Why HID.SendReport() is limited at 9 char

Using Arduino Microcontrollers
1

Hello,

Why this function is limited to 9 char, is it a specification of HID protocol or something else.

2

How do you know it's limited to 9 characters (it's actually bytes :wink: ) The function itself does not impose the limit (as far as I can see, also not if you dig deeper (into USB_Send etc)).

int HID_::SendReport(uint8_t id, const void* data, int len)
{
	auto ret = USB_Send(pluggedEndpoint, &id, 1);
	if (ret < 0) return ret;
	auto ret2 = USB_Send(pluggedEndpoint | TRANSFER_RELEASE, data, len);
	if (ret2 < 0) return ret2;
	return ret + ret2;
}

I suspect that it's actually 'imposed' by the function that calls SendReport. As you're talking about characters, I guess that you're using the keyboard library that has the struct

typedef struct
{
  uint8_t modifiers;
  uint8_t reserved;
  uint8_t keys[6];
} KeyReport;

which is 8 bytes in total.

The answer might be in HID Keyboard Without 6 Key Limit?

3

Sorry I mean char by its length, not by its definition.
And no, it's not SendReport() nor USB_Send() because I redesigned them to delete all useless code that can interfere with its first purpose.

void HID_::USBSend(const uint8_t* data, int len)
{
	USB_Send(pluggedEndpoint | TRANSFER_RELEASE, data, len);
}

//	Blocking Send of data to an endpoint
int USB_Send(u8 ep, u8* data, int len)
{
    int len_ = len;

	if (_usbSuspendState & (1<<SUSPI)) 
    {
		//send a remote wakeup
		UDCON |= (1 << RMWKUP);
	}

    LockEP lock(ep);

    while (len--)
        Send8(*data++);

    ReleaseTX();

	TXLED1;					// light the TX LED
	TxLEDPulse = TX_RX_LED_PULSE_MS;
    
	return len_;
}

And if you try that:

HID().USBSend(data, 9);

It's working, but not that:

HID().USBSend(data, 10);

It's make no sense.

4

It doex make sense if the limit is imposed ar the receiving side; the link I gave mentions it.

5

But why when you connect your USB device to your PC, the bootloader can load 233 bytes for Descriptor ect ?

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6

Because it's a different part of the usb spec ? I really don't know, you're far more advanced in this than I am.

So what do you do in your code to send the data? Keyboard? Mouse? Something else?

7

It's maybe the HID descriptor but like I don't know how to write it, I am blocked.

I am just sending random data, it's for testing.

8

arduiko:
I am just sending random data, it's for testing.

How do you send data? Maybe show a simple example.

9

HID().USBSend(data, 9);