Debugger Bug ??? (FastGPIO on NXP)

Having had problems in the past trying to debug FastGPIO, I recently tried again.

I've just spent a considerable amount of time isolating what I believe to be another interesting (and frustrating) bug in the uVision debugger. I hope this may save someone else some time...

I'm using MDK v3.20 targetting an NXP LPC2142. When I try to initialise FastGPIO, the debugger shows some very odd behaviour. Basically, the FIO0DIR register appears to change its contents following execution of an instruction that does not reference it. I say appears because I am convinced this is a debugger bug rather than a hardware problem.

A fragment of my c source looks like this :-

  // Enable FastGPIO on both I/O ports
  SCS |= (GPIO0M | GPIO1M);

  // Set masks
  FIO0MASK = ~CPLD_P0_OUTPUTS;
  FIO1MASK = DATA_BUS_MASK;

  // Set Port 0 designated bit directions
  FIO0DIR = CPLD_P0_OUTPUTS;

With optimisation set to Level 3, this results in the following generated code :-

    97:   SCS |= (GPIO0M | GPIO1M);
    98:
    99:   // Set masks
0x00004FF4  4935      LDR       R1,[PC,#0x00D4]
0x00004FF6  6A08      LDR       R0,[R1,#0x20]
0x00004FF8  2203      MOV       R2,#0x03
0x00004FFA  4310      ORR       R0,R2
0x00004FFC  6208      STR       R0,[R1,#0x20]
   100:   FIO0MASK = ~CPLD_P0_OUTPUTS;
0x00004FFE  4934      LDR       R1,[PC,#0x00D0]
0x00005000  4834      LDR       R0,[PC,#0x00D0]
0x00005002  6101      STR       R1,[R0,#0x10]
   101:   FIO1MASK = DATA_BUS_MASK;
   102:
   103:   // Set Port 0 designated bit directions
0x00005004  4934      LDR       R1,[PC,#0x00D0]
0x00005006  6301      STR       R1,[R0,#0x30]
   104:   FIO0DIR = CPLD_P0_OUTPUTS;
   105:
0x00005008  4931      LDR       R1,[PC,#0x00C4]
0x0000500A  43C9      MVN       R1,R1
0x0000500C  6001      STR       R1,[R0,#0x00]

Following single-stepping of the line at 0x00005000, which loads into R0 the base address for the Port 0 FastGPIO registers (0x3FFFC000), the FIO0DIR register (as shown in the peripheral dialog) changes to this same value (0x3FFFC000) !

When I subsequently single-step the line at 0x0000500C which should change the value in the FIO0DIR register, it is not shown to change. I can change it directly by editing it in the peripheral dialog.

This does not happen with optimisation set to Level 0.

Needless to say, when you can't depend on the debugger, debugging is made somewhat more difficult :-)

Can anyone else (Keil ?) confirm, refute or explain this behaviour ?

Regards,

David