How can I turn an GPIO to its alternate function?

Question

I want to use the altenate function of GPIO 4 namely the GPCLK0 function. How can one set a GPIO to its alternate function? Many alternate functions are summarized here: elinux-wiki

The best would be if it is possible with the wiringPi Library. But i do not mind to use arbitrary C-code if someone tells me how to do it. After entering an alternative function, i think it is needed to configure it?

Answer 1

You can use the following C Macro to change a given GPIO to an alternate function as specified by this table: http://elinux.org/RPi_BCM2835_GPIOs

volatile unsigned *gpio;

#define SET_GPIO_ALT(g,a) *(gpio+(((g)/10))) |= (((a)<=3?(a)+4:(a)==4?3:2)<<(((g)%10)*3))

Example (from http://elinux.org/RPi_Low-level_peripherals#GPIO_hardware_hacking)

The following C Macros would change GPIO g to its ALT0 function:

INP_GPIO(g);
SET_GPIO_ALT(g,0);

Always use INP_GPIO(x) before using SET_GPIO_ALT(x,y)

#define INP_GPIO(g) *(gpio+((g)/10)) &= ~(7<<(((g)%10)*3))

Answer 2

Using the pigpio library to setup GPIO 4 as ALT0 function i.e. GPCLK0:

$ pigs m 4 0

Ref.: changing mode of GPIO

Answer 3

here are some other threads (on www.raspberrypi.org) that are dealing with the GPCLK0 function.

• http://www.raspberrypi.org/phpBB3/viewtopic.php?f=44&t=11127
• http://www.raspberrypi.org/phpBB3/viewtopic.php?f=72&t=17975

the second link has an exact C example on how to set the GPCLK0 alternate function.

/* Setup GPIO 4 as ALT0 function i.e. GPCLK0 */
mov r0, #4
mov r1, #4 /* ALT0 */

Answer 4

This tutorial shows different ways you can configure the GPIO pins, such as bash, python, and C using wiringPi. It's as easy as an Arduino.

http://log.liminastudio.com/writing/tutorials/tutorial-how-to-use-your-raspberry-pi-like-an-arduino

You'll need to look into the WiringPi library a little more to know exactly what to set for the specific "alternate function" you want, eg SPI, PWM, I²C etc, since you didn't specify.

Answer 5

Use this code to set the alt function

static void bcm2708_set_gpio_alt(int pin, int alt)
{
    /*
     * This is the common way to handle the GPIO pins for
     * the Raspberry Pi.
     * TODO This is a hack. Use pinmux / pinctrl.
     */
#define INP_GPIO(g) *(gpio+((g)/10)) &= ~(7<<(((g)%10)*3))
#define SET_GPIO_ALT(g,a) *(gpio+(((g)/10))) |= (((a)<=3?(a)+4:(a)==4?3:2)        <<(((g)%10)*3))
    unsigned int *gpio;
    gpio = ioremap(GPIO_BASE, SZ_16K);
    INP_GPIO(pin);
    SET_GPIO_ALT(pin, alt);
    iounmap(gpio);
#undef INP_GPIO
#undef SET_GPIO_ALT
}

Answer 6

I found the pigpio library can do this for several languages, including python (which is where I needed it).

#!/usr/bin/env python3
import pigpio

RTS=17
pi = pigpio.pi()
if pi.connected:
    # Configure RTS0 for alt function 3
    pi.set_mode(RTS, pigpio.ALT3)

Answer 7

You can use raspi-gpio in terminal.
raspi-gpio set 4 a4 - Change GPIO4 to alternative function 4.

Another helpful commands:
raspi-gpio help
raspi-gpio get - Prints state of all GPIOs one per line.
raspi-gpio get 20 - Prints state of 20.
raspi-gpio set 20 pu - Enable GPIO20 ~50k in-pad pull up.
raspi-gpio set 20 pd - Enable GPIO20 ~50k in-pad pull down.