diff --git a/libconfig/src/lib.rs b/libconfig/src/lib.rs
index 4e5ff30..f07fb8f 100644
--- a/libconfig/src/lib.rs
+++ b/libconfig/src/lib.rs
@@ -3,7 +3,7 @@ extern crate alloc;
 
 use core::fmt;
 use alloc::{string::FromUtf8Error, string::String, vec::Vec, rc::Rc};
-use core_io::{self as io, BufRead, BufReader, Read};
+use core_io::{self as io, BufRead, BufReader, Read, Write, Seek, ErrorKind, SeekFrom};
 use libboard_zynq::sdio;
 
 pub mod sd_reader;
@@ -66,6 +66,38 @@ fn parse_config<'a>(
     }
     Err(Error::KeyNotFoundError(key))
 }
+fn delete_old_entry<'a>(
+    key: &str,
+    file: fatfs::File<sd_reader::SdReader>,
+    mut file_tmp: fatfs::File<sd_reader::SdReader>,
+) -> Result<'a, ()> {
+    let prefix = [key, "="].concat();
+    let buf_reader = BufReader::new(file);
+
+    for line in buf_reader.lines() {
+        let line = line?;
+        if !line.starts_with(&prefix) {
+            file_tmp.write_all(&[line.as_str(), "\n"].concat().as_bytes())?;
+        }
+    }
+    Ok(())
+}
+fn rename_file<'a>(dir: &fatfs::Dir<sd_reader::SdReader>, old_file_name: &str, new_file_name: &str) -> Result<'a, ()>{
+    {
+        let old_file = dir.open_file(old_file_name)?;
+        let mut new_file = dir.create_file(new_file_name)?;
+        new_file.truncate()?;
+
+        for line in BufReader::new(old_file).lines() {
+            let line = line?;
+            new_file.write_all(&[line.as_str(), "\n"].concat().as_bytes())?;
+        }
+    }
+
+    dir.remove(old_file_name)?;
+
+    Ok(())
+}
 
 pub struct Config {
     fs: Option<Rc<fatfs::FileSystem<sd_reader::SdReader>>>,
@@ -112,4 +144,76 @@ impl Config {
     pub fn read_str<'b>(&self, key: &'b str) -> Result<'b, String> {
         Ok(String::from_utf8(self.read(key)?)?)
     }
+
+    pub fn write_str<'b>(&mut self, key: &str, data: &str) -> Result<'b, ()>{
+        if let Some(fs) = &self.fs {
+            let root_dir = fs.root_dir();
+
+            let config_key_bin = &["/CONFIG/", key, ".BIN"].concat();
+            let config_txt_tmp = "/CONFIG.TMP";
+
+            if data.is_ascii() & (data.len() <= 100) {
+                match root_dir.create_file("/CONFIG.TXT") {
+                    Ok(file) => {
+                        let mut file_tmp = root_dir.create_file(config_txt_tmp)?;
+                        file_tmp.truncate()?;
+                        delete_old_entry(key, file, file_tmp)?;
+                    },
+                    Err(e) => {
+                        return Err(Error::IoError(e));
+                    }
+                };
+
+                rename_file(&root_dir, config_txt_tmp, "/CONFIG.TXT")?;
+
+                let mut file = root_dir.open_file("/CONFIG.TXT")?;
+                file.seek(SeekFrom::End(0))?;
+                file.write_all(&["\n", key, "=", data, "\n"].concat().as_bytes())?;
+
+                match root_dir.remove(config_key_bin) {
+                    Ok(_) => {},
+                    Err(e) => match e.kind() {
+                        ErrorKind::NotFound => {},
+                        _ => {
+                            return Err(Error::IoError(e));
+                        }
+                    }
+                };
+            } else {
+                root_dir.create_dir("/CONFIG")?;
+                match root_dir.create_file(config_key_bin) {
+                    Ok(mut file) => {
+                        file.truncate()?;
+                        file.write_all(&[data, "\n"].concat().as_bytes())?;
+                    },
+                    Err(e) => {
+                        return Err(Error::IoError(e));
+                    }
+                };
+
+                let mut need_to_rename = false;
+                match root_dir.open_file("/CONFIG.TXT") {
+                    Ok(file) => {
+                        need_to_rename = true;
+                        let mut file_tmp = root_dir.create_file(config_txt_tmp)?;
+                        file_tmp.truncate()?;
+                        delete_old_entry(key, file, file_tmp)?;
+                    },
+                    Err(e) => match e.kind() {
+                        ErrorKind::NotFound => {},
+                        _ => {
+                            return Err(Error::IoError(e));
+                        }
+                    }
+                };
+                if need_to_rename { // docs: ensure no ref to files
+                    rename_file(&root_dir, config_txt_tmp, "/CONFIG.TXT")?;
+                }
+            }
+
+            Ok(())
+        } else {
+            Err(Error::NoConfig)
+        }
+    }
 }