[CDC + HID] CDC interface number mismatch — tud_cdc_n_*() always requires 0

[fikririzal]

Summary
I have implemented a USB device with two interfaces, based on cdc_dual_port :

• HID Keyboard (Interface 0 in my enum)
• CDC (Interface 1 and 2 in my enum)

Everything works as expected, but I noticed something strange:
In order to make CDC communication work, I always need to pass 0 as the interface index to tud_cdc_n_*() API, even though in my descriptor the CDC interface number is not zero.

---

• TinyUSB version: 0.18.0
• MCU: STM32F407VGT6
• Board: STM32F4XX M
• OS: Windows 11 (host)
• Software: STM32CubeIDE 1.19
• Files: My Repo

---

Interface enum and descriptors

enum {
  ITF_NUM_KEYBOARD = 0,
  ITF_NUM_CDC_0,
  ITF_NUM_CDC_0_DATA,
  ITF_NUM_TOTAL
};

uint8_t const desc_fs_configuration[] = {
  TUD_CONFIG_DESCRIPTOR(1, ITF_NUM_TOTAL, 0, CONFIG_TOTAL_LEN, 0x00, 100),

  // CDC descriptor
  TUD_CDC_DESCRIPTOR(ITF_NUM_CDC_0, 4, EPNUM_CDC_0_NOTIF, 16, EPNUM_CDC_0_OUT, EPNUM_CDC_0_IN, 64),

  // HID Keyboard descriptor
  TUD_HID_DESCRIPTOR(ITF_NUM_KEYBOARD, 5, HID_ITF_PROTOCOL_KEYBOARD, sizeof(desc_hid_keyboard_report), EPNUM_KEYBOARD, CFG_TUD_HID_EP_BUFSIZE, 1),
};

---

CDC task code

static void cdc_task(void) {
    if (tud_cdc_n_available(0)) {  // <- always need to use 0 here
        uint8_t buf[64];
        uint32_t count = tud_cdc_n_read(0, buf, sizeof(buf));
        echo_serial_port(0, buf, count);
    }
}

---

HID Keyboard code
(taken from TinyUSB HID example, works fine with ITF_NUM_KEYBOARD)

if (typing) {
    if (has_keyboard_key) {
        modifier = 0;
        tud_hid_n_keyboard_report(ITF_NUM_KEYBOARD, report_id, modifier, empty_key);
        has_keyboard_key = false;
    } else {
        modifier = 0;
        uint8_t chr = text[index];
        if (conv_table[chr][0]) modifier = KEYBOARD_MODIFIER_LEFTSHIFT;
        keycode[0] = conv_table[chr][1];
        tud_hid_n_keyboard_report(ITF_NUM_KEYBOARD, report_id, modifier, keycode);
        has_keyboard_key = true;
        index++;
        if (index >= textLen) {
            typing = false;
            index = 0;
        }
    }
}

---

Question
Why does tud_cdc_n_*() require passing 0 for the interface index, instead of the actual CDC interface number from my enum? Is the parameter for tud_cdc_n_*() referring to the CDC instance index rather than the USB interface number? If so, what is the correct way to match them?

Thank you!