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

18 files changed, 1082 insertions, 103 deletions

diff --git a/.github/workflows/build.yml b/.github/workflows/build.yml
index 19a13d71..58aaf7d0 100644
--- a/.github/workflows/build.yml
+++ b/.github/workflows/build.yml
@@ -148,7 +148,7 @@ jobs:
     steps:
       - name: Checkout
         uses: actions/checkout@v4
-      
+
       - name: Install rust ${{ matrix.toolchain }}
         run: |
           rustup set profile minimal
@@ -164,6 +164,9 @@ jobs:
       - name: Cache Dependencies
         uses: Swatinem/rust-cache@v2
 
+      - name: Install flip-link
+        run: cargo install flip-link
+
       - name: Check the examples
         run: cargo xtask usage-example-build
 
@@ -275,22 +278,22 @@ jobs:
           cp -r target/doc $td/api
           echo rtic
           lychee --offline --format detailed $td/api/rtic/
-        
+
           echo rtic_common
           lychee --offline --format detailed $td/api/rtic_common/
-        
+
           echo rtic_macros
           lychee --offline --format detailed $td/api/rtic_macros/
-        
+
           echo rtic_monotonics
           lychee --offline --format detailed $td/api/rtic_monotonics/
 
           echo rtic_sync
           lychee --offline --format detailed $td/api/rtic_sync/
-        
+
           echo rtic_time
           lychee --offline --format detailed $td/api/rtic_time/
-      
+
 
       - name: Archive the API docs
         run: |
@@ -336,7 +339,7 @@ jobs:
           name: apidocs
 
       - name: Extract the API docs
-        run: tar -xf apidocs.tar 
+        run: tar -xf apidocs.tar
 
       - name: Check links
         run: |
@@ -463,7 +466,7 @@ jobs:
   # master branch.
   # As master moves on to development, the work on the
   # stable version will happen in release/v"stable_version".
-  # Thus, no need to push changes 
+  # Thus, no need to push changes
   #
   # This needs to run before book is built, as bookbuilding fetches from the branch
   pushtostablebranch:
@@ -503,8 +506,8 @@ jobs:
     runs-on: ubuntu-22.04
     needs:
       - pushtostablebranch
-      - docs      
-      - mdbookold      
+      - docs
+      - mdbookold
       - mdbook
 
     # Only run this when pushing to master branch
@@ -521,7 +524,7 @@ jobs:
         uses: taiki-e/install-action@v2
         with:
           tool: mdbook-mermaid
- 
+
       - name: mdBook Action
         uses: peaceiris/actions-mdbook@v1
         with:
@@ -537,7 +540,7 @@ jobs:
 
       - name: Extract the dev-version book and API docs
         run: |
-          tar -xf book.tar 
+          tar -xf book.tar
 
       - name: Download built old versions of books and API docs
         uses: actions/download-artifact@v3
@@ -546,7 +549,7 @@ jobs:
 
       - name: Extract the old version books and API docs
         run: |
-          tar -xf mdbookold.tar 
+          tar -xf mdbookold.tar
 
       - name: Prepare books
         shell: 'script --return --quiet --command "bash {0}"'
@@ -631,7 +634,7 @@ jobs:
 
       - name: Extract the books
         run: |
-          tar -xf bookstodeploy.tar 
+          tar -xf bookstodeploy.tar
 
