diff --git a/README.md b/README.md
index 7aaeaa1..7f3a055 100644
--- a/README.md
+++ b/README.md
@@ -19,6 +19,7 @@ git submodule update --init --recursive
 | Project | Description | Target Board |
 |---|---|---|
 | [color_switcher](color_switcher/) | Interactive color-switching firmware with USB-CDC serial commands and WS2812B LED control via PIO | Waveshare RP2040-Zero |
+| [bootloader](bootloader/) | Custom firmware bootloader — receives updates over UART/USB, writes to flash, jumps to app | RP2040 (any board) |
 
 ## Shared Components (git submodules)
 
@@ -63,6 +64,7 @@ pico/
 │   ├── MCU_PIO/                 # generic PIO driver + ws2812.pio
 │   ├── HAL_COM/                 # communication abstraction
 │   └── HAL_LED/                 # LED strip abstraction
+├── bootloader/                  # project: custom firmware bootloader
 ├── color_switcher/              # project: WS2812B color switcher
 │   ├── cmake/                   # modular CMake build system
 │   ├── src/                     # project-specific components (APP_CLSW, SYS_ECU)
diff --git a/bootloader/CLAUDE.md b/bootloader/CLAUDE.md
new file mode 100644
index 0000000..2055bb6
--- /dev/null
+++ b/bootloader/CLAUDE.md
@@ -0,0 +1,24 @@
+# CLAUDE.md
+
+This file provides guidance to Claude Code (claude.ai/code) when working with code in this repository.
+
+## Project
+
+Custom bootloader for the Raspberry Pi Pico (RP2040). Receives firmware over a communication interface, writes to flash, and jumps to the main application.
+
+## Build System
+
+All builds run inside Docker — no local toolchain required.
+
+- `docker compose run --rm pico-build bash /scripts/build.sh` — compile the firmware
+- `./flash.sh bootloader` — flash to Pico (from repo root)
+
+## Architecture
+
+- `src/APP_BOOT/{inc,prg,cfg}/` — bootloader application logic (firmware receive, flash write, app jump)
+- `src/SYS_ECU/{inc,prg,cfg}/` — system orchestrator (init sequence, main loop)
+- Shared components from `common/` (STD_TYPES, MCU_UART, MCU_USB, HAL_COM, etc.)
+
+## Conventions
+
+Same as the repo-level CLAUDE.md — MISRA-C style, STD_TYPES, Hungarian naming, descriptive comments, no magic numbers.
\ No newline at end of file
diff --git a/bootloader/Dockerfile b/bootloader/Dockerfile
new file mode 100644
index 0000000..424c0c7
--- /dev/null
+++ b/bootloader/Dockerfile
@@ -0,0 +1,23 @@
+# Base image: Ubuntu 24.04 for ARM cross-compilation toolchain
+FROM ubuntu:24.04
+
+# Install cross-compilation dependencies + clangd for dev container intellisense
+RUN apt-get update && apt-get install -y \
+    cmake \
+    gcc-arm-none-eabi \
+    libnewlib-arm-none-eabi \
+    build-essential \
+    git \
+    python3 \
+    clangd \
+    && rm -rf /var/lib/apt/lists/*
+
+# Clone the Pico SDK with submodules (TinyUSB, etc.)
+RUN git clone https://github.com/raspberrypi/pico-sdk.git /opt/pico-sdk \
+    && cd /opt/pico-sdk \
+    && git submodule update --init
+
+# Tell CMake where to find the Pico SDK
+ENV PICO_SDK_PATH=/opt/pico-sdk
+
+WORKDIR /project
\ No newline at end of file
diff --git a/bootloader/README.md b/bootloader/README.md
new file mode 100644
index 0000000..45da7cb
--- /dev/null
+++ b/bootloader/README.md
@@ -0,0 +1,26 @@
+# Bootloader
+
+A custom bootloader for the Raspberry Pi Pico (RP2040). Receives firmware updates over a communication interface, writes to flash, and jumps to the main application.
+
+## Status
+
+**Scaffolded — not yet implemented.** See `TODO.md` for the design decisions and implementation tasks.
