Oh Cheat!

I've decided to apply this project to phase 4 of Hackaday 2016 prize which involves automation. This post is to clarify/complete some information required;

The project was conceived for playing "GTA" game series in a PC computer by automate the entry of cheat codes composed by single keystrokes.

About the licenses/permissions:

The project is released underGPL 2.0.

About Third party licences/restrictions

The project is based on Digi-Spark board.

Copyright of information used on the article.

"Oh Sh*t!" game was property of Aackosoft International B.V. (now extinct)

"MSX" is a trademark fromASCII Corporation

"GTA San Andreas" is a trademark from Rockstar

Here you are a small video of Oh Cheat! in action playing GTA San Andreas.

The Chinese Digispark clones are very cheap and have a female USB connector which is good but they ship with RESET enabled which means that PB5 is not available for use.

To perform the change it is necessary to change the HFuse configuration of the ATTiny85.

I have used AVeRCADE with USBasp firmware for doing that, but any ISP programmer will do fine. The connections to a 10 pin are show below. However it is possible to use other methods, for example using an Arduino (link).

avrdude -c usbasp -p t85 -U hfuse:w:0x5f:m

I've created a standalone design for the board, single sided and using only pth components.

I've made an extension to the SendKeyStroke method of DigiKeyboard.h by adding a key down time parameter.

SendKeyStroke method now has a third parameter to allow the simulation of pressing time (in milliseconds) that the key remains pressed until is released.

void sendKeyStroke(byte keyStroke, byte modifiers, uint16_t key_down_time);

If 'key_down_time' is zero then don't release the key, it should be done manually by sendKeysStroke(0). As 'zero' means forever, the minimum press time time is 1ms and the maximum is 65535ms.

Examples:

DigiKeyboard.sendKeyStroke(KEY_B);        // Send a B to host
DigiKeyboard.sendKeyStroke(KEY_A,0,300); // Send an A to host, takes 300ms until release

DigiKeyboard.sendKeyStroke(KEY_C,0,0);    //Send a C to host and keep it pushed
DigiKeyboard.delay(2000);                 // wait 2 seconds
DigiKeyboard.sendKeyStroke(0);            // now release any key pressed.

Tried to send a pull request to original project but without success, though. Either way the extended function is below:

  void sendKeyStroke(byte keyStroke, byte modifiers, uint16_t key_down_time) {
   	while (!usbInterruptIsReady()) {
      // Note: We wait until we can send keystroke
      //       so we know the previous keystroke was
      //       sent.
    	usbPoll();
    	_delay_ms(5);
    }
    
    memset(reportBuffer, 0, sizeof(reportBuffer));
		
    reportBuffer[0] = modifiers;
    reportBuffer[1] = keyStroke;
    
    usbSetInterrupt(reportBuffer, sizeof(reportBuffer));
		
  	while (!usbInterruptIsReady()) {
      // Note: We wait until we can send keystroke
      //       so we know the previous keystroke was
      //       sent.
    	usbPoll();
    	_delay_ms(5);
    }
	
	// Pressing time (in milliseconds) that the key remains pressed
	// until is released. If is zero don't release the key (it should be
	// done manually by sendKeysStroke(0). Therefore minimum time is 1ms
	// and the maximum is 65535ms.
	if (key_down_time) {
		while (key_down_time--) {
			usbPoll();      
			_delay_ms(1);
		}
		// Now send 'no key' pressed
		memset(reportBuffer, 0, sizeof(reportBuffer));
		usbSetInterrupt(reportBuffer, sizeof(reportBuffer));	
	} 
  }

I have found the problem with GTA: The game kinda sample the keyboard state, so if the key remain pressed for a few time it won't be recognized. I got that insight when I realized that sometimes the game recognized one of the keys (like A that makes the character move left).

By analyzing Digispark.h I have noticed that the SendKeyStroke function sends a key up right after the key pressed.

  void sendKeyStroke(byte keyStroke, byte modifiers) {
   	while (!usbInterruptIsReady()) {
    	usbPoll();
    	_delay_ms(5);
    } 
    memset(reportBuffer, 0, sizeof(reportBuffer));		
    reportBuffer[0] = modifiers;
    reportBuffer[1] = keyStroke;
    usbSetInterrupt(reportBuffer, sizeof(reportBuffer));
    while (!usbInterruptIsReady()) {
    	usbPoll();
    	_delay_ms(5);
    }

         /*   HERE WAS THE PROBLEM WITH GTA   */

    // This stops endlessly repeating keystrokes:  
    memset(reportBuffer, 0, sizeof(reportBuffer));      
    usbSetInterrupt(reportBuffer, sizeof(reportBuffer));
  }

The compiler complained about a missing definition of reportBuffer variable. Strangely enough the definition of such variable is not defined on DigiKeyboard.h. (it is defined in DigiMouse.h and DigiJoystick.h though!).

To fix it I have just created the variable and it compiled (and worked as well).

////////////////////////////////////////////////
//   __   __        _      _    _
//   \ \ / /_ _ _ _(_)__ _| |__| |___ ___
//    \ V / _` | '_| / _` | '_ \ / -_|_-<
//     \_/\__,_|_| |_\__,_|_.__/_\___/__/
//
uint8_t ticks = 0;
uint8_t lastkey = 0;
uint8_t temp, key;
uchar     reportBuffer[2];

// Print a string to the host, and add a key up after a delay 
void DigiKeyboard_print ( char *str ) {
  uint8_t data;
  while (*str) {
    data = pgm_read_byte_near(ascii_to_scan_code_table + (*str++ - 8));
    sendKeyStroke_nostop(data & 0b01111111, data >> 7 ? MOD_SHIFT_RIGHT : 0);
    DigiKeyboard.delay(50);
    sendKeyStroke_nostop(0,0);
  }
}

Bug Busted!!

I still haven't figured out why do the game (GTASA) is not accepting the keystrokes, though it can really recognize some of the strokes.

I wrote my own 'print' method to put each key, then a zero but not worked. Then I've inserted a delay between keystrokes, and yet is not working. Even tried to send several zeros in between each keystroke but still no success.

void my_DigiKeyboard_print ( char *str ) {
  uint8_t data;
  
  while (*str) {
    data = pgm_read_byte_near(ascii_to_scan_code_table + (*str++ - 8));
    DigiKeyboard.sendKeyStroke(data & 0b01111111, data >> 7 ? MOD_SHIFT_RIGHT : 0); 
    DigiKeyboard.sendKeyStroke(0);
    DigiKeyboard.sendKeyStroke(0);
    DigiKeyboard.sendKeyStroke(0);    
    DigiKeyboard.delay(100);   
  }  
}

In any other application the keys are correctly injected, it is weird.. I still have more work to do.

For development of the firmware I have used a protoboard

I don't have a DigiSpark here but I have a DIY clone (named Cricket)

After I have built a keyboard with tactile switches. Sadly the perfboard only have room for 6 keys but nevermind, I don't give a cheat about 7th key.