       - name: Deploy to GH-pages
         uses: peaceiris/actions-gh-pages@v3
diff --git a/examples/nrf52840_blinky/.cargo/config.toml b/examples/nrf52840_blinky/.cargo/config.toml
new file mode 100644
index 00000000..419b43aa
--- /dev/null
+++ b/examples/nrf52840_blinky/.cargo/config.toml
@@ -0,0 +1,18 @@
+[target.'cfg(all(target_arch = "arm", target_os = "none"))']
+runner = "probe-rs run --chip nRF52840_xxAA"
+
+rustflags = [
+  "-C", "linker=flip-link",
+  "-C", "link-arg=-Tlink.x",
+  "-C", "link-arg=-Tdefmt.x",
+  # This is needed if your flash or ram addresses are not aligned to 0x10000 in memory.x
+  # See https://github.com/rust-embedded/cortex-m-quickstart/pull/95
+  "-C", "link-arg=--nmagic",
+]
+
+[build]
+target = "thumbv7em-none-eabihf" # Cortex-M4F and Cortex-M7F (with FPU)
+
+[alias]
+rb = "run --bin"
+rrb = "run --release --bin"
diff --git a/examples/nrf52840_blinky/.gitignore b/examples/nrf52840_blinky/.gitignore
new file mode 100644
index 00000000..ea8c4bf7
--- /dev/null
+++ b/examples/nrf52840_blinky/.gitignore
@@ -0,0 +1 @@
+/target
diff --git a/examples/nrf52840_blinky/.vscode/settings.json b/examples/nrf52840_blinky/.vscode/settings.json
new file mode 100644
index 00000000..c684c242
--- /dev/null
+++ b/examples/nrf52840_blinky/.vscode/settings.json
@@ -0,0 +1,9 @@
+{
+    // override the default setting (`cargo check --all-targets`) which produces the following error
+    // "can't find crate for `test`" when the default compilation target is a no_std target
+    // with these changes RA will call `cargo check --bins` on save
+    "rust-analyzer.checkOnSave.allTargets": false,
+    "rust-analyzer.checkOnSave.extraArgs": [
+        "--bins"
+    ]
+}
diff --git a/examples/nrf52840_blinky/Cargo.lock b/examples/nrf52840_blinky/Cargo.lock
new file mode 100644
index 00000000..a3d75427
--- /dev/null
+++ b/examples/nrf52840_blinky/Cargo.lock
@@ -0,0 +1,612 @@
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+ "critical-section",
+ "rtic-core",
+ "rtic-macros",
+]
+
+[[package]]
+name = "rtic-common"
+version = "1.0.1"
+dependencies = [
+ "critical-section",
+ "portable-atomic",
+]
+
+[[package]]
+name = "rtic-core"
+version = "1.0.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "d9369355b04d06a3780ec0f51ea2d225624db777acbc60abd8ca4832da5c1a42"
+
+[[package]]
+name = "rtic-macros"
+version = "2.0.1"
+dependencies = [
+ "indexmap",
+ "proc-macro-error",
+ "proc-macro2",
+ "quote",
+ "syn 1.0.109",
+]
+
+[[package]]
+name = "rtic-monotonics"
+version = "1.4.0"
+dependencies = [
+ "atomic-polyfill",
+ "cfg-if",
+ "cortex-m",
+ "critical-section",
+ "embedded-hal 1.0.0-rc.2",
+ "fugit",
+ "nrf52840-pac",
+ "rtic-time",
+]
+
+[[package]]
+name = "rtic-time"
+version = "1.1.0"
+dependencies = [
+ "critical-section",
+ "futures-util",
+ "rtic-common",
+]
+
+[[package]]
+name = "rustc_version"
+version = "0.2.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "138e3e0acb6c9fb258b19b67cb8abd63c00679d2851805ea151465464fe9030a"
+dependencies = [
+ "semver",
+]
+
+[[package]]
+name = "semver"
+version = "0.9.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "1d7eb9ef2c18661902cc47e535f9bc51b78acd254da71d375c2f6720d9a40403"
+dependencies = [
+ "semver-parser",
+]
+
+[[package]]
+name = "semver-parser"
+version = "0.7.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "388a1df253eca08550bef6c72392cfe7c30914bf41df5269b68cbd6ff8f570a3"
+
+[[package]]
+name = "stable_deref_trait"
+version = "1.2.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "a8f112729512f8e442d81f95a8a7ddf2b7c6b8a1a6f509a95864142b30cab2d3"
+
+[[package]]
+name = "syn"
+version = "1.0.109"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "72b64191b275b66ffe2469e8af2c1cfe3bafa67b529ead792a6d0160888b4237"
+dependencies = [
+ "proc-macro2",
+ "quote",
+ "unicode-ident",
+]
+
+[[package]]
+name = "syn"
+version = "2.0.39"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "23e78b90f2fcf45d3e842032ce32e3f2d1545ba6636271dcbf24fa306d87be7a"
+dependencies = [
+ "proc-macro2",
+ "quote",
+ "unicode-ident",
+]
+
+[[package]]
+name = "thiserror"
+version = "1.0.50"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "f9a7210f5c9a7156bb50aa36aed4c95afb51df0df00713949448cf9e97d382d2"
+dependencies = [
+ "thiserror-impl",
+]
+
+[[package]]
