Digispark HID via https://digistump.com/board/index.php?topic=1096.0

keyboard

#include "DigiKeyboard.h"




// Delay between keystrokes
#define KEYSTROKE_DELAY 1000


// Blinking control
int iterationCounter = 0;
#define BLINK_RATE 2


void setup() {
  
  // initialize the digital pin as an output.
  pinMode(0, OUTPUT); //LED on Model B
  pinMode(1, OUTPUT); //LED on Model A     
  
  // don't need to set anything up to use DigiKeyboard
}


void loop() {
  
  DigiKeyboard.update();


  if (iterationCounter % BLINK_RATE == 0)
      turnLedOn ();


  // this is generally not necessary but with some older systems it seems to
  // prevent missing the first character after a delay:
  DigiKeyboard.sendKeyStroke(0);


  // It's better to use DigiKeyboard.delay() over the regular Arduino delay()
  // if doing keyboard stuff because it keeps talking to the computer to make
  // sure the computer knows the keyboard is alive and connected
  DigiKeyboard.delay(KEYSTROKE_DELAY);


  
  // UP
  DigiKeyboard.sendKeyStroke(KEY_ARROW_UP);
  DigiKeyboard.delay(KEYSTROKE_DELAY);  
  
  // DOWN
  DigiKeyboard.sendKeyStroke(KEY_ARROW_DOWN);
  DigiKeyboard.delay(KEYSTROKE_DELAY);   
 


  if (iterationCounter % BLINK_RATE == 0)
      turnLedOff ();  
  
  iterationCounter++;
}


void turnLedOn ()
{
    digitalWrite(0, HIGH);   // turn the LED on (HIGH is the voltage level)
    digitalWrite(1, HIGH);
}


void turnLedOff ()
{
    digitalWrite(0, LOW);    // turn the LED off by making the voltage LOW
    digitalWrite(1, LOW); 
}

UPDATE

1. Mark device as boot device in configuration descriptor:


      #define USB_CFG_DEVICE_CLASS        0     /* set to 0 if deferred to interface */
      #define USB_CFG_DEVICE_SUBCLASS     0
      /* See USB specification if you want to conform to an existing device class.
       * Class 0xff is "vendor specific".
       */
      #define USB_CFG_INTERFACE_CLASS     3   /* define class here if not at device level */
      #define USB_CFG_INTERFACE_SUBCLASS  1
      #define USB_CFG_INTERFACE_PROTOCOL  1


2. Use a HID report descriptor indicating boot support (see http://www.usb.org/developers/devclass_docs/HID1_11.pdf , Appendix B: Boot Interface 
Descriptors) (Update USB_CFG_HID_REPORT_DESCRIPTOR_LENGTH to the new size, mine is 63 bytes):


   PROGMEM char usbHidReportDescriptor[USB_CFG_HID_REPORT_DESCRIPTOR_LENGTH] = { /* USB report descriptor */
      0x05, 0x01,                    // USAGE_PAGE (Generic Desktop)
      0x09, 0x06,                    // USAGE (Keyboard)
      0xa1, 0x01,                    // COLLECTION (Application)
      0x75, 0x01,                    //   REPORT_SIZE (1)
      0x95, 0x08,                    //   REPORT_COUNT (8)
      0x05, 0x07,                    //   USAGE_PAGE (Keyboard)(Key Codes)
      0x19, 0xe0,                    //   USAGE_MINIMUM (Keyboard LeftControl)(224)
      0x29, 0xe7,                    //   USAGE_MAXIMUM (Keyboard Right GUI)(231)
      0x15, 0x00,                    //   LOGICAL_MINIMUM (0)
      0x25, 0x01,                    //   LOGICAL_MAXIMUM (1)
      0x81, 0x02,                    //   INPUT (Data,Var,Abs) ; Modifier byte
      0x95, 0x01,                    //   REPORT_COUNT (1)
      0x75, 0x08,                    //   REPORT_SIZE (8)
      0x81, 0x03,                    //   INPUT (Cnst,Var,Abs) ; Reserved byte
      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) ; LED report
      0x95, 0x01,                    //   REPORT_COUNT (1)
      0x75, 0x03,                    //   REPORT_SIZE (3)
      0x91, 0x03,                    //   OUTPUT (Cnst,Var,Abs) ; LED report padding
      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)(Key Codes)
      0x19, 0x00,                    //   USAGE_MINIMUM (Reserved (no event indicated))(0)
      0x29, 0x65,                    //   USAGE_MAXIMUM (Keyboard Application)(101)
      0x81, 0x00,                    //   INPUT (Data,Ary,Abs)
      0xc0                           // END_COLLECTION
   };
   
3. Fill a report structure for boot protocol. Send it when needed. Up to 6 at the same time. For example
 
   typedef struct {
         uint8_t modifier;
         uint8_t reserved;
         uint8_t keycode[6];
   } keyboard_report_t;