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PODriving Beaglebone GPIO through /dev/mem
 

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  1. PODriving Beaglebone GPIO through /dev/mem
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    <p>I'm trying to write a C program for blinking a LED on the Beaglebone. I know I can use the sysfs way...but I'd like to see if it is possible to get the same result mapping the physical address space with /dev/mem.</p> <p>I have a header file, beaglebone_gpio.h wit the following contents:</p> <pre><code>#ifndef _BEAGLEBONE_GPIO_H_ #define _BEAGLEBONE_GPIO_H_ #define GPIO1_START_ADDR 0x4804C000 #define GPIO1_END_ADDR 0x4804DFFF #define GPIO1_SIZE (GPIO1_END_ADDR - GPIO1_START_ADDR) #define GPIO_OE 0x134 #define GPIO_SETDATAOUT 0x194 #define GPIO_CLEARDATAOUT 0x190 #define USR0_LED (1&lt;&lt;21) #define USR1_LED (1&lt;&lt;22) #define USR2_LED (1&lt;&lt;23) #define USR3_LED (1&lt;&lt;24) #endif </code></pre> <p>and then I have my C program, gpiotest.c</p> <pre><code>#include &lt;stdio.h&gt; #include &lt;stdlib.h&gt; #include &lt;sys/mman.h&gt; #include &lt;sys/stat.h&gt; #include &lt;fcntl.h&gt; #include "beaglebone_gpio.h" int main(int argc, char *argv[]) { volatile void *gpio_addr = NULL; volatile unsigned int *gpio_oe_addr = NULL; volatile unsigned int *gpio_setdataout_addr = NULL; volatile unsigned int *gpio_cleardataout_addr = NULL; unsigned int reg; int fd = open("/dev/mem", O_RDWR); printf("Mapping %X - %X (size: %X)\n", GPIO1_START_ADDR, GPIO1_END_ADDR, GPIO1_SIZE); gpio_addr = mmap(0, GPIO1_SIZE, PROT_READ | PROT_WRITE, MAP_SHARED, fd, GPIO1_START_ADDR); gpio_oe_addr = gpio_addr + GPIO_OE; gpio_setdataout_addr = gpio_addr + GPIO_SETDATAOUT; gpio_cleardataout_addr = gpio_addr + GPIO_CLEARDATAOUT; if(gpio_addr == MAP_FAILED) { printf("Unable to map GPIO\n"); exit(1); } printf("GPIO mapped to %p\n", gpio_addr); printf("GPIO OE mapped to %p\n", gpio_oe_addr); printf("GPIO SETDATAOUTADDR mapped to %p\n", gpio_setdataout_addr); printf("GPIO CLEARDATAOUT mapped to %p\n", gpio_cleardataout_addr); reg = *gpio_oe_addr; printf("GPIO1 configuration: %X\n", reg); reg = reg &amp; (0xFFFFFFFF - USR1_LED); *gpio_oe_addr = reg; printf("GPIO1 configuration: %X\n", reg); printf("Start blinking LED USR1\n"); while(1) { printf("ON\n"); *gpio_setdataout_addr= USR1_LED; sleep(1); printf("OFF\n"); *gpio_cleardataout_addr = USR1_LED; sleep(1); } close(fd); return 0; } </code></pre> <p>The output is:</p> <pre><code>Mapping 4804C000 - 4804DFFF (size: 1FFF) GPIO mapped to 0x40225000 GPIO OE mapped to 40225134 GPIO SEDATAOUTADDR mapped to 0x40225194 GPIO CLEARDATAOUTADDR mapped to 0x40225190 GPIO1 configuration: FE1FFFFF GPIO1 configuratino: FE1FFFFF Start blinking LED USR1 ON OFF ON OFF ... </code></pre> <p>but I can't see the led blinking.</p> <p>As you can see from the output of the program the configuration is correct, FE1FFFFF, is coherent since GPIO1_21, GPIO1_22, GPIO1_23 and GPIO1_24 are configured as outputs, each one driving a LED.</p> <p>Any idea about the reason?</p>
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    1. COI have found the solution...it is only required to use MAP_SHARED in mmap instead of MAP_PRIVATE. I leave the question anyway. Maybe it will be useful for someone else.
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    2. COIt is perfectly fine practice to answer your own question, as long as others are given a fair chance to answer it as well.
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