Adding an additional example for TTN Generic Node

Adding an additional example for TTN Generic Node to drive MLT-7525 buzzer. Example was tested on the target and works.

Author: maebli

examples/examples/buzzer.rs

@@ -0,0 +1,39 @@
+// Sounds a buzzer, will work only for TTN Generic node with STM32WL5x
+// see https://github.com/TheThingsIndustries/generic-node-se
+// The datasheet of the buzzer `MLT-7525` shows that the oscillation frequency is `2700Hz` so the period
+// is around 185us
+
+#![no_std]
+#![no_main]
+
+use defmt_rtt as _; // global logger
+use panic_probe as _; // panic handler
+
+use stm32wlxx_hal::{
+    self as hal,
+    cortex_m::{self, delay::Delay},
+    gpio::{pins, Output, PinState, PortA},
+    pac,
+    util::new_delay,
+};
+
+#[hal::cortex_m_rt::entry]
+fn main() -> ! {
+    let mut dp: pac::Peripherals = defmt::unwrap!(pac::Peripherals::take());
+    let cp: pac::CorePeripherals = defmt::unwrap!(pac::CorePeripherals::take());
+
+    let gpioa: PortA = PortA::split(dp.GPIOA, &mut dp.RCC);
+    let mut buzzer: Output<pins::A15> =
+        cortex_m::interrupt::free(|cs| Output::default(gpioa.a15, cs));
+
+    let mut delay: Delay = new_delay(cp.SYST, &dp.RCC);
+
+    defmt::info!("Starting buzzer");
+
+    loop {
+        for &level in &[PinState::High, PinState::Low] {
+            buzzer.set_level(level);
+            delay.delay_us(185);
+        }
+    }
+}