main.c

#include <stdlib.h>
#include <stdio.h>
#include <string.h>

#include "hardware/uart.h"
#include "pico/stdlib.h"
#include "bsp/board.h"
#include "tusb.h"

#define UART_ID uart0
#define BAUD_RATE 9600
#define UART_TX_PIN 0
#define UART_RX_PIN 1

static uint8_t const keycode2ascii[128][2] =  { HID_KEYCODE_TO_ASCII };

static void process_kbd_report(hid_keyboard_report_t const *report);


int main() {
  board_init();

  // init serial
  uart_init(UART_ID, BAUD_RATE);
  gpio_set_function(UART_TX_PIN, GPIO_FUNC_UART);
  gpio_set_function(UART_RX_PIN, GPIO_FUNC_UART);

  // TinyUSB, init host stack on configured RootHub Port
  tuh_init(BOARD_TUH_RHPORT);

  // turn on onboard led
  board_led_write(true);

  while(true) {
    tuh_task();  // tinyusb host task
  }

  return 0;
}


//--------------------------------------------------------------------+
// TinyUSB Callbacks
//--------------------------------------------------------------------+

// called after all tuh_hid_mount_cb
void tuh_mount_cb(uint8_t dev_addr)
{
  // application set-up
  printf("A device with address %d is mounted\r\n", dev_addr);
}

// called before all tuh_hid_unmount_cb
void tuh_umount_cb(uint8_t dev_addr)
{
  // application tear-down
  printf("A device with address %d is unmounted \r\n", dev_addr);
}


// Invoked when device with hid interface is mounted
// Report descriptor is also available for use. tuh_hid_parse_report_descriptor()
// can be used to parse common/simple enough descriptor.
// Note: if report descriptor length > CFG_TUH_ENUMERATION_BUFSIZE, it will be skipped
// therefore report_desc = NULL, desc_len = 0
void tuh_hid_mount_cb(uint8_t dev_addr, uint8_t instance, uint8_t const* desc_report, uint16_t desc_len) {
  printf("HID device address = %d, instance = %d is mounted\r\n", dev_addr, instance);

  if(tuh_hid_interface_protocol(dev_addr, instance) == HID_ITF_PROTOCOL_KEYBOARD) {
    if ( !tuh_hid_receive_report(dev_addr, instance) )
    {
      printf("Error: cannot request to receive report\r\n");
    }
  }
}

// Invoked when device with hid interface is un-mounted
void tuh_hid_umount_cb(uint8_t dev_addr, uint8_t instance) {
  printf("HID device address = %d, instance = %d is unmounted\r\n", dev_addr, instance);
}



// Invoked when received report from device via interrupt endpoint (key down and key up)
void tuh_hid_report_received_cb(uint8_t dev_addr, uint8_t instance, uint8_t const* report, uint16_t len)
{
  printf("received report from HID device address = %d, instance = %d\r\n", dev_addr, instance);

  uint8_t const itf_protocol = tuh_hid_interface_protocol(dev_addr, instance);

  switch (itf_protocol)
  {
    case HID_ITF_PROTOCOL_KEYBOARD:
      printf("HID receive boot keyboard report\r\n");
      process_kbd_report( (hid_keyboard_report_t const*) report );
    break;
  }

  // continue to request to receive report
  if ( !tuh_hid_receive_report(dev_addr, instance) )
  {
    printf("Error: cannot request to receive report\r\n");
  }
}


//--------------------------------------------------------------------+
// Keyboard
//--------------------------------------------------------------------+

// look up new key in previous keys
static inline bool find_key_in_report(hid_keyboard_report_t const *report, uint8_t keycode)
{
  for(uint8_t i=0; i<6; i++)
  {
    if (report->keycode[i] == keycode)  return true;
  }

  return false;
}

static void process_kbd_report(hid_keyboard_report_t const *report)
{
  static hid_keyboard_report_t prev_report = { 0, 0, {0} }; // previous report to check key released

  //------------- example code ignore control (non-printable) key affects -------------//
  for(uint8_t i=0; i<6; i++)
  {
    if ( report->keycode[i] )
    {
      if ( find_key_in_report(&prev_report, report->keycode[i]) )
      {
        // exist in previous report means the current key is holding
      }else
      {
        // not existed in previous report means the current key is pressed
        bool const is_shift = report->modifier & (KEYBOARD_MODIFIER_LEFTSHIFT | KEYBOARD_MODIFIER_RIGHTSHIFT);
        uint8_t ch = keycode2ascii[report->keycode[i]][is_shift ? 1 : 0];
        putchar(ch);
        putchar('\n');
        if ( ch == '\r' ) putchar('\n'); // added new line for enter key

        fflush(stdout); // flush right away, else nanolib will wait for newline
      }
    }
    // TODO example skips key released
  }

  prev_report = *report;
}