+
+## Prerequisites
+
+- [Docker](https://docs.docker.com/get-docker/) and Docker Compose
+
+## Building
+
+```bash
+cd bootloader/
+docker compose run --rm pico-build bash /scripts/build.sh
+```
+
+## Flashing
+
+From the repo root:
+
+```bash
+./flash.sh bootloader
+```
diff --git a/bootloader/TODO.md b/bootloader/TODO.md
new file mode 100644
index 0000000..3311f2f
--- /dev/null
+++ b/bootloader/TODO.md
@@ -0,0 +1,61 @@
+# Bootloader — Design Decisions & TODO
+
+## Open Design Decisions
+
+These must be resolved before implementation begins:
+
+### 1. Firmware update interface
+- [ ] UART only
+- [ ] USB-CDC only
+- [ ] Both (selectable via config or auto-detect)
+- **Notes:** We have MCU_UART and MCU_USB in common/ ready to use via HAL_COM.
+
+### 2. Transfer protocol
+- [ ] Custom simple protocol (length + CRC32 + raw binary)
+- [ ] XMODEM / YMODEM (standard, many terminal tools support it)
+- [ ] Custom framed protocol (start byte, length, payload, CRC, end byte)
+- **Notes:** Custom is simplest to implement. XMODEM is widely supported by serial tools.
+
+### 3. Flash memory layout
+- [ ] Define bootloader size (e.g., first 16 KB, 32 KB, or 64 KB of flash)
+- [ ] Define application start address
+- [ ] Define metadata region (app version, CRC, valid flag)
+- **Notes:** RP2040 has 2 MB flash. Pico SDK linker script needs modification to place the bootloader and app at different addresses.
+
+### 4. Bootloader entry trigger
+- [ ] Always enter bootloader, wait N seconds for firmware, then jump to app
+- [ ] Check a GPIO pin (button held = stay in bootloader)
+- [ ] Check a magic value in flash/RAM (set by the application to request update)
+- [ ] Combination: check button first, then magic value, then jump to app
+- **Notes:** "Button + timeout" is the most user-friendly approach.
+
+### 5. Application jump mechanism
+- [ ] Read the app's vector table at the app start address
+- [ ] Set the MSP (main stack pointer) from the vector table
+- [ ] Jump to the reset handler from the vector table
+- **Notes:** Standard Cortex-M0+ boot sequence. Need to disable interrupts and peripherals before jumping.
+
+### 6. Integrity verification
+- [ ] CRC32 check on the received firmware before writing to flash
+- [ ] CRC32 check on the stored application before jumping (boot-time validation)
+- [ ] Signature verification (stretch goal — not needed for initial version)
+
+### 7. Flash write strategy
+- [ ] Erase + write as chunks arrive (streaming)
+- [ ] Buffer entire firmware in RAM, then erase + write at once (limited by RAM size)
+- [ ] Dual-bank: write to alternate region, swap on success (A/B update)
+- **Notes:** Streaming is most memory-efficient. A/B is safest (rollback on failure) but uses 2x flash.
+
+## Implementation Tasks
+
+Once design decisions are made:
+
+- [ ] Create linker scripts (bootloader.ld, application.ld) with correct flash regions
+- [ ] Implement APP_BOOT_enuInit — check entry trigger, decide boot vs update mode
+- [ ] Implement firmware receive via HAL_COM (protocol parsing, CRC validation)
+- [ ] Implement flash write using Pico SDK hardware_flash API
+- [ ] Implement application jump (disable interrupts, set MSP, branch to reset handler)
+- [ ] Implement boot-time app validation (CRC check before jumping)
+- [ ] Modify color_switcher's linker script to start at the app address (not 0x10000000)
+- [ ] Test: flash bootloader, then send color_switcher firmware over UART/USB
+- [ ] Test: corrupt firmware scenario — bootloader should stay in update mode
\ No newline at end of file
diff --git a/bootloader/cmake/CMakeLists.txt b/bootloader/cmake/CMakeLists.txt
new file mode 100644
index 0000000..7ba900d
--- /dev/null
+++ b/bootloader/cmake/CMakeLists.txt
@@ -0,0 +1,28 @@
+# ============================================================================
+# Top-level CMakeLists.txt for the bootloader project
+# ============================================================================
+# Same phase-ordered structure as color_switcher.
