How do I know the length of receive data on the USB HID?

RE: How do I know the length of receive data on the USB HID?

Dmitry — Thu, 29 Oct 2020 10:20:20 GMT

As your input data doesn't match the format of report descriptor, other side can choose whether do reject it, or pad it with zeroes up to 64 bytes - your PC chose second option.

[quote userid="83275" url="~/f/nordic-q-a/67629/how-do-i-know-the-length-of-receive-data-on-the-usb-hid/277567#277567"]Can I know the exact length of the received data?[/quote]

For example, you can use first byte or your message as a length field - to send two bytes, fill buffer with [2,byte1,byte2].

RE: How do I know the length of receive data on the USB HID?

moon.kim — Thu, 29 Oct 2020 10:14:07 GMT

Thank you.

I have one more question.

Why is the received data size different?

I sent 2 bytes from the PC to the device. However, the size of the received data is 64 bytes.

Can I know the exact length of the received data?

 recv_buf = (uint8_t *)app_usbd_hid_generic_out_report_get(&m_app_hid_generic,&recv_size);

NRF_LOG_INFO("received data length:%d", recv_size);
NRF_LOG_HEXDUMP_INFO(recv_buf, recv_size);

RE: How do I know the length of receive data on the USB HID?

Dmitry — Thu, 29 Oct 2020 09:59:15 GMT

Yes, you should send an exact number of bytes as configured in report descriptor (read this thread for more information).

You can configure several report descriptors of different sizes with different IDs, but this would complicate things a lot - it's much easier to just send 64 bytes of data everytime, no matter how many bytes are used actually.

RE: How do I know the length of receive data on the USB HID?

moon.kim — Thu, 29 Oct 2020 01:39:23 GMT

Hi.

When I transmit 2 bytes, if abSize is set to 2, data is not transmitted.

However, if it is set to 64, 2 bytes are transmitted and the remaining 62 bytes are transmitted as <NUL>.

RE: How do I know the length of receive data on the USB HID?

ovrebekk — Wed, 28 Oct 2020 12:09:21 GMT

What do you mean the abSize parameter has to be set to 64?

Do you get an error message if you set it to something else?

Best regards
Torbjørn

RE: How do I know the length of receive data on the USB HID?

moon.kim — Mon, 26 Oct 2020 23:53:21 GMT

Hi,

Sorry. I can't understand.

I use'app_usbd_hid_generic_out_report_get' and'app_usbd_hid_generic_in_report_set' to send and receive data.

When receiving data, the data size (recv_size) is always 64. I want to know the size of the data actually received.

recv_buf = (uint8_t *)app_usbd_hid_generic_out_report_get(&m_app_hid_generic,&recv_size);

And when data is transmitted, data size(abSize) must be set to 64 for transmission. Can't I set the length of the data I want to send?

ret = app_usbd_hid_generic_in_report_set(
        &m_app_hid_generic,
        (uint8_t *)apvData,
        abSize);

RE: How do I know the length of receive data on the USB HID?

Dmitry — Mon, 26 Oct 2020 11:25:49 GMT

Hi,

report length is always the same size - the one found in report descriptor (in your case, there is a single report of Report Count * Report Size bits). You can fill only part of report buffer and store the real size, for example, in the first byte of your message.