+name = "thiserror-impl"
+version = "1.0.50"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "266b2e40bc00e5a6c09c3584011e08b06f123c00362c92b975ba9843aaaa14b8"
+dependencies = [
+ "proc-macro2",
+ "quote",
+ "syn 2.0.39",
+]
+
+[[package]]
+name = "typenum"
+version = "1.17.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "42ff0bf0c66b8238c6f3b578df37d0b7848e55df8577b3f74f92a69acceeb825"
+
+[[package]]
+name = "unicode-ident"
+version = "1.0.12"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "3354b9ac3fae1ff6755cb6db53683adb661634f67557942dea4facebec0fee4b"
+
+[[package]]
+name = "usb-device"
+version = "0.2.9"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "1f6cc3adc849b5292b4075fc0d5fdcf2f24866e88e336dd27a8943090a520508"
+
+[[package]]
+name = "vcell"
+version = "0.1.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "77439c1b53d2303b20d9459b1ade71a83c716e3f9c34f3228c00e6f185d6c002"
+
+[[package]]
+name = "version_check"
+version = "0.9.4"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "49874b5167b65d7193b8aba1567f5c7d93d001cafc34600cee003eda787e483f"
+
+[[package]]
+name = "void"
+version = "1.0.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "6a02e4885ed3bc0f2de90ea6dd45ebcbb66dacffe03547fadbb0eeae2770887d"
+
+[[package]]
+name = "volatile-register"
+version = "0.2.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "de437e2a6208b014ab52972a27e59b33fa2920d3e00fe05026167a1c509d19cc"
+dependencies = [
+ "vcell",
+]
diff --git a/examples/nrf52840_blinky/Cargo.toml b/examples/nrf52840_blinky/Cargo.toml
new file mode 100644
index 00000000..8b136493
--- /dev/null
+++ b/examples/nrf52840_blinky/Cargo.toml
@@ -0,0 +1,68 @@
+[package]
+authors = ["Finomnis <finomnis@gmail.com>"]
+name = "nrf52840-blinky"
+edition = "2021"
+version = "0.1.0"
+
+[workspace]
+
+[lib]
+harness = false
+
+[dependencies]
+cortex-m = { version = "0.7", features = ["critical-section-single-core"] }
+cortex-m-rt = "0.7"
+defmt = "0.3"
+defmt-rtt = "0.4"
+panic-probe = { version = "0.3", features = ["print-defmt"] }
+cortex-m-semihosting = "0.5.0"
+nrf52840-hal = "0.16.0"
+fugit = { version = "0.3.7", features = ["defmt"] }
+
+[dependencies.rtic]
+path = "../../rtic"
+version = "2.0.1"
+features = ["thumbv7-backend"]
+
+[dependencies.rtic-monotonics]
+path = "../../rtic-monotonics"
+version = "1.4.0"
+features = ["nrf52840"]
+
+# cargo build/run
+[profile.dev]
+codegen-units = 1
+debug = 2
+debug-assertions = true # <-
+incremental = false
+opt-level = 'z'         # <-
+overflow-checks = true  # <-
+
+# cargo test
+[profile.test]
+codegen-units = 1
+debug = 2
+debug-assertions = true # <-
+incremental = false
+opt-level = 3           # <-
+overflow-checks = true  # <-
+
+# cargo build/run --release
+[profile.release]
+codegen-units = 1
+debug = 2
+debug-assertions = false # <-
+incremental = false
+lto = 'fat'
+opt-level = 3            # <-
+overflow-checks = false  # <-
+
+# cargo test --release
+[profile.bench]
+codegen-units = 1
+debug = 2
+debug-assertions = false # <-
+incremental = false
+lto = 'fat'
+opt-level = 3            # <-
+overflow-checks = false  # <-
diff --git a/examples/nrf52840_blinky/README.md b/examples/nrf52840_blinky/README.md
new file mode 100644
index 00000000..56eac32a
--- /dev/null
+++ b/examples/nrf52840_blinky/README.md
@@ -0,0 +1,33 @@
+# `nrf52840_blinky`
+
+An RTIC blinky example intended for [nrf52840-dongle].
+
+[nrf52840-dongle]: https://www.nordicsemi.com/Products/Development-hardware/nrf52840-dongle
+
+## Dependencies
+
+#### 1. `flip-link`:
+
+```console
+$ cargo install flip-link
+```
+
+#### 2. `probe-rs`:
+
+``` console
+$ # make sure to install v0.2.0 or later
+$ cargo install probe-rs --features cli
+```
+
+
+## Run
+
+The [nrf52840-dongle] needs to be connected to the computer via an SWD probe, like a [J-Link EDU Mini].
+
+Then, run:
+
+```
+cargo run --release --bin blinky_timer
+```
+
+[J-Link EDU Mini]: https://www.segger.com/products/debug-probes/j-link/models/j-link-edu-mini/
diff --git a/examples/nrf52840_blinky/src/bin/blinky_rtc.rs b/examples/nrf52840_blinky/src/bin/blinky_rtc.rs
new file mode 100644
index 00000000..e515659e
--- /dev/null
+++ b/examples/nrf52840_blinky/src/bin/blinky_rtc.rs
@@ -0,0 +1,69 @@
+#![deny(unsafe_code)]
+#![deny(warnings)]
+#![no_main]