+
+cmake_minimum_required(VERSION 3.13)
+
+get_filename_component(PROJECT_ROOT_DIR "${CMAKE_CURRENT_SOURCE_DIR}/.." ABSOLUTE)
+
+list(APPEND CMAKE_MODULE_PATH ${CMAKE_CURRENT_SOURCE_DIR}/cmake_config)
+
+include(project_config)
+include(mcu_config)
+
+mcu_init()
+
+project(${PROJECT_NAME}
+        VERSION   ${PROJECT_VERSION}
+        LANGUAGES ${PROJECT_LANGUAGES})
+
+mcu_sdk_config()
+
+include(sources_config)
+
+add_executable(${PROJECT_NAME} ${PROJECT_SOURCES})
+target_include_directories(${PROJECT_NAME} PRIVATE ${PROJECT_INCLUDE_DIRS})
+
+mcu_link_target(${PROJECT_NAME})
\ No newline at end of file
diff --git a/bootloader/cmake/cmake_config/mcu_config.cmake b/bootloader/cmake/cmake_config/mcu_config.cmake
new file mode 100644
index 0000000..d9f7f08
--- /dev/null
+++ b/bootloader/cmake/cmake_config/mcu_config.cmake
@@ -0,0 +1,34 @@
+# ============================================================================
+# mcu_config.cmake — RP2040 / Pico SDK configuration for the bootloader
+# ============================================================================
+
+# Step 1/3: called BEFORE project() — bootstrap the Pico SDK toolchain
+macro(mcu_init)
+    include(${CMAKE_SOURCE_DIR}/pico_sdk_import.cmake)
+endmacro()
+
+# Step 2/3: called AFTER project() — register SDK targets
+macro(mcu_sdk_config)
+    set(CMAKE_C_STANDARD          ${PROJECT_C_STANDARD})
+    set(CMAKE_C_STANDARD_REQUIRED ON)
+    pico_sdk_init()
+endmacro()
+
+# Step 3/3: called AFTER add_executable() — link SDK libs + configure target
+function(mcu_link_target target)
+    # Libraries needed by the bootloader.
+    # hardware_flash is critical — the bootloader writes firmware to flash.
+    target_link_libraries(${target} PRIVATE
+        pico_stdlib
+        hardware_uart
+        hardware_dma
+        hardware_flash
+    )
+
+    # Route stdio over USB-CDC for bootloader status messages
+    pico_enable_stdio_usb(${target}  1)
+    pico_enable_stdio_uart(${target} 0)
+
+    # Generate .uf2 for initial bootloader flashing via BOOTSEL
+    pico_add_extra_outputs(${target})
+endfunction()
diff --git a/bootloader/cmake/cmake_config/project_config.cmake b/bootloader/cmake/cmake_config/project_config.cmake
new file mode 100644
index 0000000..4147bf9
--- /dev/null
+++ b/bootloader/cmake/cmake_config/project_config.cmake
@@ -0,0 +1,11 @@
+# ============================================================================
+# project_config.cmake — Bootloader project identity and language settings
+# ============================================================================
+
+set(PROJECT_NAME       Bootloader_PICO)
+set(PROJECT_VERSION    0.1.0)
+set(PROJECT_LANGUAGES  C CXX ASM)
+set(PROJECT_C_STANDARD 11)
+
+# Generate compile_commands.json for clangd intellisense
+set(CMAKE_EXPORT_COMPILE_COMMANDS ON)
diff --git a/bootloader/cmake/cmake_config/sources_config.cmake b/bootloader/cmake/cmake_config/sources_config.cmake
new file mode 100644
index 0000000..56864ce
--- /dev/null
+++ b/bootloader/cmake/cmake_config/sources_config.cmake
@@ -0,0 +1,37 @@
+# ============================================================================
+# sources_config.cmake — Source files and include paths for the bootloader
+# ============================================================================
+
