GPIO and sysfs Post: 303030530

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Top Forums UNIX for Advanced & Expert Users GPIO and sysfs Post 303030530 by Circuits on Tuesday 12th of February 2019 11:37:23 AM

02-12-2019

Registered User

GPIO and sysfs

I was recently working on a project where some gpio pins were being toggled from within the user space:

Code:

const char *const amplifierGPIO = "/sys/class/gpio/gpio107/value";

    void amplifierUnmute()
    {
      std::ofstream amp(amplifierGPIO);
      if (amp.is_open())
      {
        amp << "1";
        amp.close();
      }
    }

Now I am wondering how this works. For instance, is there some file where every pin which has been instantiated as a gpio pin is listed s/t the a user can access it like in the example above. Or does one have to go beyond instantiating the pin as gpio? For instance, lets say I build a pin as a gpio:

Code:

MX51_PIN_EIM_A24 = _MXC_BUILD_GPIO_PIN_MX51(1, 18, 1, 0xBC, 0x450),

Now how to access that pin inside the user space... it doesn't seem obvious to me how to go from instantiating a pin as a gpio pin to accessing it from the user space. It also seems foolish to believe that it is hard. I am going to start with this kernels Documentation/gpio.txt

Last edited by RudiC; 02-12-2019 at 01:07 PM..

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LEARN ABOUT FREEBSD

gpiobus

GPIO(4) 						   BSD Kernel Interfaces Manual 						   GPIO(4)

NAME

     gpiobus -- GPIO bus system

SYNOPSIS

     To compile these devices into your kernel and use the device hints, place the following lines in your kernel configuration file:

	   device gpio
	   device gpioc
	   device gpioiic
	   device gpioled

     Additional device entries for the ARM architecture include:

	   device a10_gpio
	   device bcm_gpio
	   device imx51_gpio
	   device lpcgpio
	   device mv_gpio
	   device ti_gpio
	   device gpio_avila
	   device gpio_cambria
	   device zy7_gpio
	   device pxagpio

     Additional device entries for the MIPS architecture include:

	   device ar71xxx_gpio
	   device octeon_gpio
	   device rt305_gpio

     Additional device entries for the POWERPC architecture include:

	   device wiigpio
	   device macgpio

DESCRIPTION

     The gpiobus system provides a simple interface to the GPIO pins that are usually available on embedded architectures and can provide bit
     banging style devices to the system.

     The acronym GPIO means ``General-Purpose Input/Output.''

     The BUS physically consists of multiple pins that can be configured for input/output, IRQ delivery, SDA/SCL iicbus use, etc.

     On some embedded architectures (like MIPS), discovery of the bus and configuration of the pins is done via device.hints(5) in the platform's
     kernel config(5) file.

     On some others (like ARM), where FDT(4) is used to describe the device tree, the bus discovery is done via the DTS passed to the kernel,
     being either statically compiled in, or by a variety of ways where the boot loader (or Open Firmware enabled system) passes the DTS blob to
     the kernel at boot.

     The following device.hints(5) are only provided by the ar71xx_gpio driver:

     hint.gpio.%d.pinmask   This is a bitmask of pins on the GPIO board that we would like to expose for use to the host operating system.  To
			    expose pin 0, 4 and 7, use the bitmask of 10010001 converted to the hexadecimal value 0x0091.

     hint.gpio.%d.pinon     This is a bitmask of pins on the GPIO board that will be set to ON at host start.  To set pin 2, 5 and 13 to be set ON
			    at boot, use the bitmask of 10000000010010 converted to the hexadecimal value 0x2012.

     hint.gpio.function_set

     hint.gpio.function_clear
			    These are bitmasks of pins that will remap a pin to handle a specific function (USB, UART TX/RX, etc) in the Atheros
			    function registers.  This is mainly used to set/clear functions that we need when they are set up or not set up by
			    uBoot.

     Simply put, each pin of the GPIO interface is connected to an input/output of some device in a system.

SEE ALSO

     gpioiic(4), gpioled(4), iicbus(4), gpioctl(8)

HISTORY

     The gpiobus manual page first appeared in FreeBSD 10.0.

AUTHORS

     This manual page was written by Sean Bruno <sbruno@FreeBSD.org>.

BSD
								 November 5, 2013							       BSD