diff --git a/openemc-firmware/memory-big.x b/openemc-firmware/memory-big.x
index f1c983b..4d325f9 100644
--- a/openemc-firmware/memory-big.x
+++ b/openemc-firmware/memory-big.x
@@ -31,4 +31,13 @@ SECTIONS
 }
 INSERT BEFORE .data;
 
+SECTIONS
+{
+  .program_end (NOLOAD) : ALIGN(4)
+  {
+    __flash_program_end = .;
+  } > FLASH
+}
+INSERT AFTER .gnu.sgstubs;
+
 __flash_end = ORIGIN(FLASH) + LENGTH(FLASH);
diff --git a/openemc-firmware/memory-normal.x b/openemc-firmware/memory-normal.x
index 1bd341a..e0c156f 100644
--- a/openemc-firmware/memory-normal.x
+++ b/openemc-firmware/memory-normal.x
@@ -31,4 +31,13 @@ SECTIONS
 }
 INSERT BEFORE .data;
 
+SECTIONS
+{
+  .program_end (NOLOAD) : ALIGN(4)
+  {
+    __flash_program_end = .;
+  } > FLASH
+}
+INSERT AFTER .gnu.sgstubs;
+
 __flash_end = ORIGIN(FLASH) + LENGTH(FLASH);
diff --git a/openemc-firmware/memory-standalone.x b/openemc-firmware/memory-standalone.x
index d52e9ae..03120d8 100644
--- a/openemc-firmware/memory-standalone.x
+++ b/openemc-firmware/memory-standalone.x
@@ -31,4 +31,13 @@ SECTIONS
 }
 INSERT BEFORE .data;
 
+SECTIONS
+{
+  .program_end (NOLOAD) : ALIGN(4)
+  {
+    __flash_program_end = .;
+  } > FLASH
+}
+INSERT AFTER .gnu.sgstubs;
+
 __flash_end = ORIGIN(FLASH) + LENGTH(FLASH);
diff --git a/openemc-firmware/src/cfg.rs b/openemc-firmware/src/cfg.rs
index 8a77f8e..b7e6dca 100644
--- a/openemc-firmware/src/cfg.rs
+++ b/openemc-firmware/src/cfg.rs
@@ -3,7 +3,7 @@
 use core::{mem::MaybeUninit, ptr::addr_of_mut};
 use defmt::Format;
 
-use crate::flash_data::Validate;
+use crate::flash_data::FlashData;
 
 /// Behavior when charger is attached while device is powered off.
 #[derive(Default, Format, Clone, Copy, PartialEq, Eq)]
@@ -29,7 +29,7 @@ impl TryFrom<u8> for ChargerAttached {
 }
 
 /// Configuration stored in flash.
-#[derive(Format, Clone, Copy)]
+#[derive(Format, Clone, Copy, PartialEq, Eq)]
 #[repr(C)]
 pub struct Cfg {
     /// Behavior when charger is attached while device is powered off.
@@ -40,14 +40,7 @@ pub struct Cfg {
     pub prohibit_power_off: bool,
 }
 
-impl Cfg {
-    /// Log configuration change.
-    pub fn log(&self) {
-        defmt::info!("Configuration changed:    {:?}", self);
-    }
-}
-
-impl Validate for Cfg {
+impl FlashData for Cfg {
     fn validate(mut this: MaybeUninit<Self>) -> Self {
         unsafe {
             let ptr = this.as_mut_ptr();
@@ -65,4 +58,8 @@ impl Validate for Cfg {
             this.assume_init()
         }
     }
+
+    fn notify_changed(&self) {
+        defmt::info!("Configuration changed:    {:?}", self);
+    }
 }
diff --git a/openemc-firmware/src/flash_data.rs b/openemc-firmware/src/flash_data.rs
index f0d0b44..d63129f 100644
--- a/openemc-firmware/src/flash_data.rs
+++ b/openemc-firmware/src/flash_data.rs
@@ -2,7 +2,7 @@
 
 use core::{
     mem::{size_of, zeroed, MaybeUninit},
-    ops::{Deref, DerefMut},
+    ops::Deref,
     slice,
 };
 use defmt::assert;
@@ -11,10 +11,12 @@ use stm32f1xx_hal::flash::FlashWriter;
 
 use crate::{flash_page_size, util::FlashUtil};
 
