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AHB for GPIO

Hi,

I want to get high speed GPIO. I set the GPIO port to use APB, while the system clock run at 50MHz, and then the GPIO  toggle up to 12.5MHz.

Now I set the GPIO port to use AHP for higher speed, but GPIO toggle speed is only 12.5MHz too, What's Wrong with the CODE?

    //
    // Set the system clock to run at 50MHz from the PLL
    //
    SysCtlClockSet(SYSCTL_SYSDIV_4 | SYSCTL_USE_PLL | SYSCTL_OSC_MAIN |
                       SYSCTL_XTAL_16MHZ);

    //
    // Enable the GPIO port that is used for the on-board LED.
    //
    SysCtlPeripheralEnable(SYSCTL_PERIPH_GPIOF);
 SysCtlGPIOAHBEnable(SYSCTL_PERIPH_GPIOF);

    //
    // Enable the GPIO pin for the LED (PF3).  Set the direction as output, and
    // enable the GPIO pin for digital function.
    //
    GPIOPinTypeGPIOOutput(GPIO_PORTF_AHB_BASE, GPIO_PIN_3);

    //
    // Loop forever.
    //
    while(1)
    {
        //
        // Turn on the LED.
        //
     HWREG(GPIO_PORTF_AHB_BASE + GPIO_O_DATA + (GPIO_PIN_3 << 2)) = GPIO_PIN_3;


        //
        // Turn off the LED.
        //                      
     HWREG(GPIO_PORTF_AHB_BASE + GPIO_O_DATA + (GPIO_PIN_3 << 2)) = 0; 
    }