEY_KP_9          0xE9
#define KEY_KP_0          0xEA
#define KEY_KP_DOT        0xEB

// Function keys
#define KEY_F1            0xC2
#define KEY_F2            0xC3
#define KEY_F3            0xC4
#define KEY_F4            0xC5
#define KEY_F5            0xC6
#define KEY_F6            0xC7
#define KEY_F7            0xC8
#define KEY_F8            0xC9
#define KEY_F9            0xCA
#define KEY_F10           0xCB
#define KEY_F11           0xCC
#define KEY_F12           0xCD
#define KEY_F13           0xF0
#define KEY_F14           0xF1
#define KEY_F15           0xF2
#define KEY_F16           0xF3
#define KEY_F17           0xF4
#define KEY_F18           0xF5
#define KEY_F19           0xF6
#define KEY_F20           0xF7
#define KEY_F21           0xF8
#define KEY_F22           0xF9
#define KEY_F23           0xFA
#define KEY_F24           0xFB

#define INPUT_NUM         0x1a

// Supported keyboard layouts
extern const uint8_t KeyboardLayout_de_DE[];
extern const uint8_t KeyboardLayout_en_US[];
extern const uint8_t KeyboardLayout_es_ES[];
extern const uint8_t KeyboardLayout_fr_FR[];
extern const uint8_t KeyboardLayout_it_IT[];
extern const uint8_t KeyboardLayout_pt_PT[];
extern const uint8_t KeyboardLayout_sv_SE[];
extern const uint8_t KeyboardLayout_da_DK[];
extern const uint8_t KeyboardLayout_hu_HU[];

// Low level key report: up to 6 keys and shift, ctrl etc at once
typedef struct
{
  uint8_t modifiers;
  uint8_t reserved;
  uint8_t keys[INPUT_NUM];
} KeyReport;

class Keyboard_ : public Print
{
private:
  KeyReport _keyReport;
  const uint8_t *_asciimap;
  void sendReport(KeyReport* keys);
public:
  Keyboard_(void);
  void begin(const uint8_t *layout = KeyboardLayout_en_US);
  void end(void);
  size_t write(uint8_t k);
  size_t write(const uint8_t *buffer, size_t size);
  size_t press(uint8_t k);
  size_t release(uint8_t k);
  void releaseAll(void);
};
extern Keyboard_ Keyboard;

#endif
#endif

Keyboard.cpp

/*
  Keyboard.cpp

  Copyright (c) 2015, Arduino LLC
  Original code (pre-library): Copyright (c) 2011, Peter Barrett

  This library is free software; you can redistribute it and/or
  modify it under the terms of the GNU Lesser General Public
  License as published by the Free Software Foundation; either
  version 2.1 of the License, or (at your option) any later version.

  This library is distributed in the hope that it will be useful,
  but WITHOUT ANY WARRANTY; without even the implied warranty of
  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  Lesser General Public License for more details.

  You should have received a copy of the GNU Lesser General Public
  License along with this library; if not, write to the Free Software
  Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA
*/

#include "Keyboard.h"
#include "KeyboardLayout.h"

#if defined(_USING_HID)

//================================================================================
//================================================================================
//  Keyboard

static const uint8_t _hidReportDescriptor[] PROGMEM = {

    //  Keyboard
    0x05, 0x01,                    // USAGE_PAGE (Generic Desktop)  // 47
    0x09, 0x06,                    // USAGE (Keyboard)
    0xa1, 0x01,                    // COLLECTION (Application)
    0x85, 0x02,                    //   REPORT_ID (2)
    0x05, 0x07,                    //   USAGE_PAGE (Keyboard)

    0x19, 0xe0,                    //   USAGE_MINIMUM (Keyboard LeftControl)
    0x29, 0xe7,                    //   USAGE_MAXIMUM (Keyboard Right GUI)
    0x15, 0x00,                    //   LOGICAL_MINIMUM (0)
    0x25, 0x01,                    //   LOGICAL_MAXIMUM (1)
    0x75, 0x01,                    //   REPORT_SIZE (1)

    0x95, 0x08,                    //   REPORT_COUNT (8)
    0x81, 0x02,                    //   INPUT (Data,Var,Abs)
    0x95, 0x01,                    //   REPORT_COUNT (1)
    0x75, 0x08,                    //   REPORT_SIZE (8)
    0x81, 0x03,                    //   INPUT (Cnst,Var,Abs)

    0x95, INPUT_NUM,               //   REPORT_COUNT (INPUT_NUM) 
    0x75, 0x08,                    //   REPORT_SIZE (8)
    0x15, 0x00,                    //   LOGICAL_MINIMUM (0)
    0x25, 0x73,                    //   LOGICAL_MAXIMUM (115)
    0x05, 0x07,                    //   USAGE_PAGE (Keyboard)

