Controlling Jetson TX2 pin states

Controlling the pin states on the Jetson TX2 SoM

There's two ways:

• Through bootloader configuration.
• Through using the virtual /sys filesystem in userspace.

Pin settings in bootloader configuration

Summary

You need to do the following:

1. Download an Microsoft Excel sheet(!) containing some macros(!!) and the L4T ("Linux For Tegra") package from Nvidias downloadcenter. Note: For this you need a Nvidia developer account.
2. In the Excel sheet, select the desired pin configuration using cell dropdown menus. Use the embedded macro to write out some device tree files.
3. Use a Python script which comes with L4T to convert the device tree files into something file the bootloader can understand.
4. Embed the bootloader configuration in the Yocto source tree.

Detailed steps

As an example, the following guide walks you through reconfiguring pin A9 from it's default state (output GND) to input with weak pull-up.

1. The MS Excel part:
   1. Visit the Nvidia developer download center and search for Jetson TX2 Series Pinmux. Here's a direct link for 1.08. Download and run it with macros enabled.
   2. On the second sheet you'll find the configuration for pin A9 on (at the time of writing) line 246. Cells in columns AR and AS define it as output grounding the signal. Change these cells to Input and Int PU.
   3. At the very top of the sheet, click the button labeled Generate DT file. Some dialogues will pop up which asks for stuff and have an effect on the filename.
2. The Python part:
   1. Go to the Nvidia developer download center and search for Jetson Linux Driver Package (L4T). Follow the link to the L4T x.y.z Release Page. (For example, here's the one for R32.4.3.) There, you should find a link labeled L4T Driver Package (BSP) leading to some tarball named similar to Tegra186_Linux_Rx.y.z_aarch64.tbz2. (Again, as an example here's the one for R32.4.3.). Uncompress it and change to Linux_for_Tegra/kernel/pinmux/t186/ inside.
   2. Run the pinmux-dts2cfg.py in the following way:

   python pinmux-dts2cfg.py \
       --pinmux \
       addr_info.txt \
       gpio_addr_info.txt \
       por_val.txt \
       --mandatory_pinmux_file mandatory_pinmux.txt \
       /path/to/your/excel-created/tegra18x-jetson-tx2-config-template-*-pinmux.dtsi \
       /path/to/your/excel-created/tegra18x-jetson-tx2-config-template-*-gpio-*.dtsi \
       1.0 \
       > /tmp/new.cfg
   

   If it throws errors, it might be related to this.
3. Add a patch in your distro layer reflecting the pin settings in your /tmp/new.cfg created above.

Controlling/reading the pin state from userspace

You can control/read the pin value from the virtual /sys filesystem but not the pull up/down state.

Software-wise, the GPIOs have other names than on the schematic. Nvidia doesn't make it easy to go from schematic name (like A9) to the /sys name (like gpio488). The following user contributed posts explain it better than anything Nvidia has come up with so far:

• https://forums.developer.nvidia.com/t/gpio-doesnt-work/49203/14
• https://forums.developer.nvidia.com/t/gpio-doesnt-work/49203/2
• This post contains the equations in the links above solved for all possible input values.

Having found out the /sys name for your pin, you can take following snippets as an example:

The following snippet sets the gpio to output-low.

# GPIO488 is A9 on the SoM
pin=488
echo $pin > /sys/class/gpio/export
echo out  > /sys/class/gpio/gpio$pin/direction
echo 0    > /sys/class/gpio/gpio$pin/value

The following snippet sets the pin to input and reads its logical state:

# GPIO488 is A9 on the SoM
pin=488
echo $pin > /sys/class/gpio/export
echo in   > /sys/class/gpio/gpio$pin/direction
cat         /sys/class/gpio/gpio$pin/value