+set(COMMON_DIR "${PROJECT_ROOT_DIR}/../common")
+
+# Collect .c files from project src/ and common submodules
+file(GLOB_RECURSE PROJECT_SOURCES CONFIGURE_DEPENDS
+     "${PROJECT_ROOT_DIR}/src/*.c"
+     "${COMMON_DIR}/STD_TYPES/*.c"
+     "${COMMON_DIR}/MCU_UART/*.c"
+     "${COMMON_DIR}/MCU_USB/*.c"
+     "${COMMON_DIR}/HAL_COM/*.c")
+
+set(PROJECT_INCLUDE_DIRS
+    # Common components (git submodules)
+    ${COMMON_DIR}/STD_TYPES/inc
+
+    ${COMMON_DIR}/MCU_UART/inc
+    ${COMMON_DIR}/MCU_UART/prg
+    ${COMMON_DIR}/MCU_UART/cfg
+
+    ${COMMON_DIR}/MCU_USB/inc
+    ${COMMON_DIR}/MCU_USB/prg
+    ${COMMON_DIR}/MCU_USB/cfg
+
+    ${COMMON_DIR}/HAL_COM/inc
+    ${COMMON_DIR}/HAL_COM/prg
+    ${COMMON_DIR}/HAL_COM/cfg
+
+    # Project-specific components
+    ${PROJECT_ROOT_DIR}/src/APP_BOOT/inc
+    ${PROJECT_ROOT_DIR}/src/APP_BOOT/prg
+    ${PROJECT_ROOT_DIR}/src/APP_BOOT/cfg
+
+    ${PROJECT_ROOT_DIR}/src/SYS_ECU/prg
+)
\ No newline at end of file
diff --git a/bootloader/cmake/pico_sdk_import.cmake b/bootloader/cmake/pico_sdk_import.cmake
new file mode 100644
index 0000000..d493cc2
--- /dev/null
+++ b/bootloader/cmake/pico_sdk_import.cmake
@@ -0,0 +1,121 @@
+# This is a copy of <PICO_SDK_PATH>/external/pico_sdk_import.cmake
+
+# This can be dropped into an external project to help locate this SDK
+# It should be include()ed prior to project()
+
+# Copyright 2020 (c) 2020 Raspberry Pi (Trading) Ltd.
+#
+# Redistribution and use in source and binary forms, with or without modification, are permitted provided that the
+# following conditions are met:
+#
+# 1. Redistributions of source code must retain the above copyright notice, this list of conditions and the following
+# disclaimer.
+#
+# 2. Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following
+# disclaimer in the documentation and/or other materials provided with the distribution.
+#
+# 3. Neither the name of the copyright holder nor the names of its contributors may be used to endorse or promote products
+# derived from this software without specific prior written permission.
+#
+# THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES,
+# INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+# DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+# SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+# SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY,
+# WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
+# THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+if (DEFINED ENV{PICO_SDK_PATH} AND (NOT PICO_SDK_PATH))
+    set(PICO_SDK_PATH $ENV{PICO_SDK_PATH})
+    message("Using PICO_SDK_PATH from environment ('${PICO_SDK_PATH}')")
+endif ()
+
+if (DEFINED ENV{PICO_SDK_FETCH_FROM_GIT} AND (NOT PICO_SDK_FETCH_FROM_GIT))
+    set(PICO_SDK_FETCH_FROM_GIT $ENV{PICO_SDK_FETCH_FROM_GIT})
+    message("Using PICO_SDK_FETCH_FROM_GIT from environment ('${PICO_SDK_FETCH_FROM_GIT}')")
+endif ()
+
+if (DEFINED ENV{PICO_SDK_FETCH_FROM_GIT_PATH} AND (NOT PICO_SDK_FETCH_FROM_GIT_PATH))
+    set(PICO_SDK_FETCH_FROM_GIT_PATH $ENV{PICO_SDK_FETCH_FROM_GIT_PATH})
+    message("Using PICO_SDK_FETCH_FROM_GIT_PATH from environment ('${PICO_SDK_FETCH_FROM_GIT_PATH}')")
+endif ()
+
+if (DEFINED ENV{PICO_SDK_FETCH_FROM_GIT_TAG} AND (NOT PICO_SDK_FETCH_FROM_GIT_TAG))
