Lpc2000

The engineering dept. at the particular company that I work for is
looking into using the Philips LPC as the foundation for our future
devices. We have been playing with an Olimex LPC-H212X evaluation
board. I've written various small programs to test the features we
need from the processor.

We are using a 14.7456 MHz crystal and have CCLK and PCLK set to
58.9824 MHz.

The errata listed for the LPC2129 helped to get the external
interrupts going. The result was:

{
    /* Do PLL, UART0 and interrupt vector 0 initialization... */
    PINSEL1 |= 1;
    tmp = VPBDIV;
    tmp = VPBDIV;
    VPBDIV = 0;
    EXTMODE = 1;
    VPBDIV = 1;
    VPBDIV = 0;
    EXTPOLAR = 0;
    VPBDIV = 0;
    VPBDIV = tmp;
    /* Remainder of program... */
}

This seemed to work fine. However, when I tried using the external
interrupts with a more complicated program, I began to get odd
behavior. After playing around with it for a while, I discovered that
the following would work.

{
    volatile int vol;
    int i;
    /* Do PLL, UART0 and interrupt vector 0 initialization. */
    PINSEL1 |= 1;
    tmp = VPBDIV;
    tmp = VPBDIV;

    for(i = 0; i < 3; i++){
       vol = 0;
    }

    VPBDIV = 0;
    EXTMODE = 1;
    VPBDIV = 1;
    VPBDIV = 0;
    EXTPOLAR = 0;
    VPBDIV = 0;
    VPBDIV = tmp;
    /* Remainder of program. */
}

If I bumped the number of iterations down to 2, the program would
exhibit odd behavior.

Could anyone explain to me why this is?

Thanks,

Josh