+#![no_std]
+#![feature(type_alias_impl_trait)]
+
+use nrf52840_blinky::hal;
+
+#[rtic::app(device = hal::pac, dispatchers = [SWI0_EGU0])]
+mod app {
+    use super::*;
+
+    use hal::gpio::{Level, Output, Pin, PushPull};
+    use hal::prelude::*;
+
+    use rtic_monotonics::nrf::rtc::Rtc0 as Mono;
+    use rtic_monotonics::nrf::rtc::*;
+    use rtic_monotonics::Monotonic;
+
+    #[shared]
+    struct Shared {}
+
+    #[local]
+    struct Local {
+        led: Pin<Output<PushPull>>,
+    }
+
+    #[init]
+    fn init(cx: init::Context) -> (Shared, Local) {
+        // Configure low frequency clock
+        hal::clocks::Clocks::new(cx.device.CLOCK).start_lfclk();
+
+        // Initialize Monotonic
+        let token = rtic_monotonics::create_nrf_rtc0_monotonic_token!();
+        Mono::start(cx.device.RTC0, token);
+
+        // Setup LED
+        let port0 = hal::gpio::p0::Parts::new(cx.device.P0);
+        let led = port0.p0_06.into_push_pull_output(Level::Low).degrade();
+
+        // Schedule the blinking task
+        blink::spawn().ok();
+
+        (Shared {}, Local { led })
+    }
+
+    #[task(local = [led])]
+    async fn blink(cx: blink::Context) {
+        let blink::LocalResources { led, .. } = cx.local;
+
+        let mut next_tick = Mono::now();
+        let mut blink_on = false;
+        loop {
+            let now = Mono::now();
+            let now_ms: fugit::SecsDurationU64 = now.duration_since_epoch().convert();
+            defmt::println!("Timer {} ({})", now_ms, now.ticks());
+
+            blink_on = !blink_on;
+            if blink_on {
+                led.set_high().unwrap();
+            } else {
+                led.set_low().unwrap();
+            }
+
+            next_tick += 1000.millis();
+            Mono::delay_until(next_tick).await;
+        }
+    }
+}
diff --git a/examples/nrf52840_blinky/src/bin/blinky_timer.rs b/examples/nrf52840_blinky/src/bin/blinky_timer.rs
new file mode 100644
index 00000000..53ccf4e7
--- /dev/null
+++ b/examples/nrf52840_blinky/src/bin/blinky_timer.rs
@@ -0,0 +1,66 @@
+#![deny(unsafe_code)]
+#![deny(warnings)]
+#![no_main]
+#![no_std]
+#![feature(type_alias_impl_trait)]
+
+use nrf52840_blinky::hal;
+
+#[rtic::app(device = hal::pac, dispatchers = [SWI0_EGU0])]
+mod app {
+    use super::*;
+
+    use hal::gpio::{Level, Output, Pin, PushPull};
+    use hal::prelude::*;
+
+    use rtic_monotonics::nrf::timer::Timer0 as Mono;
+    use rtic_monotonics::nrf::timer::*;
+    use rtic_monotonics::Monotonic;
+
+    #[shared]
+    struct Shared {}
+
+    #[local]
+    struct Local {
+        led: Pin<Output<PushPull>>,
+    }
+
+    #[init]
+    fn init(cx: init::Context) -> (Shared, Local) {
+        // Initialize Monotonic
+        let token = rtic_monotonics::create_nrf_timer0_monotonic_token!();
+        Mono::start(cx.device.TIMER0, token);
+
+        // Setup LED
+        let port0 = hal::gpio::p0::Parts::new(cx.device.P0);
+        let led = port0.p0_06.into_push_pull_output(Level::Low).degrade();
+
+        // Schedule the blinking task
+        blink::spawn().ok();
+
+        (Shared {}, Local { led })
+    }
+
+    #[task(local = [led])]
+    async fn blink(cx: blink::Context) {
+        let blink::LocalResources { led, .. } = cx.local;
+
+        let mut next_tick = Mono::now();
+        let mut blink_on = false;
+        loop {
+            let now = Mono::now();
+            let now_ms: fugit::SecsDurationU64 = now.duration_since_epoch().convert();
+            defmt::println!("Timer {} ({})", now_ms, now.ticks());
+
+            blink_on = !blink_on;
+            if blink_on {
+                led.set_high().unwrap();
+            } else {
+                led.set_low().unwrap();
+            }
+
+            next_tick += 1000.millis();
+            Mono::delay_until(next_tick).await;
+        }
+    }
+}
diff --git a/examples/nrf52840_blinky/src/lib.rs b/examples/nrf52840_blinky/src/lib.rs
new file mode 100644
index 00000000..7795f2da
--- /dev/null
+++ b/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);
+    }
+}
diff --git a/examples/teensy4_blinky/Cargo.lock b/examples/teensy4_blinky/Cargo.lock
index 408bfe7d..314eae06 100644
--- a/examples/teensy4_blinky/Cargo.lock
+++ b/examples/teensy4_blinky/Cargo.lock
@@ -133,9 +133,9 @@ dependencies = [
 
