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I am using beaglebone to access digital input from specific pin using sysfs interface. And I can change the output states but not the input :(. What I did is, I have two pins pinA and pinB. pinA I made it output and pinB I made input. Connected pinA to pinB. Configured pinA as output pin by sending out to direction attribute in sysfs and pinB as input by passing in. And I changed value of PinA to 1 and it is giving 1 as output (I tested using LED). But when I read the value of PinB it is giving 0 only, even I pass 0 to value of pinA. what may be the reason ?

Thank you :)

duslabo
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    Could you paste precisely the sequence of sysfs operations you are using please? – marko Aug 20 '12 at 09:46
  • Thanks for the reply @Marko. See the steps I followed. echo 7 > /sys/kernel/debug/omap_mux/gpmc_ad6 , echo 38 > /sys/class/gpio/export , echo in > /sys/class/gpio/gpio38/direction cat /sys/class/gpio/gpio38/value . It shows 0 if i give input 3.3v to the pin. – duslabo Aug 20 '12 at 11:26
  • Didn't know you could actually alter muxing using sysfs. I wonder about that `7` you are setting the pin's mux to - on OMAP4 that's safe-mode with disconnects the IO pin entirely. Probably the same on OMAP3. You probably ought not to be connecting 3.3v directly to those pins either - they are probably 1.8v IO only. – marko Aug 20 '12 at 11:35
  • But according to [Beaglebone_SRM](http://beagleboard.org/static/beaglebone/latest/Docs/Hardware/BONE_SRM.pdf) mode 7 is for GPIO. And ya I tried with 1.8 volts too. – duslabo Aug 20 '12 at 11:46
  • Looks as if this is [right](http://www.ti.com/lit/ds/sprs717d/sprs717d.pdf) - Page 31. Having looked at the Beaglebone documentation it doesn't strike me as abundantly clear which pins are corresponding to gpio ids as seen in sysfs. Might be worth double checking that you are actually reading the right pin. Guess you know you can get the raw states of all GPIO pins from sysfs don't you? `cat /sys/kernel/debug/gpio` – marko Aug 20 '12 at 13:07
  • Yeah Marko, I did cross check with the pins. It is working for output but. not for input. So there is no worries that I chose right pin and right sysfs. – duslabo Aug 20 '12 at 13:29

3 Answers3

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As I understood, the steps you followed:

echo 7 > /sys/kernel/debug/omap_mux/gpmc_ad6
echo 38 > /sys/class/gpio/export
echo in > /sys/class/gpio/gpio38/direction
cat /sys/class/gpio/gpio38/value

I also did the same mistake and it took me hours, but the answer was simple: The first line starting with "echo 7" is the problem. Look at the muxing bits:

Bit 5: 1 - Input, 0 - Output

Bit 4: 1 - Pull up, 0 - Pull down

Bit 3: 1 - Pull disabled, 0 - Pull enabled

Bit 2 \

Bit 1 |- Mode

Bit 0 /

You were entering echo 7 which is --> 0 0 0111 and it means: bit 0,1 and 2 is 1, so the mode is set. No problem. However you just forgot to set whether it's an input or output. And it should be like this:

echo 0x27 > /sys/kernel/debug/omap_mux/gpmc_ad6

your bits are now: 1 0 0111 binary which is 0x27 (hex).

When you write "cat /sys/class/gpio/gpio38/value" while giving input, you can see a wonderful 1 :) I’m sure you will be very happy as much as I was :)

Also, one more thing, you are right for Analog input about 1.8V, but GPIO operates with 3.3v.

Chris Desjardins
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Ibrahim Arli
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Several possible causes:

1) Did you set the IO direction of the input pin?

eg. echo "in" > /sys/class/gpio/gpioN/direction

2) (less likely) Is the GPIO pin you're using as an input multiplexed as a GPIO line and in the right direction? Most of the GPIO pins on the OMAP SoCs are multi-function. You're kernel might have set it for an alternate function.

You can check it with:

cat /sys/kernel/debug/omap_mux/board/core

Which dumps the configurations of all IO pins. The output looks like this:

OMAP4_MUX(CSI22_DY1, OMAP_PIN_INPUT | OMAP_MUX_MODE0),
/* gpio_81 */
OMAP4_MUX(CAM_SHUTTER, OMAP_PIN_OUTPUT | OMAP_MUX_MODE3),
OMAP4_MUX(CAM_STROBE, OMAP_PIN_OUTPUT | OMAP_MUX_MODE0),
/* gpio_83 */

In this case, CAM_SHUTTER is set an output, and routed as to the GPIO module (OMAP_MUX_MODE3)

[Caveat: this is from my OMAP4 board - without having the OMAP3 data sheet to hand - there will be a fair amount of similarity]

You can't change this through sysfs - instead you'll need to modify either your kernel (or possibly boot-loader if the kernel uses the configuration it set up).

In the board-file for your system - which I think in your case will be in <linux_source_root>/arch/arm/mach-omap2/board-omap3beagle.c - you'll find a initialiser for the MUX table. You will need the board's schematics, the kernel source tree and the SoC data sheet to get between the primary function name of the pin (in my example above CAM_SHUTTER) and a GPIO number.

3) I was a bit confused by even I pass 0 to value of pinA - I wonder whether you meant that? This does however point to another thing to watch for - there is the programmable pull-up or -down on each IO pin. These are set with the MUX settings. There may conceivably be an external one as well - again you'll need the schematics to be sure.

marko
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  • Hey, I didn't check this. What I feel is there is some problem with pull-up/pull-down resistors, is there any idea to enable/disable pull-up resistor ? – duslabo Aug 20 '12 at 11:31
  • I've posted the response to this as another answer as it's a bit long for here. – marko Aug 20 '12 at 11:45
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Yes. The internal pull up and down is configured in the same register as the mux-mode - so it might be that you can configure this in the same way you're setting the mux-mode. Get the AM335x TRM (for OMAP4 the chapter is called Control Module).

In terns of the kernel, look in <linux_source_root>/arch/arm/mach-omap2/mux.h where a bunch of macros are defined

As an example for use I have in my board file:

/* PIC -> OMAP4 interrupt line 2 - GPIO81 */
OMAP4_MUX(CAM_SHUTTER, OMAP_MUX_MODE3 | OMAP_PIN_INPUT_PULLUP),

and

OMAP4_MUX(GPMC_AD11, OMAP_MUX_MODE0 | OMAP_PIN_INPUT_PULLDOWN),

From memory you get a choice of either a pull-up or pull-down but can't select neither.

marko
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