A13-OLinuXino playing with GPIOs

Today I will show you how to access A13-OLinuXino GPIOs.

First make sure you have installed the GPIO drivers by

# lsmod

you will see the list of the installed kernel modules and if “sun4i_gpio” is there everything is OK

If you got A13-OLinuXino-SD with the pre-installed Debian everything is already setup for you, if not you can follow the instructions of this post and make one SD card yourself: https://olimex.wordpress.com/2012/10/12/building-bootable-sd-card-with-debian-linux-image-for-a13-olinuxino/ do not forget when compile the kernel to enable SUN4I_GPIO_UGLY = y inside .config , why UGLY? you will understand soon 😉

Allwinner do not care too much for the GPIOs as they primary support Android and their major market is the tablet manufacturers. This SUN4I-GPIO is made by Tom Cubie who work at Allwinner and made available to community, this driver though is not so flexible as the sysfs and you can’t switch between input and output easily.

With this driver you permanently assign one port to be input or output in the script.bin and when this gpio kernel module is loading it reads from script.bin the assignments and make them permanently, so if you want to change some GPIO to input or output you have to unload the driver make change in the script.bin and load it again and this is ugly 🙂

Anyway with the card we ship we made these gpios defined:

pe4, pe5, pe6,  pe7: inputs

pe8,  pe9,  pg10,  pg11,  pg9, pb10,  pb3,  pb4,  pe10,  pe11:  outputs

pg9 is GPIO output with the green status LED connected to it

to switch LED on you can do:

# echo 1 > /sys/devices/virtual/misc/sun4i-gpio/pin/pg9

and to switch LED off you can do:

# echo 0 > /sys/devices/virtual/misc/sun4i-gpio/pin/pg9

Now let’s see how fast we can toggle LED from shell script:

# vi gpio

and enter this code:

while [ true]

do

echo 1 > /sys/devices/virtual/misc/sun4i-gpio/pin/pg9

echo 0 > /sys/devices/virtual/misc/sun4i-gpio/pin/pg9

done

# chmod +x gpio

#./gpio

with the oscilloscope is seen square wave with 220 uS period i.e. 45 454 Hz! this is quite more fast than the 300Hz we achieved on iMX233 GPIOs from shell https://olimex.wordpress.com/2012/09/07/imx233-olinuxino-working-with-gpios/

now let’s try same in C:

# vi gpioc.c

#include <stdio.h>
#include <fcntl.h>
#include <string.h>

int main() {

char s_0[] = “0”;
char s_1[] = “1”;
int fd;

//toggle pg9 LED gpio

if((fd=open(“/sys/devices/virtual/misc/sun4i-gpio/pin/pg9”,O_RDWR))<0) return(1);
else {

while(1) {

if(write(fd,s_1,strlen(s_1))<0) return(1);

if(write(fd,s_0,strlen(s_0))<0) return(1);
};

if(close(fd)<0) return(1);

}
}

# cc gpioc.c -o gpioc

# chmod +x gpioc

# ./gpioc

the oscilloscope show 4.2 uS period square wave i.e. about 238 095Hz not so fast than iMX233 140kHz. But obviously if we chase speed should not use file system.

Now next step is to make access via /dev/mem as mmgpio with iMX233 https://olimex.wordpress.com/2012/09/11/imx233-olinuxino-gpios-faster-and-faster/

Using directly /dev/mem iMX233 achieved up to 5.7 Mhz GPIO toggle, I do wonder how fast we can go with A13, but this will be done in my next blog about it.