 [[package]]
 name = "embedded-hal"
-version = "1.0.0-rc.1"
+version = "1.0.0-rc.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "2894bc2f0457b8ca3d6b8ab8aad64d9337583672494013457f86c5a9146c0e22"
+checksum = "3e57ec6ad0bc8eb967cf9c9f144177f5e8f2f6f02dad0b8b683f9f05f6b22def"
 
 [[package]]
 name = "equivalent"
@@ -440,12 +440,12 @@ dependencies = [
 
 [[package]]
 name = "rtic-monotonics"
-version = "1.3.0"
+version = "1.4.0"
 dependencies = [
  "atomic-polyfill",
  "cfg-if",
  "cortex-m",
- "embedded-hal 1.0.0-rc.1",
+ "embedded-hal 1.0.0-rc.2",
  "fugit",
  "imxrt-ral",
  "rtic-time",
@@ -453,7 +453,7 @@ dependencies = [
 
 [[package]]
 name = "rtic-time"
-version = "1.0.0"
+version = "1.1.0"
 dependencies = [
  "critical-section",
  "futures-util",
diff --git a/rtic-monotonics/CHANGELOG.md b/rtic-monotonics/CHANGELOG.md
index eda2351d..d6b43e2e 100644
--- a/rtic-monotonics/CHANGELOG.md
+++ b/rtic-monotonics/CHANGELOG.md
@@ -7,6 +7,12 @@ For each category, *Added*, *Changed*, *Fixed* add new entries at the top!
 
 ## Unreleased
 
+### Fixed
+
+- Race condition in `nrf::timer`.
+- Race condition in `nrf::rtc`.
+- Add internal counter integrity check to all half-period based monotonics.
+
 ## v1.4.0 - 2023-12-04
 
 ### Fixed
diff --git a/rtic-monotonics/src/imxrt.rs b/rtic-monotonics/src/imxrt.rs
index 5f9fc088..ecf9129b 100644
--- a/rtic-monotonics/src/imxrt.rs
+++ b/rtic-monotonics/src/imxrt.rs
@@ -141,13 +141,15 @@ macro_rules! make_timer {
                 // so it gets combined with rollover interrupt
                 ral::write_reg!(ral::gpt, gpt, OCR[1], 0x0000_0000);
 
+                // Initialize timer queue
+                $tq.initialize(Self {});
+
                 // Enable the timer
                 ral::modify_reg!(ral::gpt, gpt, CR, EN: 1);
                 ral::modify_reg!(ral::gpt, gpt, CR,
                     ENMOD: 0,   // Keep state when disabled
                 );
 
-                $tq.initialize(Self {});
 
                 // SAFETY: We take full ownership of the peripheral and interrupt vector,
                 // plus we are not using any external shared resources so we won't impact
@@ -244,13 +246,15 @@ macro_rules! make_timer {
                 let (rollover, half_rollover) = ral::read_reg!(ral::gpt, gpt, SR, ROV, OF1);
 
                 if rollover != 0 {
-                    $period.fetch_add(1, Ordering::Relaxed);
+                    let prev = $period.fetch_add(1, Ordering::Relaxed);
                     ral::write_reg!(ral::gpt, gpt, SR, ROV: 1);
+                    assert!(prev % 2 == 1, "Monotonic must have skipped an interrupt!");
                 }
 
                 if half_rollover != 0 {
-                    $period.fetch_add(1, Ordering::Relaxed);
+                    let prev = $period.fetch_add(1, Ordering::Relaxed);
                     ral::write_reg!(ral::gpt, gpt, SR, OF1: 1);
+                    assert!(prev % 2 == 0, "Monotonic must have skipped an interrupt!");
                 }
             }
         }
diff --git a/rtic-monotonics/src/nrf/rtc.rs b/rtic-monotonics/src/nrf/rtc.rs
index 1f4e34f5..884b523a 100644
--- a/rtic-monotonics/src/nrf/rtc.rs
+++ b/rtic-monotonics/src/nrf/rtc.rs
@@ -44,6 +44,7 @@ use crate::{Monotonic, TimeoutError, TimerQueue};
 use atomic_polyfill::{AtomicU32, Ordering};
 use core::future::Future;
 pub use fugit::{self, ExtU64, ExtU64Ceil};
+use rtic_time::half_period_counter::calculate_now;
 
 #[doc(hidden)]
 #[macro_export]
@@ -92,6 +93,16 @@ macro_rules! create_nrf_rtc2_monotonic_token {
     }};
 }
 
+struct TimerValueU24(u32);
+impl rtic_time::half_period_counter::TimerValue for TimerValueU24 {
+    const BITS: u32 = 24;
+}
+impl From<TimerValueU24> for u64 {
+    fn from(value: TimerValueU24) -> Self {
+        Self::from(value.0)
+    }
+}
+
 macro_rules! make_rtc {
     ($mono_name:ident, $rtc:ident, $overflow:ident, $tq:ident$(, doc: ($($doc:tt)*))?) => {
         /// Monotonic timer queue implementation.
@@ -107,13 +118,49 @@ macro_rules! make_rtc {
             /// Start the timer monotonic.
             pub fn start(rtc: $rtc, _interrupt_token: impl crate::InterruptToken<Self>) {
                 unsafe { rtc.prescaler.write(|w| w.bits(0)) };
-                rtc.intenset.write(|w| w.compare0().set().ovrflw().set());
-                rtc.evtenset.write(|w| w.compare0().set().ovrflw().set());
 
