Faster IO on ARM Cortex-M3 (Sam3x8e) than PIO_SODR and PIO_CODR?

Question

I have a very simple C program that toggles a pin on a Cortex-M3 board (SAM3X8E) with a 84MHz clock speed. I built this using Atmel Studio 6.1.

#include <asf.h>

int main (void)
{
    board_init();
    PIOB->PIO_PER = PIO_PB27;  //Enable PIO
    PIOB->PIO_OER = PIO_PB27;  //Set to OUTPUT
    PIOB->PIO_PUDR = PIO_PB27; //Disable the pull up resistor

    while(1) {
        PIOB->PIO_SODR = PIO_PB27; //Enable output
        PIOB->PIO_CODR = PIO_PB27; //Disable output
    }
}

Now when I load this onto the board, and hook up an oscilloscope up to the pin, I see the signal pulsating at about 700kHz. I would have thought that the speed of the cycle would have been much quicker with this minimalistic code.

Is it possible to get a GPIO frequency higher than a MHz using this setup? Is it my build environment that is not producing optimal code? Is it possible to get faster GPIO using another technique?

Answer 1

I don't know the SAM3X family but the Atmel Studio IDE and the UC3 Family from Atmel.

Try adding the Clock Module to your project, configure it via the config_clock.h file, then add a sysclk_init() call to your main and it should be going a lot faster.

The SAM3X by default clocks at 4MHz if I'm not mistaken.

Answer 2

Try using direct port access and not like you did via the PIO struct. Also enable the OWER flag for your PIO pins and write your state directly to REG_PIOX_ODSR which saves you some extra instructions and improves the overall speed of your code. Your code could look similar to this:

int main (void)
{
    board_init();
    sysclk_init();
    PIOB->PIO_PER = PIO_PB27;  //Enable PIO
    PIOB->PIO_OER = PIO_PB27;  //Set to OUTPUT
    PIOB->PIO_OWER = PIO_PB27; //Enable direct bit set

    while(1) {
        REG_PIOB_ODSR = (~(REG_PIOB_ODSR & PIO_PB27)) & PIO_PB27; //Enable output
    }
}

If you want to go even faster try to use timer interrupts which can handle speeds to half the clock speed of the cpu in the case of the sam3x up to 42MHz

Answer 3

Remember the while loop itself would have some impact too. A good way to minimize the impact of this is to duplicate the meat of the action (actually setting the pins) a few hundred times.