diff --git a/CHANGELOG.md b/CHANGELOG.md
index d2be11b..b8b0ed3 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -15,6 +15,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Add Windows native driver support, #39
 - Use loaded memory address from ELF file or ihex file
 
+### Fixed
+
+- Merge gaps in firmware sections, #56
+
 ### Changed
 
 - No erase by default when flashing
diff --git a/src/commands/mod.rs b/src/commands/mod.rs
index a2cefd0..d5e8960 100644
--- a/src/commands/mod.rs
+++ b/src/commands/mod.rs
@@ -91,7 +91,7 @@ impl<const N: u8> fmt::Debug for RawCommand<N> {
     }
 }
 
-/// Set address and offset of the firmware, 0x01.
+/// 0x01 - Set address and offset of the firmware
 #[derive(Debug)]
 pub struct SetWriteMemoryRegion {
     // 0x08000000 or 0x00000000
@@ -126,14 +126,14 @@ impl Command for SetReadMemoryRegion {
     }
 }
 
-/// 0x02 subset
+/// 0x02 - Flash or Memory operations
 #[derive(Copy, Clone, Debug, PartialEq, Eq)]
 pub enum Program {
     // wlink_erase
     EraseFlash = 0x01,
     // Before write firmware bytes, choice between 0x02 and 0x04
     WriteFlash = 0x02,
-    // after write flash
+    // Write flash
     WriteFlashAndVerify = 0x04,
     /// Write Flash OP
     WriteFlashOP = 0x05,
diff --git a/src/firmware.rs b/src/firmware.rs
index 2269ef4..64896dc 100644
--- a/src/firmware.rs
+++ b/src/firmware.rs
@@ -39,7 +39,7 @@ pub enum Firmware {
 }
 
 impl Firmware {
-    /// Merge sections w/ <= 256 bytes gap
+    /// Merge sections, and fill gap with 0xff
     pub fn merge_sections(self) -> Result<Self> {
         if let Firmware::Sections(mut sections) = self {
             sections.sort_by_key(|s| s.address);
@@ -49,16 +49,14 @@ impl Firmware {
             let mut last = it
                 .next()
                 .expect("firmware must has at least one section; qed");
+
             for sect in it {
                 if let Some(gap) = sect.address.checked_sub(last.end_address()) {
-                    if gap > 256 {
-                        merged.push(last);
-                        last = sect.clone();
-                        continue;
-                    } else {
-                        last.data.resize(last.data.len() + gap as usize, 0);
-                        last.data.extend_from_slice(&sect.data);
+                    if gap > 0 {
+                        log::debug!("Merge firmware sections with gap: {}", gap);
                     }
+                    last.data.resize(last.data.len() + gap as usize, 0xff); // fill gap with 0xff
+                    last.data.extend_from_slice(&sect.data);
                 } else {
                     return Err(anyhow::format_err!(
                         "section address overflow: {:#010x} + {:#x}",