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ktgpio-example-app

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Kotlin/Native example app using ktgpio to turn GPIO Raspberry PI pins on and off

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# ktgpio Example App

A Kotlin/Native example app which uses the [ktgpio](https://github.com/ktgpio/ktgpio) to control
[Raspberry PI GPIO](https://pinout.xyz/#) pins.

This example app only activates the GPIO17 (pin #11) for a second, then turns it off.
See the [Main.kt](src/nativeMain/kotlin/Main.kt) script for more details.

For a more complex example please take a look at the sources of the [ktgpio-samples](https://github.com/ktgpio/ktgpio-samples/)
example app.

Licensed under the MIT license.

## Prerequisites

Kotlin/Native at the moment doesn't support building on arm64: you'll get
"unknown host target: linux aarch64" error if you try. See the
[getting 'unknown host target: linux aarch64'](https://discuss.kotlinlang.org/t/kotlin-native-getting-unknown-host-target-linux-aarch64-on-raspberry-pi-3b-ubuntu-21-04-aarch64/22874)
forum and also [KT-42445](https://youtrack.jetbrains.com/issue/KT-42445) for more details.

Therefore, you can not build this project on the Raspberry PI itself - you'll need to build this project
on an x86-64 machine (Intel/AMD) via a process called "cross-compiling" (that is, compiling a binary which runs on a CPU with an architecture different to the one performing the build).
The cross-compiling itself is handled automatically by the Kotlin plugin behind the scenes, there's nothing you need to do.
You only need to remember to build the project on a x86 machine.

You can use any major operating system to build this project. I'm using Ubuntu Linux x86-64 OS, however this
project builds on Windows and MacOS as well.

> See [Kotlin/Multiplatform Supported Platforms](https://kotlinlang.org/docs/mpp-supported-platforms.html) for
> a list of supported build combinations of host and target architectures. Note you're building for the `linuxArm64` target architecture.

You need to install Java SDK on the build machine, in order to be able to run the Gradle build script. You
don't need to install Gradle itself - the `gradlew` script will download Gradle and all
necessary files automatically, you only need to have an internet access.

Obviously you'll need a Raspberry PI in order to run the binary produced by the build.
By default this project builds an arm64 binary which only works
on 64-bit Linux (I'm running Ubuntu 21.04 arm64 on my RPI 3B with 1G of RAM but the binary should work on any arm64 Linux, say on
RPI 4). If you'd like to build this project for an arm32 Linux, see below on how to do this.

Make sure to have the `libgpio` and `libi2c` libraries installed on your Raspberry PI before you try to run the binary. You
can install those by running
`sudo apt install libi2c0 libgpiod gpiod`.

## Building

Simply run `./gradlew`. The project should build fine and the binary .kexe should be present
in the `build/bin/native/releaseExecutable/` folder.

As stated above, I'm building this project on my Ubuntu 21.04 x86-64 machine and it works properly,
however the build should work properly even when running on MacOsX or Windows.

## Running

The binary will obviously only run on an arm64 Linux. It needs to have the `libi2c0` and `libgpiod` libraries
installed (those are needed by the ktgpio library) - make sure to install them first as stated above.
Now, simply copy the `ktgpio-example-app.kexe` binary from your desktop machine to your
Raspberry PI, for example via ssh by using the scp program.

You'll probably need to run this app with sudo, in order to be able to access
`/dev/mem` and `/dev/gpiomem`. See
https://raspberrypi.stackexchange.com/questions/40105/access-gpio-pins-without-root-no-access-to-dev-mem-try-running-as-root
for more details. Simply run the binary with `sudo ./ktgpio-example-app.kexe` and you should be fine.

By default the program will perform a LED blinking. See below for other examples.

## Development

Simply download Intellij IDEA (Community Edition is enough) and open this project in order to develop this example app further.
Intellij offers amazing autocomplete capabilities, it's definitely worth a try.

The development cycle is as follows:

1. Make changes to the example app
2. Build new binary by running `./gradlew`
3. scp it to your RPI
4. Run it in your RPI
5. Goto 1

You can simply run steps 2. and 3. by running `./gradlew deploy` - it will rebuild the binary and
deploy to your RPI.

Make sure to visit the excellent [GPIO Zero: Basic Recipes Guide](https://gpiozero.readthedocs.io/en/stable/recipes.html)
to experiment with your RPI.

### LED blinking

See the [LED example](https://gpiozero.readthedocs.io/en/stable/recipes.html#led) on how
to connect the LED. Then, make sure the `ledExample(gpio)` function is called in `Main.kt`,
build and deploy this app via `./gradlew deploy`, then run it on your RPI.

### Waiting for Button
See the [Button example](https://gpiozero.readthedocs.io/en/stable/recipes.html#button)
on how to connect the button. Then, make sure the `buttonExample(gpio)` function
is called in `Main.kt`.

The `buttonExample2(gpio)` instead repeatedly measures the button press.

## More documentation

Please read the [Python GPIOZero excellent documentation](https://gpiozero.readthedocs.io/en/stable/recipes.html)
for more fun. This project implements some of the classes present in the GPIOZero project.

## Compiling a 32bit binary

This project by default builds a 64bit binary for arm64. If you are running a 32bit Linux
on your RPI, you must reconfigure this project to produce a 32bit binary:

1. Edit `build.gradle.kts` and replace `linuxArm64("native")` with `linuxArm32Hfp("native")`.
2. Replace both of the `libs/*.so` library files by their 32bit counterparts downloaded from
  your arm32 Linux running on your RPI. See [libs/README.txt](libs/README.txt) for more details
  on where to obtain the files.

That's it! Now run `./gradlew` and hopefully the arm32 binary will be produced.

## FAQ/Troubleshooting

Q: The build fails with `aarch64-unknown-linux-gnu/bin/ld.gold: error: cannot find -li2c`

A: Make sure that the `libi2c.so` file is present in the `libs/` folder and is matching
  the target arch exactly - arm64 when building arm64 binary, arm32 when building arm32 binary.
  Similarly with the issue `cannot find -lgpiod`: the `libgpiod.so` must be present in the `libs/` folder.

Q: The binary fails with `kotlin.IllegalStateException: Can not access specified GPIO chip: 0`

A: The binary doesn't have rights to access `/dev/mem` and `/dev/gpiomem`. Run the binary
   with `sudo`.

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