    0x19, 0x00,                    //   USAGE_MINIMUM (Reserved (no event indicated))
    0x29, 0x73,                    //   USAGE_MAXIMUM (Keyboard Application)
    0x81, 0x00,                    //   INPUT (Data,Ary,Abs)
    0xc0,                          // END_COLLECTION
};

Keyboard_::Keyboard_(void)
{
	static HIDSubDescriptor node(_hidReportDescriptor, sizeof(_hidReportDescriptor));
	HID().AppendDescriptor(&node);
	_asciimap = KeyboardLayout_en_US;
}

void Keyboard_::begin(const uint8_t *layout)
{
	_asciimap = layout;
}

void Keyboard_::end(void)
{
}

void Keyboard_::sendReport(KeyReport* keys)
{
	HID().SendReport(2,keys,sizeof(KeyReport));
}

uint8_t USBPutChar(uint8_t c);

// press() adds the specified key (printing, non-printing, or modifier)
// to the persistent key report and sends the report.  Because of the way
// USB HID works, the host acts like the key remains pressed until we
// call release(), releaseAll(), or otherwise clear the report and resend.
size_t Keyboard_::press(uint8_t k)
{
	uint8_t i;
	uint8_t j;
	uint8_t flag;
	if (k >= 136) {			// it's a non-printing key (not a modifier)
		k = k - 136;
	} else if (k >= 128) {	// it's a modifier key
		_keyReport.modifiers |= (1<<(k-128));
		k = 0;
	} else {				// it's a printing key
		k = pgm_read_byte(_asciimap + k);
		if (!k) {
			setWriteError();
			return 0;
		}
		if ((k & ALT_GR) == ALT_GR) {
			_keyReport.modifiers |= 0x40;   // AltGr = right Alt
			k &= 0x3F;
		} else if ((k & SHIFT) == SHIFT) {
			_keyReport.modifiers |= 0x02;	// the left shift modifier
			k &= 0x7F;
		}
		if (k == ISO_REPLACEMENT) {
			k = ISO_KEY;
		}
	}

	for(j = 0; j < INPUT_NUM; j++){
		if(_keyReport.keys[j] == k){
			flag = 0;
			break;
		}
		flag = 1;
	}
	
	// Add k to the key report only if it's not already present
	// and if there is an empty slot.
	if (flag == 1) {
		for (i=0; i<INPUT_NUM; i++) {
			if (_keyReport.keys[i] == 0x00) {
				_keyReport.keys[i] = k;
				break;
			}
		}
		if (i == INPUT_NUM) {
			setWriteError();
			return 0;
		}
	}
	sendReport(&_keyReport);
	return 1;
}

// release() takes the specified key out of the persistent key report and
// sends the report.  This tells the OS the key is no longer pressed and that
// it shouldn't be repeated any more.
size_t Keyboard_::release(uint8_t k)
{
	uint8_t i;
	if (k >= 136) {			// it's a non-printing key (not a modifier)
		k = k - 136;
	} else if (k >= 128) {	// it's a modifier key
		_keyReport.modifiers &= ~(1<<(k-128));
		k = 0;
	} else {				// it's a printing key
		k = pgm_read_byte(_asciimap + k);
		if (!k) {
			return 0;
		}
		if ((k & ALT_GR) == ALT_GR) {
			_keyReport.modifiers &= ~(0x40);   // AltGr = right Alt
			k &= 0x3F;
		} else if ((k & SHIFT) == SHIFT) {
			_keyReport.modifiers &= ~(0x02);	// the left shift modifier
			k &= 0x7F;
		}
		if (k == ISO_REPLACEMENT) {
			k = ISO_KEY;
		}
	}

	// Test the key report to see if k is present.  Clear it if it exists.
	// Check all positions in case the key is present more than once (which it shouldn't be)
	for (i=0; i<INPUT_NUM; i++) {
		if (0 != k && _keyReport.keys[i] == k) {
			_keyReport.keys[i] = 0x00;
		}
	}

	sendReport(&_keyReport);
	return 1;
}

void Keyboard_::releaseAll(void)
{
	int i;
	for(i = 0; i < INPUT_NUM; i++){
		_keyReport.keys[i] = 0;
	}
	_keyReport.modifiers = 0;
	sendReport(&_keyReport);
}

size_t Keyboard_::write(uint8_t c)
{
	uint8_t p = press(c);	// Keydown
	release(c);		// Keyup
	return p;		// just return the result of press() since release() almost always returns 1
}

size_t Keyboard_::write(const uint8_t *buffer, size_t size) {
	size_t n = 0;
	while (size--) {
		if (*buffer != '\r') {
			if (write(*buffer)) {
				n++;
			} else {
				break;
			}
		}
		buffer++;
	}
	return n;
}

Keyboard_ Keyboard;

#endif

私も原理がよくわかってないので必要ない部分も変更してる可能性があります。
keyboard.hの113行目に追加したINPUT_NUMを変更することで上限を変更できます。↑では0x1a == 26個まで同時に押すことができます。