Accessing LEDs and Switches in Xillinux

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

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

Thanks Eli, that perfectly explained it.

Just a follow up, I don't really need to, but is it possible to use LD0-3 easily? Would I just change this line:

NET  "GPIO_LED[0]"          LOC = T22  | IOSTANDARD=LVCMOS33;  # "LD0"

to something like this:

NET  PS_GPIO[56] LOC = T22 | IOSTANDARD = LVCMOS33;t# LD0

or would that cause problems with GPIO_LED[0] not being able to be found?

Thanks again.

Hi,

There's no problem taking over the four LEDs that are currently used by the Xillybus IP core (heartbeat + activity indicators).

I would suggest assigning a vacant pin to the GPIO_LED[] port that has been removed from its led, or turn it into a wire (Verilog) or signal (VHDL) rather than a port in the toplevel HDL module.

Either way, Xillinux is configured for 56 GPIOs, so there is no PS_GPIO[56]. You'll have to take one from the PMODs or something.

Which brings me to the simplest solution -- just swap pins between a GPIO_LED[] and a PS_GPIO[] pin.

Regards,
   Eli

Good to know, thank you.

Sorry for all the questions, but you've been very helpful so far, so I figure I may as well ask...

I've been following this guide: http://forums.xilinx.com/xlnx/attachments/xlnx/ELINUX/8467/1/zedboard_CTT_v2013_2_130807.pdf particularly part 5.4 Controlling LEDs and Switches in Linux Example. So instead of accessing the LEDs directly, I'm controlling them from within the PL.

I'm fine with the creation of the design, but when I put this on the SD card, it doesn't appear to boot up properly. I've tried replacing both the boot.bin and xillydemo.bit files. What should I be doing?

Thanks

Hello,

I'm afraid I'm not going to be so helpful this time...

When you mix binaries from different settings (i.e. Xillinux' and Xilinx' sample designs), there are so many things that can go wrong, that it's quite hopeless (and even more so, pointless) to attempt debugging it remotely.

If you're interested in controlling the LEDs as shown in Xilinx' guide, I suggest adopting the spirit shown there, and implement it on Xillinux' development bundle. Or go 100% for Xilinx' sample design.

Alternatively, you can adopt Xillinux' method of talking with the hardware -- through Xillybus pipes.

Regards,
   Eli

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.