STM32 HID Keyboard - stm32

Question

I am trying to use and STM32F0-disco as a keyboard for a Windows PC. I'm having a problem with the characters that are being printed.
The code below waits until the onboard button is pressed then should print the three characters once.
/* USER CODE BEGIN 2 */
USBDelay = USBD_HID_GetPollingInterval(&hUsbDeviceFS);
while (HAL_GPIO_ReadPin(B1_GPIO_Port, B1_Pin) == 0);
if (HAL_GPIO_ReadPin(B1_GPIO_Port, B1_Pin) == 1) {
HAL_GPIO_TogglePin(LD3_GPIO_Port, LD3_Pin);
buff[0] = 0x00;
buff[2] = 0x04;
buff[3] = 0x05;
buff[4] = 0x06;
USBD_HID_SendReport(&hUsbDeviceFS, buff, 8);
HAL_Delay(USBDelay);
buff[0] = 0x00;
buff[2] = 0x00;
buff[3] = 0x00;
buff[4] = 0x00;
USBD_HID_SendReport(&hUsbDeviceFS, buff, 8);
HAL_GPIO_TogglePin(LD4_GPIO_Port, LD4_Pin);
HAL_Delay(1000);
}
while (HAL_GPIO_ReadPin(B1_GPIO_Port, B1_Pin) == 1);
/* USER CODE END 2 */
The problem I'm having is that the only the characters in the first two key positions of the buffer (which would be the third and fourth elements of the array since element one is the modifier and the second element is reserved). Also, instead of printing once, the characters print continuously without stopping.
Based on the LED's I've added for testing purposes, the characters that I want to print don't print until the second USB_SendReport, which should be sending blank characters to stop the printing.
I've made the necessary changes to usbd_hid.c and usbd_hid.h, as well as the Clock configuration and enabling USB and such. I've put my keyboard descriptor below.
0x05, 0x01, // USAGE_PAGE (Generic Desktop)
0x09, 0x06, // USAGE (Keyboard)
0xa1, 0x01, // COLLECTION (Application)
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, 0x05, // REPORT_COUNT (5)
0x75, 0x01, // REPORT_SIZE (1)
0x05, 0x08, // USAGE_PAGE (LEDs)
0x19, 0x01, // USAGE_MINIMUM (Num Lock)
0x29, 0x05, // USAGE_MAXIMUM (Kana)
0x91, 0x02, // OUTPUT (Data,Var,Abs)
0x95, 0x01, // REPORT_COUNT (1)
0x75, 0x03, // REPORT_SIZE (3)
0x91, 0x03, // OUTPUT (Cnst,Var,Abs)
0x95, 0x06, // REPORT_COUNT (6)
0x75, 0x08, // REPORT_SIZE (8)
0x15, 0x00, // LOGICAL_MINIMUM (0)
0x25, 0x65, // LOGICAL_MAXIMUM (101)
0x05, 0x07, // USAGE_PAGE (Keyboard)
0x19, 0x00, // USAGE_MINIMUM (Reserved (no event indicated))
0x29, 0x65, // USAGE_MAXIMUM (Keyboard Application)
0x81, 0x00, // INPUT (Data,Ary,Abs)
0xc0 // END_COLLECTION
Any ideas?

Answer 1

The USB HID protocol works differently than you assume. If the input report contains multiple pressed keys, the host (your Windows PC) assumes that they have been pressed at the same time. So they can be inserted in any order. And it might also be the reason the third key is not inserted.
So you need to send a separate output report for each key. And after the last report you need to send another empty one (with all 0s) to indicate the keys have been released (as you are already doing). Such an additional report is also needed between two presses of the same key.
The reason the keys are inserted continuously indicates the empty report was not received. Even though the function uses the term send (USBD_HID_SendReport()), the USB protocol works such that the host needs to pick up the input report. And since it is an USB interrupt endpoint, this only happens ever so often. If a report hasn't been picked up yet, USBD_HID_SendReport() does nothing. It not even returns an error.
While querying to polling interval (USBD_HID_GetPollingInterval()) looks ok at first, it's the polling interval declared in the USB endpoint descriptor. However, Windows only supports a few interval values and rounds up to the next supported value. So you have to increase the value.