-                rtc.tasks_clear.write(|w| unsafe { w.bits(1) });
-                rtc.tasks_start.write(|w| unsafe { w.bits(1) });
-
-                $tq.initialize(Self {});
+                // Disable interrupts, as preparation
+                rtc.intenclr.write(|w| w
+                    .compare0().clear()
+                    .compare1().clear()
+                    .ovrflw().clear()
+                );
+
+                // Configure compare registers
+                rtc.cc[0].write(|w| unsafe { w.bits(0) }); // Dynamic wakeup
+                rtc.cc[1].write(|w| unsafe { w.bits(0x80_0000) }); // Half-period
+
+                // Timing critical, make sure we don't get interrupted
+                critical_section::with(|_|{
+                    // Reset the timer
+                    rtc.tasks_clear.write(|w| unsafe { w.bits(1) });
+                    rtc.tasks_start.write(|w| unsafe { w.bits(1) });
+
+                    // Clear pending events.
+                    // Should be close enough to the timer reset that we don't miss any events.
+                    rtc.events_ovrflw.write(|w| w);
+                    rtc.events_compare[0].write(|w| w);
+                    rtc.events_compare[1].write(|w| w);
+
+                    // Make sure overflow counter is synced with the timer value
+                    $overflow.store(0, Ordering::SeqCst);
+
+                    // Initialized the timer queue
+                    $tq.initialize(Self {});
+
+                    // Enable interrupts.
+                    // Should be close enough to the timer reset that we don't miss any events.
+                    rtc.intenset.write(|w| w
+                        .compare0().set()
+                        .compare1().set()
+                        .ovrflw().set()
+                    );
+                    rtc.evtenset.write(|w| w
+                        .compare0().set()
+                        .compare1().set()
+                        .ovrflw().set()
+                    );
+                });
 
                 // SAFETY: We take full ownership of the peripheral and interrupt vector,
                 // plus we are not using any external shared resources so we won't impact
@@ -130,12 +177,6 @@ macro_rules! make_rtc {
                 &$tq
             }
 
-            #[inline(always)]
-            fn is_overflow() -> bool {
-                let rtc = unsafe { &*$rtc::PTR };
-                rtc.events_ovrflw.read().bits() == 1
-            }
-
             /// Timeout at a specific time.
             #[inline]
             pub async fn timeout_at<F: Future>(
@@ -181,31 +222,24 @@ macro_rules! make_rtc {
             type Duration = fugit::TimerDurationU64<32_768>;
 
             fn now() -> Self::Instant {
-                // In a critical section to not get a race between overflow updates and reading it
-                // and the flag here.
-                critical_section::with(|_| {
-                    let rtc = unsafe { &*$rtc::PTR };
-                    let cnt = rtc.counter.read().bits();
-                    // OVERFLOW HAPPENS HERE race needs to be handled
-                    let ovf = if Self::is_overflow() {
-                        $overflow.load(Ordering::Relaxed) + 1
-                    } else {
-                        $overflow.load(Ordering::Relaxed)
-                    } as u64;
-
-                    // Check and fix if above race happened
-                    let new_cnt = rtc.counter.read().bits();
-                    let cnt = if new_cnt >= cnt { cnt } else { new_cnt } as u64;
-
-                    Self::Instant::from_ticks((ovf << 24) | cnt)
-                })
+                let rtc = unsafe { &*$rtc::PTR };
+                Self::Instant::from_ticks(calculate_now(
+                    $overflow.load(Ordering::Relaxed),
+                    || TimerValueU24(rtc.counter.read().bits())
+                ))
             }
 
             fn on_interrupt() {
                 let rtc = unsafe { &*$rtc::PTR };
-                if Self::is_overflow() {
-                    $overflow.fetch_add(1, Ordering::SeqCst);
+                if rtc.events_ovrflw.read().bits() == 1 {
                     rtc.events_ovrflw.write(|w| unsafe { w.bits(0) });
+                    let prev = $overflow.fetch_add(1, Ordering::Relaxed);
+                    assert!(prev % 2 == 1, "Monotonic must have skipped an interrupt!");
+                }
+                if rtc.events_compare[1].read().bits() == 1 {
+                    rtc.events_compare[1].write(|w| unsafe { w.bits(0) });
+                    let prev = $overflow.fetch_add(1, Ordering::Relaxed);
+                    assert!(prev % 2 == 0, "Monotonic must have skipped an interrupt!");
                 }
             }
 
