how to read GPIO values from command line?

[Sascha Hauer]

On Thu, Feb 09, 2012 at 11:34:55AM -0500, Robert P. J. Day wrote:
> 
>   i'm looking at the docs and tried the obvious but it didn't seem to
> work.  in order to recognize the version of beagleboard, i can see
> what happens in the u-boot code:
> 
> /*
>  * Routine: get_board_revision
>  * Description: Detect if we are running on a Beagle revision Ax/Bx,
>  *              C1/2/3, C4 or xM. This can be done by reading
>  *              the level of GPIO173, GPIO172 and GPIO171. This should
>  *              result in
>  *              GPIO173, GPIO172, GPIO171: 1 1 1 => Ax/Bx
>  *              GPIO173, GPIO172, GPIO171: 1 1 0 => C1/2/3
>  *              GPIO173, GPIO172, GPIO171: 1 0 1 => C4
>  *              GPIO173, GPIO172, GPIO171: 0 0 0 => xM
>  */
> int get_board_revision(void)
> {
>         int revision;
> 
>         if (!gpio_request(171, "") &&
>             !gpio_request(172, "") &&
>             !gpio_request(173, "")) {
> 
>                 gpio_direction_input(171);
>                 gpio_direction_input(172);
>                 gpio_direction_input(173);
> 
>                 revision = gpio_get_value(173) << 2 |
>                            gpio_get_value(172) << 1 |
>                            gpio_get_value(171);
>         } else {
>                 printf("Error: unable to acquire board revision GPIOs\n");
>                 revision = -1;
>         }
> 
>         return revision;
> }
> 
>   however, in the latest kernel tree, it seems things have changed
> slightly:
> 
> /*
>  * OMAP3 Beagle revision
>  * Run time detection of Beagle revision is done by reading GPIO.
>  * GPIO ID -
>  *      AXBX    = GPIO173, GPIO172, GPIO171: 1 1 1
>  *      C1_3    = GPIO173, GPIO172, GPIO171: 1 1 0
>  *      C4      = GPIO173, GPIO172, GPIO171: 1 0 1
>  *      XMA/XMB = GPIO173, GPIO172, GPIO171: 0 0 0
>  *      XMC = GPIO173, GPIO172, GPIO171: 0 1 0
>  */
> 
>   now that's useful since there are now allegedly different values for
> xM A/B, and xM C, and i have a rev C.
> 
>   so what exactly does one do to read those values?  i tried:
> 
> barebox at Texas Instrument's Beagle:/ gpio_direction_input 171
> barebox at Texas Instrument's Beagle:/ gpio_get_value 171
> 
> but what do i check?  the shell variable $?  i'm not sure what i
> should do next.

Yes, check $?. This assumes that your iomux is set up correctly as Eric
already mentioned. I wonder that there is no iomux setup for the panda
board. You can copy it from U-Boot. It has to arrays of iomux setup:
core_padconf_array_essential and core_padconf_array_non_essential

Sascha

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