A System Tick problem in the bitband example of the EK-TM4C1294XL

Hello Henry,

The problem lies in the initialization of the SysTick reload value. As it states in the driver library user guide and in the header of the flash version, the max value that can be programmed/passed is 16, 777, 216 since the SysTick counter is a 24-bit counter. The example is passing the value returned by SysCtlClockFreqSet which will be 120000000. This results in the incorrect value being programmed into the reload register and thus an incorrect time for the roll over of the counter.

Note that the value of 1000 used in the Delay function for the compare of the return value of ROM_SysTickValueGet in the while loop is arbitrary since the main goal of this code is to delay for one wrap around cycle without using interrupts. Certainly this could be improved by enabling the SysTick interrupt but for the simplicity of this example it did not make sense to do so.

To resolve the issue, I changed the reload register init to divide the system clock by 10 changing the reload value to 12,000,000 which is now within the acceptable range. This, however, changes the timer from a seconds timer to a 0.1 seconds timer so all values passed would need to be multiplied by 10 to get 1 second.

Below is the resulting main function after the change.

//*****************************************************************************
//
// This example demonstrates the use of bit-banding to set individual bits
// within a word of SRAM.
//
//*****************************************************************************
int
main(void)
{
    uint32_t ui32Errors, ui32Idx;

    //
    // Set the clocking to run directly from the crystal at 120MHz.
    //
    g_ui32SysClock = SysCtlClockFreqSet((SYSCTL_XTAL_25MHZ |
                                         SYSCTL_OSC_MAIN | SYSCTL_USE_PLL |
                                         SYSCTL_CFG_VCO_480), 120000000);

    //
    // Initialize the UART interface.
    //
    ConfigureUART();
    UARTprintf("\033[2J\033[H");
    UARTprintf("Bit banding...\n");

    //
    // Set up and enable the SysTick timer.  It will be used as a reference
    // for delay loops.  The SysTick timer period will be set up for one
    // second.
    //
    ROM_SysTickPeriodSet((g_ui32SysClock/10)); // Divide the system clock by 10 to bring into acceptable range.
    ROM_SysTickEnable();

    //
    // Set the value and error count to zero.
    //
    g_ui32Value = 0;
    ui32Errors = 0;

    //
    // Print the initial value to the UART.
    //
    UARTprintf("\r%08x", g_ui32Value);

    //
    // Delay for 0.1 second.
    //
    Delay(1);

    //
    // Set the value to 0xdecafbad using bit band accesses to each individual
    // bit.
    //
    for(ui32Idx = 0; ui32Idx < 32; ui32Idx++)
    {
        //
        // Set this bit.
        //
        HWREGBITW(&g_ui32Value, 31 - ui32Idx) = (0xdecafbad >>
                (31 - ui32Idx)) & 1;

        //
        // Print the current value to the UART.
        //
        UARTprintf("\r%08x", g_ui32Value);

        //
        // Delay for 1 second.
        //
        Delay(10);
    }

    //
    // Make sure that the value is 0xdecafbad.
    //
    if(g_ui32Value != 0xdecafbad)
    {
        ui32Errors++;
    }

    //
    // Make sure that the individual bits read back correctly.
    //
    for(ui32Idx = 0; ui32Idx < 32; ui32Idx++)
    {
        if(HWREGBITW(&g_ui32Value, ui32Idx) != ((0xdecafbad >> ui32Idx) & 1))
        {
            ui32Errors++;
        }
    }

    //
    // Print out the result.
    //
    if(ui32Errors)
    {
        UARTprintf("\nErrors!\n");
    }
    else
    {
        UARTprintf("\nSuccess!\n");
    }

    //
    // Loop forever.
    //
    while(1)
    {
    }
}