iPhone app crashes when coming to foreground

I have an application that is communicating to a device through the accessory port. When the device is spitting data (at a rate of one 13-byte packet every 20 ms, so not very quickly) to the iPhone, if I press the home button to send the app to the background, then open it again it will crash. I have added listeners to the UIApplicationDidBecomeActiveNotification and UIApplicationWillResignActiveNotification to shut down the data transmission when the app goes to the background, and turn it back on when it comes back, and now it doesn't crash all the time, but irregularly. When it crashes, it crashes even before it sends the UIApplicationDidBecomeActiveNotification.

Here's the Device Log from the error:

Exception Type:  EXC_BAD_ACCESS (SIGBUS)  
Exception Codes: KERN_PROTECTION_FAILURE at 0x00000004  
Crashed Thread:  3

Thread 0:  
0   libSystem.B.dylib               0x000016a8 semaphore_wait_trap + 8  
1   libSystem.B.dylib               0x0002ed9e semaphore_wait + 2  
2   libSystem.B.dylib               0x000d7ce4 _dispatch_semaphore_wait_slow + 296  
3   libSystem.B.dylib               0x000d64a0 _dispatch_barrier_sync_f_slow + 128  
4   CoreFoundation                  0x0001511e CFMachPortCreateRunLoopSource + 118  
5   QuartzCore                      0x00005466 CA::Context::client_port() + 78  
6   QuartzCore                      0x00005310 CA::Context::connect_remote() + 244  
7   QuartzCore                      0x00004fda -[CAContextImpl initRemoteWithOptions:] + 26  
8   QuartzCore                      0x00004f9c +[CAContext remoteContextWithOptions:] + 28  
9   UIKit                           0x0000dae0 -[UIWindow _createContext] + 532  
10  UIKit                           0x0015957e _UIWindowUpdateVisibleContextOrder + 98  
11  UIKit                           0x0015962a +[UIWindow _prepareWindowsForAppResume] + 10  
12  UIKit                           0x00152824 -[UIApplication _handleApplicationResumeEvent:] + 68  
13  UIKit                           0x0005da80 -[UIApplication handleEvent:withNewEvent:] + 3772  
14  UIKit                           0x0005ca56 -[UIApplication sendEvent:] + 38  
15  UIKit                           0x0005c500 _UIApplicationHandleEvent + 5016  
16  GraphicsServices                0x00004140 PurpleEventCallback + 660  
17  GraphicsServices                0x000041ca PurpleEventSignalCallback + 10  
18  CoreFoundation                  0x00071a86 __CFRUNLOOP_IS_CALLING_OUT_TO_A_SOURCE0_PERFORM_FUNCTION_ + 6  
19  CoreFoundation                  0x000736ac _CFRunLoopDoSources0 + 188  
20  CoreFoundation                  0x00074504 _CFRunLoopRun + 224  
21  CoreFoundation                  0x0001d8e4 CFRunLoopRunSpecific + 224  
22  CoreFoundation                  0x0001d7ec CFRunLoopRunInMode + 52  
23  GraphicsServices                0x000036e8 GSEventRunModal + 108  
24  GraphicsServices                0x00003794 GSEventRun + 56  
25  UIKit                           0x000062a0 -[UIApplication _run] + 396  
26  UIKit                           0x00004e10 UIApplicationMain + 664  
27  TestRaspLayer                   0x00003034 main (main.m:14)  
28  TestRaspLayer                   0x00002fe8 start + 32  

Thread 1:  
0   libSystem.B.dylib               0x0002dc4c kevent + 24  
1   libSystem.B.dylib               0x000d6e44 _dispatch_mgr_invoke + 88  
2   libSystem.B.dylib               0x000d6894 _dispatch_queue_invoke + 96  
3   libSystem.B.dylib               0x000d6a34 _dispatch_worker_thread2 + 120  
4   libSystem.B.dylib               0x0007ad82 _pthread_wqthread + 258  
5   libSystem.B.dylib               0x00073fcc start_wqthread + 0  