-/// Validate data loaded from flash.
-pub trait Validate: Sized {
+/// Data storable in flash.
+pub trait FlashData: Sized + Copy + Eq + Sized + 'static {
     /// Validate and fix data as needed so that it becomes a valid Rust value.
     fn validate(this: MaybeUninit<Self>) -> Self;
+    /// Notify data that it was changed (for logging).
+    fn notify_changed(&self) {}
 }
 
 #[derive(Clone, Copy, PartialEq, Eq)]
@@ -35,12 +37,14 @@ pub struct FlashBackened<T> {
 
 impl<T> FlashBackened<T>
 where
-    T: Validate + Copy + Sized + 'static,
+    T: FlashData,
 {
     const HEADER_SIZE: usize = 8;
 
     /// Create copies at specified offsets in flash.
-    pub fn new_at(flash: &mut FlashWriter, flash_offset1: u32, flash_offset2: u32, reserved: usize) -> Self {
+    pub fn new_at(
+        flash: &mut FlashWriter, flash_offset1: u32, flash_offset2: u32, reserved: usize, erase: bool,
+    ) -> Self {
         assert!(size_of::<T>() % 2 == 0, "data must have even size");
         assert!(reserved % flash_page_size() == 0, "reserved size must be a multiple of flash page size");
         assert!(
@@ -58,15 +62,21 @@ where
             version: 0,
             next_save: FlashCopy::Copy1,
         };
-        this.load(flash);
+
+        if erase {
+            this.erase(flash);
+        } else {
+            this.load(flash);
+        }
+
         this
     }
 
     /// Create copies with end offset in flash specified.
-    pub fn new_at_end(flash: &mut FlashWriter, reserved: usize, end_offset: u32) -> Self {
+    pub fn new_at_end(flash: &mut FlashWriter, reserved: usize, end_offset: u32, erase: bool) -> Self {
         let flash_offset2 = end_offset - reserved as u32;
         let flash_offset1 = flash_offset2 - reserved as u32;
-        Self::new_at(flash, flash_offset1, flash_offset2, reserved)
+        Self::new_at(flash, flash_offset1, flash_offset2, reserved, erase)
     }
 
     /// Read copy from flash and verify CRC32.
@@ -125,11 +135,12 @@ where
 
         self.version = version;
         self.data = T::validate(data);
+        self.data.notify_changed();
         self.next_save = next_save;
     }
 
     /// Save data to flash.
-    pub fn save(&mut self, flash: &mut FlashWriter) {
+    fn save(&mut self, flash: &mut FlashWriter) {
         self.version = self.version.wrapping_add(1);
         self.next_save = match self.next_save {
             FlashCopy::Copy1 => {
@@ -143,10 +154,28 @@ where
         };
     }
 
-    /// Erase data in flash.
-    pub fn erase(&self, flash: &mut FlashWriter) {
-        flash.erase_unwrap(self.flash_offset1, self.reserved + Self::HEADER_SIZE);
-        flash.erase_unwrap(self.flash_offset2, self.reserved + Self::HEADER_SIZE);
+    /// Erase data in flash and init with defaults.
+    pub fn erase(&mut self, flash: &mut FlashWriter) {
+        flash.erase_unwrap(self.flash_offset1, self.reserved);
+        flash.erase_unwrap(self.flash_offset2, self.reserved);
+
+        self.version = 0;
+        self.data = T::validate(unsafe { zeroed() });
+        self.data.notify_changed();
+        self.next_save = FlashCopy::Copy1;
+    }
+
+    /// Modify data and save to flash if changed.
+    pub fn modify(&mut self, flash: &mut FlashWriter, modify: impl FnOnce(&mut T)) {
+        let old_data = self.data;
+        modify(&mut self.data);
+
+        if old_data == self.data {
+            return;
+        }
+
+        self.data.notify_changed();
+        self.save(flash);
     }
 }
 