@@ -221,7 +255,7 @@ macro_rules! make_rtc {
 
             fn set_compare(instant: Self::Instant) {
                 let rtc = unsafe { &*$rtc::PTR };
-                unsafe { rtc.cc[0].write(|w| w.bits(instant.ticks() as u32 & 0xffffff)) };
+                unsafe { rtc.cc[0].write(|w| w.bits(instant.ticks() as u32 & 0xff_ffff)) };
             }
 
             fn clear_compare_flag() {
diff --git a/rtic-monotonics/src/nrf/timer.rs b/rtic-monotonics/src/nrf/timer.rs
index 0488ca9b..2f83f40b 100644
--- a/rtic-monotonics/src/nrf/timer.rs
+++ b/rtic-monotonics/src/nrf/timer.rs
@@ -30,6 +30,7 @@ use crate::{Monotonic, TimeoutError, TimerQueue};
 use atomic_polyfill::{AtomicU32, Ordering};
 use core::future::Future;
 pub use fugit::{self, ExtU64, ExtU64Ceil};
+use rtic_time::half_period_counter::calculate_now;
 
 #[cfg(feature = "nrf52810")]
 use nrf52810_pac::{self as pac, Interrupt, TIMER0, TIMER1, TIMER2};
@@ -139,17 +140,45 @@ macro_rules! make_timer {
                 // 1 MHz
                 timer.prescaler.write(|w| unsafe { w.prescaler().bits(4) });
                 timer.bitmode.write(|w| w.bitmode()._32bit());
-                timer
-                    .intenset
-                    .modify(|_, w| w.compare0().set().compare1().set());
-                timer.cc[1].write(|w| unsafe { w.cc().bits(0) }); // Overflow
-                timer.tasks_clear.write(|w| unsafe { w.bits(1) });
-                timer.tasks_start.write(|w| unsafe { w.bits(1) });
 
-                $tq.initialize(Self {});
+                // Disable interrupts, as preparation
+                timer.intenclr.modify(|_, w| w
+                    .compare0().clear()
+                    .compare1().clear()
+                    .compare2().clear()
+                );
+
+                // Configure compare registers
+                timer.cc[0].write(|w| unsafe { w.cc().bits(0) }); // Dynamic wakeup
+                timer.cc[1].write(|w| unsafe { w.cc().bits(0x0000_0000) }); // Overflow
+                timer.cc[2].write(|w| unsafe { w.cc().bits(0x8000_0000) }); // Half-period
+
+                // Timing critical, make sure we don't get interrupted
+                critical_section::with(|_|{
+                    // Reset the timer
+                    timer.tasks_clear.write(|w| unsafe { w.bits(1) });
+                    timer.tasks_start.write(|w| unsafe { w.bits(1) });
+
+                    // Clear pending events.
+                    // Should be close enough to the timer reset that we don't miss any events.
+                    timer.events_compare[0].write(|w| w);
+                    timer.events_compare[1].write(|w| w);
+                    timer.events_compare[2].write(|w| w);
 
-                timer.events_compare[0].write(|w| w);
-                timer.events_compare[1].write(|w| w);
+                    // Make sure overflow counter is synced with the timer value
+                    $overflow.store(0, Ordering::SeqCst);
+
+                    // Initialized the timer queue
+                    $tq.initialize(Self {});
+
+                    // Enable interrupts.
+                    // Should be close enough to the timer reset that we don't miss any events.
+                    timer.intenset.modify(|_, w| w
+                        .compare0().set()
+                        .compare1().set()
+                        .compare2().set()
+                    );
+                });
 
                 // SAFETY: We take full ownership of the peripheral and interrupt vector,
                 // plus we are not using any external shared resources so we won't impact
@@ -166,12 +195,6 @@ macro_rules! make_timer {
                 &$tq
             }
 
-            #[inline(always)]
-            fn is_overflow() -> bool {
-                let timer = unsafe { &*$timer::PTR };
-                timer.events_compare[1].read().bits() & 1 != 0
-            }
-
             /// Timeout at a specific time.
             #[inline]
             pub async fn timeout_at<F: Future>(
@@ -216,44 +239,34 @@ macro_rules! make_timer {
             type Duration = fugit::TimerDurationU64<1_000_000>;
 