Thread 2:  
0   libSystem.B.dylib               0x00001658 mach_msg_trap + 20  
1   libSystem.B.dylib               0x00003724 mach_msg + 44  
2   CoreFoundation                  0x000722c8 _CFRunLoopServiceMachPort + 88  
3   CoreFoundation                  0x00074582 _CFRunLoopRun + 350  
4   CoreFoundation                  0x0001d8e4 CFRunLoopRunSpecific + 224  
5   CoreFoundation                  0x0001d7ec CFRunLoopRunInMode + 52  
6   WebCore                         0x00003056 RunWebThread(void*) + 362  
7   libSystem.B.dylib               0x0007a986 _pthread_start + 242  
8   libSystem.B.dylib               0x000700e4 thread_start + 0  

Thread 3 Crashed:  
0   libicucore.A.dylib              0x00001bd0 uhash_get + 24  
1   libicucore.A.dylib              0x00055b10 ures_hasNext + 720  
2   libicucore.A.dylib              0x00055d08 ures_hasNext + 1224  
3   libicucore.A.dylib              0x00055e64 ures_hasNext + 1572  
4   libicucore.A.dylib              0x00003a28 ures_openDirect + 80  
5   libicucore.A.dylib              0x000b4dee icu::TimeZone::getGMT() + 442  
6   libicucore.A.dylib              0x0000861a icu::TimeZone::createSystemTimeZone(icu::UnicodeString const&) + 22  
7   libicucore.A.dylib              0x0000cf90 icu::TimeZone::createTimeZone(icu::UnicodeString const&) + 4  
8   libicucore.A.dylib              0x000bc6f8 ucal_setAttribute + 96  
9   libicucore.A.dylib              0x0000d650 ucal_open + 32  
10  CoreFoundation                  0x000a7968 _CFCalendarCreateUCalendar + 160  
11  CoreFoundation                  0x00029cde CFCalendarCreateWithIdentifier + 362  
12  CoreFoundation                  0x0008d128 _CFLogCString + 472  
13  CoreFoundation                  0x00029b52 _CFLogvEx + 166  
14  CoreFoundation                  0x0003d63e CFLog + 26  
15  CoreFoundation                  0x00015184 _CFAutoreleasePoolPop + 52  
16  Foundation                      0x0000422a NSPopAutoreleasePool + 2  
17  Foundation                      0x0000de16 -[_NSOperationInternal start] + 910  
18  Foundation                      0x0000da78 -[NSOperation start] + 16  
19  Foundation                      0x0007e5b4 _startOperations_block_invoke_2 + 40  
20  libSystem.B.dylib               0x000d5c54 _dispatch_call_block_and_release + 12  
21  libSystem.B.dylib               0x000d6a34 _dispatch_worker_thread2 + 120  
22  libSystem.B.dylib               0x0007ad82 _pthread_wqthread + 258  
23  libSystem.B.dylib               0x00073fcc start_wqthread + 0  

Thread 4:  
0   libSystem.B.dylib               0x00001f8e memcpy + 718  
1   TestRaspLayer                   0x00007718 _inline_memcpy_chk (_string.h:58)  
2   TestRaspLayer                   0x00007678 -[SERIALPort processRxBuffer]   (SERIALPort.m:243)   
3   CoreFoundation                  0x0009f06c _invoking_ + 60  
4   CoreFoundation                  0x00032706 -[NSInvocation invoke] + 102  
5   Foundation                      0x0007eb50 -[NSInvocationOperation main] + 72  
6   Foundation                      0x0000dd14 -[_NSOperationInternal start] + 652  
7   Foundation                      0x0000da78 -[NSOperation start] + 16  
8   Foundation                      0x0007e5b4 _startOperations_block_invoke_2 + 40  
9   libSystem.B.dylib               0x000d5c54 _dispatch_call_block_and_release + 12  
10  libSystem.B.dylib               0x000d6a34 _dispatch_worker_thread2 + 120  
11  libSystem.B.dylib               0x0007ad82 _pthread_wqthread + 258  
12  libSystem.B.dylib               0x00073fcc start_wqthread + 0  

Thread 3 crashed with ARM Thread State:  
    r0: 0x00000000    r1: 0x0000007f      r2: 0x00000077      r3: 0x80000001  
    r4: 0x0013fc50    r5: 0x0038003c      r6: 0x0038012c      r7: 0x0037ff70  
    r8: 0x00000000    r9: 0xffffffff     r10: 0x0013fcb4     r11: 0x00000000  
    ip: 0x00000077    sp: 0x0037ff68      lr: 0x339fbe09      pc: 0x339b8bd0  
  cpsr: 0x50000030  

Will your app crash when the device is disconnected or idle, or does the crash only occur when the device is sending data?