c# create barcode 4: The Game Controller in Objective-C

Printing QR in Objective-C 4: The Game Controller

CHAPTER 4: The Game Controller
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} - (IBAction)turnOnGreenLED:(id)sender { const uint8_t buf[2] = {0x98, 0x03}; [[EADAccessoryController sharedController] writeData:[NSData dataWithBytes:buf length:2]]; } - (IBAction)turnOffGreenLED:(id)sender { const uint8_t buf[2] = {0x98, 0x04}; [[EADAccessoryController sharedController] writeData:[NSData dataWithBytes:buf length:2]]; }
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From the discussion of protocols earlier, you see that each function sends a two-byte packet to the accessory each packet containing a control byte and a parameter byte. The control byte 0x98 tells the accessory that this is a command to be executed and the parameter byte (either 0x01, 0x02, 0x03, or 0x04) tells the accessory which specific LED to turn on or off. But how do you actually send the data to the accessory Look at the second line of each method. The first part of each statement returns the shared accessory controller and to that controller you pass the writeData method with two parameters: the buffer containing the command packet and the length (two bytes) to be sent. NOTE: At the time of this writing, the source code for Apple s External Accessory Demo was only available when you purchased the 2009 WWDC proceedings. Copyright restrictions prevent its duplication in this book. Check with Apple for more information. Now that you re able to send data to the accessory, you need to consider how to get data from the accessory to the iPhone and the iPhone application. As mentioned earlier, the accessory sends two different data types to the iPhone. The first data type sent to the iPhone tells the application when the button is pressed by the user. The button press interrupts the normal programming loop executing on the firmware in the accessory. As can be seen in Figure 4 16, as soon as the accessory has initialized, it enters a loop that does nothing.
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CHAPTER 4: The Game Controller
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Figure 4 16. Interrupt routine for pushbutton
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This null loop continually loops on itself until it is interrupted by an external event. In Figure 4 16, that event would be when the user presses the pushbutton. Internal to the accessory game controller, the normal 3.0 volts is reduced to 0 volts. This negative transition (3 to 0) is interpreted as an interrupt by the PIC16LF1936 processor. Within the PIC processor, the code jumps to a table of interrupt vectors (addresses of specific routines) determined by the base address of the table plus an offset indicated by the type of interrupt that occurred. As shown in Figure 4 17, when the PIC controller powers on, it first jumps over tables that include the addresses of interrupt routines to the start of the main code. Doing it this way provides a known set of addresses that the programmer sets up for the interrupt routines. If a pushbutton interrupt occurs while executing the null loop, after saving state, the address at the start of the interrupt table offset by the number of the interrupt is loaded into the program counter. This code executes, and, in your case, sends the 0x10 value over the serial connection to the iPhone telling the iPhone app that the button was pressed.
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CHAPTER 4: The Game Controller
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Figure 4 17. Accessory firmware interrupt vector table concept
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On the iPhone side, the application sees any incoming data as an NSStream, which I discussed earlier. Both the input (to iPhone from accessory) and output streams are set up when you open the session to your accessory. The openSession routine sets the delegate to itself and attempts to open the session for the indicated protocol. If successful, meaning the session variable is not null, the routine opens the input and output streams.
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- (BOOL)openSession { [accessory setDelegate:self]; session = [[EASession alloc] initWithAccessory:_accessory forProtocol:_protocolString]; if (session) { [[session inputStream] setDelegate:self]; [[session inputStream] scheduleInRunLoop:[NSRunLoop currentRunLoop] forMode:NSDefaultRunLoopMode]; [[session inputStream] open]; [[session outputStream] setDelegate:self]; [[session outputStream] scheduleInRunLoop:[NSRunLoop currentRunLoop] forMode:NSDefaultRunLoopMode]; [[session outputStream] open]; } else { // handle failed open session
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