@@ -157,9 +186,3 @@ impl<T> Deref for FlashBackened<T> {
         &self.data
     }
 }
-
-impl<T> DerefMut for FlashBackened<T> {
-    fn deref_mut(&mut self) -> &mut Self::Target {
-        &mut self.data
-    }
-}
diff --git a/openemc-firmware/src/main.rs b/openemc-firmware/src/main.rs
index c2cddce..4beabd9 100644
--- a/openemc-firmware/src/main.rs
+++ b/openemc-firmware/src/main.rs
@@ -149,6 +149,9 @@ static mut BOOTLOADER_LOG_REF: Option<
 > = None;
 
 extern "C" {
+    /// End of program in flash (from linker).
+    pub static __flash_program_end: c_void;
+
     /// End of flash (from linker).
     pub static __flash_end: c_void;
 
@@ -156,6 +159,11 @@ extern "C" {
     pub static __flash_page_size: c_void;
 }
 
+/// End of program in flash.
+pub fn flash_program_end() -> usize {
+    unsafe { &__flash_program_end as *const _ as usize }
+}
+
 /// End of user flash.
 pub fn flash_end() -> usize {
     unsafe { &__flash_end as *const _ as usize }
@@ -357,8 +365,14 @@ mod app {
         let bi = BootInfo::get();
 
         // Load configuration from flash.
+        let cfg_size = flash_page_size();
+        defmt::assert!(flash_program_end() <= flash_end() - 2 * cfg_size, "no space for configuration in flash");
+        let erase_cfg = bi.boot_reason == BootReason::FactoryReset as _;
+        if erase_cfg {
+            defmt::info!("Erasing configuration due to factory reset");
+        }
         let mut fw = FlashWriter::new(&mut flash);
-        let cfg = FlashBackened::new_at_end(&mut fw, flash_page_size(), flash_end() as u32 - FLASH_START);
+        let cfg = FlashBackened::new_at_end(&mut fw, cfg_size, flash_end() as u32 - FLASH_START, erase_cfg);
 
         // Create board handler.
         defmt::info!("board new");
@@ -1409,20 +1423,18 @@ mod app {
                 respond_u8(cx.shared.cfg.lock(|cfg| cfg.prohibit_power_off.into()))
             }
             Event::Write { reg: reg::POWER_OFF_PROHIBITED, value } => {
-                (cx.shared.flash, cx.shared.cfg).lock(|mut flash, cfg| {
-                    cfg.prohibit_power_off = value.as_u8() != 0;
-                    cfg.save(&mut FlashWriter::new(&mut flash));
-                    cfg.log();
+                (cx.shared.flash, cx.shared.cfg).lock(|flash, cfg| {
+                    cfg.modify(&mut FlashWriter::new(flash), |cfg| cfg.prohibit_power_off = value.as_u8() != 0);
                 })
             }
             Event::Read { reg: reg::POWER_ON_BY_CHARGING } => {
                 respond_u8(cx.shared.cfg.lock(|cfg| cfg.charger_attached as u8))
             }
             Event::Write { reg: reg::POWER_ON_BY_CHARGING, value } => {
-                (cx.shared.flash, cx.shared.cfg).lock(|mut flash, cfg| {
-                    cfg.charger_attached = value.as_u8().try_into().unwrap_or_default();
-                    cfg.save(&mut FlashWriter::new(&mut flash));
-                    cfg.log();
+                (cx.shared.flash, cx.shared.cfg).lock(|flash, cfg| {
+                    cfg.modify(&mut FlashWriter::new(flash), |cfg| {
+                        cfg.charger_attached = value.as_u8().try_into().unwrap_or_default()
+                    });
                 })
             }
             Event::Read { reg: reg::PWM_TIMERS } => respond_u8(cx.local.pwm_timers.len() as u8),
diff --git a/openemc-firmware/src/util.rs b/openemc-firmware/src/util.rs
index 47b7459..e1a00db 100644
--- a/openemc-firmware/src/util.rs
+++ b/openemc-firmware/src/util.rs
@@ -85,6 +85,7 @@ pub fn unique_device_id() -> u128 {
 /// Flash utility function.
 pub trait FlashUtil {
     /// Create flash writer.
+    #[allow(clippy::new_ret_no_self)]
     fn new(parts: &mut flash::Parts) -> FlashWriter;
 
     /// Read and panic if failed.