Technical problem concerning the microcontroller of the RCX brick

Hi

I am a member of a project group at Aalborg University, Denmark, and the goal
for our project on this semester is to code an operating system for the RCX
brick from Lego.

During a whole week, I have been strugling with the 8 bit timers of the H8/3292
(which is a member of the H8/3297 series). I have in my hand section 9 of the
hardware manual from Renesa (entitled "8-Bit Timers"), which should be enough to
make the timers work. I had no problems making the 16 bit timer work using
section 8 of the same manual. If someone is interested, my collections of links
can found here: http://martintoft.dk/rcx

Let us jump right into the problem: I follow section 9 of the hardware manual to
the letter, but I cannot make the 8 bit timerswork completely. I am able to set
up one of channels, such that the overflow interrupt is requested on counter
overflow, but when I try enabling either CMIA or CMIB (Compare-Match Interrupt
Request A/B, done by setting bit 6/7 of the TCR, Timer Control Register) the
brick fails to continue. I have tried executing my program in BrickEmu and on a
real RCX brick - I get no error message from BrickEmu, and the RCX brick just
ceases operation (it does not power off, just stops further execution). I know
it is not a problem with my interrupt handler, because I used the same handler
when I successfully enabled the "on-counter-overflow-interrupt-request".

I use these typedefs:

    typedef unsigned char byte;
    typedef unsigned int word;

I use these defines:

    // Timer Control Register
    #define C0_TCR   *((volatile byte*)0xffc8)
    // Timer Control/Status Register
    #define C0_TCSR  *((volatile byte*)0xffc9)
    // Time Constant Register A
    #define C0_TCORA *((volatile byte*)0xffca)
    // Serial/Timer Control Register
    #define C01_STCR *((volatile byte*)0xffc3)
    // Compare-Match A Interrupt Vector
    #define C0_CMIA  *((volatile word*)0xfda8)

Here is my main function, which sets up channel 0 of the timer and "installs"
the interrupt handler:

    void _main(void)
    {
      // Make sure channel 0 of the timer is turned off
      C0_TCR = 0;
      // Set time constant register A to 100
      // (this value is not important)
      C0_TCORA = 100;
      // Clear counter on compare-match A
      C0_TCR &= ~0x08;   // Clear bit 3
      C0_TCR |= 0x10;    // Set bit 4
      // Choose slowest internal clock source (Oe_p/1024)
      C0_TCR |= 0x03;    // Set bit 0+1
      C0_TCR &= ~0x04;   // Clear bit 2
      C01_STCR &= ~0x01; // Clear bit 0
      // Make sure compare-match flag A is cleared
      C0_TCSR &= ~0x40;  // Clear bit 6
      // Set interrupt vector
      C0_CMIA = (word)&handler;
      // Enable compare-match interrupt request A
      C0_TCR |= 0x40;    // Set bit 6
      // Busy wait forever
      while (1);
    }

This is my interrupt handler:

    void handler(void)
    {
      asm("push r0 \n\t" // Save registers
          "push r1 \n\t" // CCR and r6 are saved by ROM dispatcher
          "push r2 \n\t"
          "push r3 \n\t"
          "push r4 \n\t"
          "push r5 \n\t"
          "mov.w #0xf000, r0 \n\t" // Start motor A
          "bset #7, @r0 \n\t"
          "bclr #6, @r0 \n\t"
          "bclr #6, @0xc3:8 \n\t"  // Clear CMFA (bit 6 of TCSR)
          "pop r5 \n\t" // Restore registers
          "pop r4 \n\t"
          "pop r3 \n\t"
          "pop r2 \n\t"
          "pop r1 \n\t"
          "pop r0"
          );
    }

I expect motor A to start, when the interrupt is requested by the timer, but
nothing happens.

Any help would be appreciated.

Best regards,
Martin Toft