             fn now() -> Self::Instant {
-                // In a critical section to not get a race between overflow updates and reading it
-                // and the flag here.
-                critical_section::with(|_| {
-                    let timer = unsafe { &*$timer::PTR };
-                    timer.tasks_capture[2].write(|w| unsafe { w.bits(1) });
-                    let cnt = timer.cc[2].read().bits();
-
-                    let unhandled_overflow = if Self::is_overflow() {
-                        // The overflow has not been handled yet, so add an extra to the read overflow.
-                        1
-                    } else {
-                        0
-                    };
-
-                    timer.tasks_capture[2].write(|w| unsafe { w.bits(1) });
-                    let new_cnt = timer.cc[2].read().bits();
-                    let cnt = if new_cnt >= cnt { cnt } else { new_cnt } as u64;
-
-                    Self::Instant::from_ticks(
-                        (unhandled_overflow + $overflow.load(Ordering::Relaxed) as u64) << 32
-                            | cnt as u64,
-                    )
-                })
+                let timer = unsafe { &*$timer::PTR };
+
+                Self::Instant::from_ticks(calculate_now(
+                    $overflow.load(Ordering::Relaxed),
+                    || {
+                        timer.tasks_capture[3].write(|w| unsafe { w.bits(1) });
+                        timer.cc[3].read().bits()
+                    }
+                ))
             }
 
             fn on_interrupt() {
                 let timer = unsafe { &*$timer::PTR };
 
                 // If there is a compare match on channel 1, it is an overflow
-                if Self::is_overflow() {
+                if timer.events_compare[1].read().bits() & 1 != 0 {
                     timer.events_compare[1].write(|w| w);
-                    $overflow.fetch_add(1, Ordering::SeqCst);
+                    let prev = $overflow.fetch_add(1, Ordering::Relaxed);
+                    assert!(prev % 2 == 1, "Monotonic must have skipped an interrupt!");
                 }
-            }
 
-            fn enable_timer() {}
-
-            fn disable_timer() {}
+                // If there is a compare match on channel 2, it is a half-period overflow
+                if timer.events_compare[2].read().bits() & 1 != 0 {
+                    timer.events_compare[2].write(|w| w);
+                    let prev = $overflow.fetch_add(1, Ordering::Relaxed);
+                    assert!(prev % 2 == 0, "Monotonic must have skipped an interrupt!");
+                }
+            }
 
             fn set_compare(instant: Self::Instant) {
                 let timer = unsafe { &*$timer::PTR };
diff --git a/rtic-monotonics/src/stm32.rs b/rtic-monotonics/src/stm32.rs
index c86005ef..601196a3 100644
--- a/rtic-monotonics/src/stm32.rs
+++ b/rtic-monotonics/src/stm32.rs
@@ -272,12 +272,14 @@ macro_rules! make_timer {
                 // Full period
                 if $timer.sr().read().uif() {
                     $timer.sr().modify(|r| r.set_uif(false));
-                    $overflow.fetch_add(1, Ordering::Relaxed);
+                    let prev = $overflow.fetch_add(1, Ordering::Relaxed);
+                    assert!(prev % 2 == 1, "Monotonic must have missed an interrupt!");
                 }
                 // Half period
                 if $timer.sr().read().ccif(2) {
                     $timer.sr().modify(|r| r.set_ccif(2, false));
-                    $overflow.fetch_add(1, Ordering::Relaxed);
+                    let prev = $overflow.fetch_add(1, Ordering::Relaxed);
+                    assert!(prev % 2 == 0, "Monotonic must have missed an interrupt!");
                 }
             }
         }
diff --git a/rtic-time/CHANGELOG.md b/rtic-time/CHANGELOG.md
index 15ebf5b2..2e02a9d0 100644
--- a/rtic-time/CHANGELOG.md
+++ b/rtic-time/CHANGELOG.md
@@ -11,6 +11,8 @@ For each category, *Added*, *Changed*, *Fixed* add new entries at the top!
 
 ### Changed
 
+- Docs: Add sanity check to `half_period_counter` code example
+
 ### Fixed
 
 ## v1.1.0 - 2023-12-04
diff --git a/rtic-time/src/half_period_counter.rs b/rtic-time/src/half_period_counter.rs
index 5d3bf752..0aaec5e2 100644
--- a/rtic-time/src/half_period_counter.rs
+++ b/rtic-time/src/half_period_counter.rs
@@ -96,11 +96,13 @@
 //!     fn on_interrupt() {
 //!         if overflow_interrupt_happened() {
 //!             clear_overflow_interrupt();
-//!             HALF_PERIOD_COUNTER.fetch_add(1, Ordering::Relaxed);
+//!             let prev = HALF_PERIOD_COUNTER.fetch_add(1, Ordering::Relaxed);
+//!             assert!(prev % 2 == 1, "Monotonic must have skipped an interrupt!");
 //!         }
 //!         if compare_interrupt_happened() {
 //!             clear_compare_interrupt();
-//!             HALF_PERIOD_COUNTER.fetch_add(1, Ordering::Relaxed);
+//!             let prev = HALF_PERIOD_COUNTER.fetch_add(1, Ordering::Relaxed);
+//!             assert!(prev % 2 == 0, "Monotonic must have skipped an interrupt!");
 //!         }
 //!     }
 //!