Fix nrf monotonics (#852)

* Fix nrf::timer

* Bootstrap nrf52840-blinky example

* More work on nrf blinky example

* Fix README

* Add asserts for correct timer functionality

* Add correctness check to other monotonics as well

* Update Changelog

* Fix potential timing issues

* Fix race condition in nrf::rtc

* Add changelog

* Add rtc blinky example

* Change rtc example to RC lf clock source

* Add changelog to rtic-time

* Add changelog

* Attempt to fix CI

* Update teensy4-blinky Cargo.lock

examples/nrf52840_blinky/src/lib.rs

@@ -0,0 +1,37 @@
+#![no_main]
+#![no_std]
+
+use cortex_m_semihosting::debug;
+
+use defmt_rtt as _; // global logger
+
+pub use nrf52840_hal as hal; // memory layout
+
+use panic_probe as _;
+
+// same panicking *behavior* as `panic-probe` but doesn't print a panic message
+// this prevents the panic message being printed *twice* when `defmt::panic` is invoked
+#[defmt::panic_handler]
+fn panic() -> ! {
+    cortex_m::asm::udf()
+}
+
+/// Terminates the application and makes a semihosting-capable debug tool exit
+/// with status code 0.
+pub fn exit() -> ! {
+    loop {
+        debug::exit(debug::EXIT_SUCCESS);
+    }
+}
+
+/// Hardfault handler.
+///
+/// Terminates the application and makes a semihosting-capable debug tool exit
+/// with an error. This seems better than the default, which is to spin in a
+/// loop.
+#[cortex_m_rt::exception]
+unsafe fn HardFault(_frame: &cortex_m_rt::ExceptionFrame) -> ! {
+    loop {
+        debug::exit(debug::EXIT_FAILURE);
+    }
+}