Reset SAADC in Drop impl for nrf52, workaround for anomaly 241.

3 files changed, 76 insertions, 0 deletions

diff --git a/embassy-nrf/CHANGELOG.md b/embassy-nrf/CHANGELOG.md
index b8d03a1f8..3ad3c8005 100644
--- a/embassy-nrf/CHANGELOG.md
+++ b/embassy-nrf/CHANGELOG.md
@@ -17,6 +17,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - changed: impl Drop for Timer
 - added: expose `regs` for timer driver
 - added: timer driver CC `clear_events` method
+- changed: Saadc reset in Drop impl, anomaly 241 - high power usage
 
 ## 0.7.0 - 2025-08-26
 
diff --git a/embassy-nrf/src/saadc.rs b/embassy-nrf/src/saadc.rs
index 92b6fb01f..d84246572 100644
--- a/embassy-nrf/src/saadc.rs
+++ b/embassy-nrf/src/saadc.rs
@@ -457,6 +457,19 @@ impl<'d> Saadc<'d, 1> {
 
 impl<'d, const N: usize> Drop for Saadc<'d, N> {
     fn drop(&mut self) {
+        // Reset of SAADC.
+        //
+        // This is needed when more than one pin is sampled to avoid needless power consumption.
+        // More information can be found in [nrf52 Anomaly 241](https://docs.nordicsemi.com/bundle/errata_nRF52810_Rev1/page/ERR/nRF52810/Rev1/latest/anomaly_810_241.html).
+        // The workaround seems like it copies the configuration before reset and reapplies it after.
+        // This method consumes the instance forcing a reconfiguration at compile time, hence we only
+        // call what is the reset portion of the workaround.
+        #[cfg(feature = "_nrf52")]
+        {
+            unsafe { core::ptr::write_volatile(0x40007FFC as *mut u32, 0) }
+            unsafe { core::ptr::read_volatile(0x40007FFC as *const ()) }
+            unsafe { core::ptr::write_volatile(0x40007FFC as *mut u32, 1) }
+        }
         let r = Self::regs();
         r.enable().write(|w| w.set_enable(false));
         for i in 0..N {
diff --git a/examples/nrf52810/src/bin/saadc_lowpower.rs b/examples/nrf52810/src/bin/saadc_lowpower.rs
new file mode 100644
index 000000000..d7e7f09a4
--- /dev/null
+++ b/examples/nrf52810/src/bin/saadc_lowpower.rs
@@ -0,0 +1,62 @@
+//! Run SAADC on multiple pins only every 3rd time, to show anomaly 241 workaround.
+//!
+//! To correctly measure the MCU current on the NRF52DK follow the instructions
+//! <https://docs.nordicsemi.com/bundle/ug_nrf52832_dk/page/UG/dk/prepare_board.html>
+//! otherwise you will measure the whole board, including the segger j-link chip for example
+
+#![no_std]
+#![no_main]
+
+use defmt::info;
+use embassy_executor::Spawner;
+use embassy_nrf::gpio::{Level, Output, OutputDrive};
+use embassy_nrf::saadc::{Oversample, Saadc};
+use embassy_nrf::{bind_interrupts, saadc};
+use embassy_time::Timer;
+use {defmt_rtt as _, panic_probe as _};
+
+bind_interrupts!(struct Irqs {
+        SAADC => saadc::InterruptHandler;
+});
+
+#[embassy_executor::main]
+async fn main(_p: Spawner) {
+    let mut p = embassy_nrf::init(Default::default());
+
+    // For PPK2 digital channel plot to track when SAADC is on/off.
+    let mut ppk2_d0 = Output::new(p.P0_27, Level::Low, OutputDrive::Standard);
+    let mut num_loops: usize = 0;
+    loop {
+        num_loops += 1;
+        if num_loops.is_multiple_of(3) {
+            ppk2_d0.set_high();
+            let battery_pin = p.P0_02.reborrow();
+            let sensor1_pin = p.P0_03.reborrow();
+            let mut adc_config = saadc::Config::default();
+            adc_config.oversample = Oversample::OVER4X;
+            let battery = saadc::ChannelConfig::single_ended(battery_pin);
+            let sensor1 = saadc::ChannelConfig::single_ended(sensor1_pin);
+            let mut saadc = Saadc::new(p.SAADC.reborrow(), Irqs, adc_config, [battery, sensor1]);
+            // Indicated: wait for ADC calibration.
+            saadc.calibrate().await;
+            let mut buf = [0; 2];
+            info!("sampling...");
+            saadc.sample(&mut buf).await;
+            info!("data: {:x}", buf);
+
+            // Sleep to show the high power usage on the plot, even though sampling is done.
+            Timer::after_millis(100).await;
+            ppk2_d0.set_low();
+            // disable the following line to show the anomaly on the power profiler plot.
+            core::mem::drop(saadc);
+            // Sleep to show the power usage when drop did not happen.
+            Timer::after_millis(100).await;
+            // worst case drop happens here
+        } else {
+            info!("waiting");
+        }
+        // Sleep for 1 second. The executor ensures the core sleeps with a WFE when it has nothing to do.
+        // During this sleep, the nRF chip should only use ~3uA
+        Timer::after_secs(1).await;
+    }
+}