Basics for accessing peripherals from code

July 28, 2011, 10:31:44 AM

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AwareWolf

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Basics for accessing peripherals from code

I need help with the very basics of using this Glomation board. I am fairly new to developing apps that run in the context of Linux on an "embedded" controller, so I'm not sure if I'm even asking the right questions, but:

To access (read/write) the GPIO pins, e.g. to toggle an LED, would I be writing to a device driver, or accessing the processor registers directly, as if there were no OS?

Is there sample code somewhere to give me a clue how to do this? I know on the Glomation support site there are code samples, but they are for a different board/processor. For what it's worth, those examples seem to be hitting the processor registers directly, or am I mistaken about that?

July 28, 2011, 05:34:31 PM

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Re: Basics for accessing peripherals from code

The Linux kernel 2.6.30 and up includes GPIO drivers. Writing hardware registers from user space application is not safe. Please upgrade to newer kernel and use the GPIO driver as shown here, https://www.ridgerun.com/developer/wiki/index.php/How_to_use_GPIO_signals

The GPIO lines may require allocation in the board init code as following to ensure proper operation,

   at91_set_GPIO_periph(AT91_PIN_PB0, 1);
   at91_set_GPIO_periph(AT91_PIN_PB1, 1);
   at91_set_GPIO_periph(AT91_PIN_PB2, 1);
   at91_set_GPIO_periph(AT91_PIN_PB3, 1);
   at91_set_GPIO_periph(AT91_PIN_PB8, 1);
   at91_set_GPIO_periph(AT91_PIN_PB9, 1);
   at91_set_GPIO_periph(AT91_PIN_PC4, 1);
   at91_set_GPIO_periph(AT91_PIN_PC5, 1);
   at91_set_GPIO_periph(AT91_PIN_PC6, 1);
   at91_set_GPIO_periph(AT91_PIN_PC7, 1);

in file arch/arm/mach-at91/board-sam9260ek.c

August 01, 2011, 07:10:19 AM

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AwareWolf

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Re: Basics for accessing peripherals from code

OK, thanks.

How do I access the GPIO pins if I don't upgrade to 2.6.30? The board came with 2.6.27.4. Does this version not have GPIO drivers?

August 01, 2011, 08:28:45 AM

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Re: Basics for accessing peripherals from code

Direct GPIO control register manipulation can be used to control the GPIO from user space if GPIO driver is not available. But it is not recommended.

It's better to upgrade the kerenl to 2.6.30 and higher with built-in GPIO support.