First, let's create a simple led blinking application.

## Preparing for launch

We will use integrated LED. Look at the schematic:

![](https://github.com/Flipper-Zero/flipperzero-firmware-community/raw/master/wiki_static/application_examples/leds.png)
![](https://github.com/Flipper-Zero/flipperzero-firmware-community/raw/master/wiki_static/application_examples/gpio_pa8.png)

This LED is connected between power rail and GPIO PA8, and we should configure this pin as an open drain to properly control the LED behaviour.

You can find GPIO API in `target_*/flipper_hal.h`. Or if you prefer to use Arduino API, you can find the bindings in `core/flipper.h`.

To work with the pin we should:

1. Create `GpioPin` instance and specify pin and port.
2. Configure mode of pin by `pinMode` function.
3. Control state of pin by `digitalWrite` function.

## Creating application

1. Create a new file (for example, `blink.c`) in `applications` folder.
2. Create code like this:

```C
#include "flipper.h"

void application_blink(void* p) {
    // create pin
    GpioPin led = {.pin = GPIO_PIN_8, .port = GPIOA};

    // configure pin
    pinMode(led, GpioModeOpenDrain);

    while(1) {
        digitalWrite(led, HIGH);
        delay(500);
        digitalWrite(led, LOW);
        delay(500);
    }
}
```
3. To start your application on Flipper startup, add it to the autorun:
    * in `applications/startup.h` add prototype of main application function:

    ```C
    void application_blink(void* p);
    ```

    * add entry to `FLIPPER_STARTUP` array (pointer to application function and application name):

    ```C
    const FlipperStartupApp FLIPPER_STARTUP[] = {
        #ifdef TEST
        {.app = flipper_test_app, .name = "test app"}
        #endif

        // user applications:

        , {.app = application_blink, .name = "blink"}  
    };
    ```

4. Add your application file to Makefile (for each target, `target_lo/Makefile` and `target_f1/Makefile`, we'll add one common makefile later):

```
# User application

C_SOURCES += ../applications/blink.c
```

Build and run for linux (target_lo):

`docker-compose exec dev make -C target_lo`

Run:

`docker-compose exec dev target_lo/build/target_lo`.

Linux version has no LED or GPIO, but we can see debug messages of how state of GPIO is changing:

![](https://github.com/Flipper-Zero/flipperzero-firmware-community/raw/master/wiki_static/application_examples/example_blink.gif)

_You can also find source of this example in `applications/examples/blink.c` and run it by `docker-compose exec dev make -C target_lo example_blink`_

Build for Flipper (board F1):

`docker-compose exec dev make -C target_f1`

Upload to microcontroller:

`./target_f1/deploy.sh`

Blink!

![](https://github.com/Flipper-Zero/flipperzero-firmware-community/raw/master/wiki_static/application_examples/example_blink_hw.gif)

_You can also compile by `docker-compose exec dev make -C target_f1 example_blink`_