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Accessing LEDs and Switches in Xillinux

Former Member

Hi there,

I've installed Xillinux on my ZedBoard, and I'm wanting to simply read switch values and write LED values, like in the ZedBoard Getting Started Guide, but I don't see anything in /sys/class/gpio, only export, unexport and gpiochip0. I've read and tried a lot of things to get it working, but don't seem to be getting anywhere.

Can anybody please explain to me how to go about achieving this?

Thanks

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    Hello,

    Xillinux uses Linux' standard GPIO convention for accessing LEDs and switches (and PMODs for that matter)

    Please refer to the xillydemo.ucf file under verilog/src (or vhdl/src, they're the same). Even if you don't understand all the details, it's clear that the following segment

    <------- Cut here -------->

    # On-board LEDs. Note that only for LEDs are allocated, as opposed to
    # Digilent's eight, and all placements that follow are shifted by four.
    # There was no other choice, as the tools don't allow unplaced PS GPIO pins.

    NET  PS_GPIO[7] LOC = V22 | IOSTANDARD = LVCMOS33;    # LD4
    NET  PS_GPIO[8] LOC = W22 | IOSTANDARD = LVCMOS33;    # LD5
    NET  PS_GPIO[9] LOC = U19 | IOSTANDARD = LVCMOS33;    # LD6
    NET  PS_GPIO[10] LOC = U14 | IOSTANDARD = LVCMOS33;    # LD7

    <------- Cut here -------->

    tells us that one of the LEDs is mapped at GPIO7. In order to access the LED from Linux, the standard command line interface can be used. There is an offset between the GPIO number in the UCF file and the one used in Linux, and it's 54 on Xillinux. So in Linux, the GPIO number for this LED should be 54+7=61.

    Which means that the following sequence should turn the LED on and off.

    At shell prompt, try

    echo 61 > /sys/class/gpio/export
    echo out > /sys/class/gpio/gpio61/direction
    echo 1 > /sys/class/gpio/gpio61/value
    echo 0 > /sys/class/gpio/gpio61/value

    There is plenty of documentation on Linux GPIO interface. There's also a faster API for application programming.

    For example, see http://falsinsoft.blogspot.co.il/2012/11/access-gpio-from-linux-user-space.html

    Hope this helped,
       Eli

  • 0 in reply to Former Member

    How do you determine the base offset to add to the number obtained from the UCF file. (the 54 in the Xillinux case)

    I searched everywhere but I wasn't able to find a reference for this information.

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