USB Class In Function: Sending Keycodes to Host

The usb_class_in() function handles sending keycodes from a USB device to a host after the device is properly configured and the USB is ready.

Steps:

1. Configuration Check: The function first verifies if the USB device is configured using the DeviceState variable. If not, it returns immediately.

2. Idle State and Key Press Check: It then checks if the USB is idle (UsbInBusy) and if a key press event has occurred (fKeyOK).

3. Initialization:

   • fKeyOK is set to 0 to indicate that the key press has been processed.
   • The key array is cleared, ensuring a clean transmission buffer.

4. Keycode Mapping: Based on the bKeyCode value, which represents the combination of pressed keys on pins P3.2 to P3.5, the key array is populated with the corresponding keycodes. This mapping is essential to translate the device's physical button presses into standardized keycodes recognized by the host.

   • For example, if bKeyCode is 0x0e (P3.2 and P3.3 pressed), key[2] is set to 0xE0 (Ctrl keycode) and key[3] is set to 0x06 (C keycode).

5. Transmission:

   • The USB is temporarily disabled (EUSB = 0).
   • UsbInBusy is set to 1, indicating that the USB is now transmitting data.
   • The usb_write_reg() function is used to write data to specific registers: INDEX, FIFO1, and INCSR1. This function is crucial for controlling the USB communication.
   • The keycodes are sent to the host via the FIFO1 register in a loop.
   • The INCSR1 register is set to INIPRDY, signaling that the data is ready to be sent.
   • Finally, the USB is re-enabled (EUSB = 1).

The scan_key() function continuously monitors the state of P3.2 to P3.5, triggering a key press event when a change is detected.

Steps:

1. Key Status Acquisition: The function obtains the current state of P3.2 to P3.5 pins using bitwise operations and stores the value in the keys variable.

2. Change Detection: It compares the current key state (keys) with the previous state (prevKeys). If there's a difference, it signifies a key press event.

3. Key Press Handling:

   • The prevKeys variable is updated with the current key state.
   • The bKeyCode variable is set to the current keycode.
   • The bKeyDebounce variable is initialized to 20, establishing a 20ms debounce delay to filter out spurious button press events.

4. Debouncing:

   • If bKeyDebounce is not zero, it indicates that the debounce delay is still active. The value is decremented until it reaches 0, after which the fKeyOK flag is set to 1, signifying that a valid key press event has occurred.