+    set(PICO_SDK_FETCH_FROM_GIT_TAG $ENV{PICO_SDK_FETCH_FROM_GIT_TAG})
+    message("Using PICO_SDK_FETCH_FROM_GIT_TAG from environment ('${PICO_SDK_FETCH_FROM_GIT_TAG}')")
+endif ()
+
+if (PICO_SDK_FETCH_FROM_GIT AND NOT PICO_SDK_FETCH_FROM_GIT_TAG)
+  set(PICO_SDK_FETCH_FROM_GIT_TAG "master")
+  message("Using master as default value for PICO_SDK_FETCH_FROM_GIT_TAG")
+endif()
+
+set(PICO_SDK_PATH "${PICO_SDK_PATH}" CACHE PATH "Path to the Raspberry Pi Pico SDK")
+set(PICO_SDK_FETCH_FROM_GIT "${PICO_SDK_FETCH_FROM_GIT}" CACHE BOOL "Set to ON to fetch copy of SDK from git if not otherwise locatable")
+set(PICO_SDK_FETCH_FROM_GIT_PATH "${PICO_SDK_FETCH_FROM_GIT_PATH}" CACHE FILEPATH "location to download SDK")
+set(PICO_SDK_FETCH_FROM_GIT_TAG "${PICO_SDK_FETCH_FROM_GIT_TAG}" CACHE FILEPATH "release tag for SDK")
+
+if (NOT PICO_SDK_PATH)
+    if (PICO_SDK_FETCH_FROM_GIT)
+        include(FetchContent)
+        set(FETCHCONTENT_BASE_DIR_SAVE ${FETCHCONTENT_BASE_DIR})
+        if (PICO_SDK_FETCH_FROM_GIT_PATH)
+            get_filename_component(FETCHCONTENT_BASE_DIR "${PICO_SDK_FETCH_FROM_GIT_PATH}" REALPATH BASE_DIR "${CMAKE_SOURCE_DIR}")
+        endif ()
+        FetchContent_Declare(
+                pico_sdk
+                GIT_REPOSITORY https://github.com/raspberrypi/pico-sdk
+                GIT_TAG ${PICO_SDK_FETCH_FROM_GIT_TAG}
+        )
+
+        if (NOT pico_sdk)
+            message("Downloading Raspberry Pi Pico SDK")
+            # GIT_SUBMODULES_RECURSE was added in 3.17
+            if (${CMAKE_VERSION} VERSION_GREATER_EQUAL "3.17.0")
+                FetchContent_Populate(
+                        pico_sdk
+                        QUIET
+                        GIT_REPOSITORY https://github.com/raspberrypi/pico-sdk
+                        GIT_TAG ${PICO_SDK_FETCH_FROM_GIT_TAG}
+                        GIT_SUBMODULES_RECURSE FALSE
+
+                        SOURCE_DIR ${FETCHCONTENT_BASE_DIR}/pico_sdk-src
+                        BINARY_DIR ${FETCHCONTENT_BASE_DIR}/pico_sdk-build
+                        SUBBUILD_DIR ${FETCHCONTENT_BASE_DIR}/pico_sdk-subbuild
+                )
+            else ()
+                FetchContent_Populate(
+                        pico_sdk
+                        QUIET
+                        GIT_REPOSITORY https://github.com/raspberrypi/pico-sdk
+                        GIT_TAG ${PICO_SDK_FETCH_FROM_GIT_TAG}
+
+                        SOURCE_DIR ${FETCHCONTENT_BASE_DIR}/pico_sdk-src
+                        BINARY_DIR ${FETCHCONTENT_BASE_DIR}/pico_sdk-build
+                        SUBBUILD_DIR ${FETCHCONTENT_BASE_DIR}/pico_sdk-subbuild
+                )
+            endif ()
+
+            set(PICO_SDK_PATH ${pico_sdk_SOURCE_DIR})
+        endif ()
+        set(FETCHCONTENT_BASE_DIR ${FETCHCONTENT_BASE_DIR_SAVE})
+    else ()
+        message(FATAL_ERROR
+                "SDK location was not specified. Please set PICO_SDK_PATH or set PICO_SDK_FETCH_FROM_GIT to on to fetch from git."
+                )
+    endif ()
+endif ()
+
+get_filename_component(PICO_SDK_PATH "${PICO_SDK_PATH}" REALPATH BASE_DIR "${CMAKE_BINARY_DIR}")
+if (NOT EXISTS ${PICO_SDK_PATH})
+    message(FATAL_ERROR "Directory '${PICO_SDK_PATH}' not found")
+endif ()
+
+set(PICO_SDK_INIT_CMAKE_FILE ${PICO_SDK_PATH}/pico_sdk_init.cmake)
+if (NOT EXISTS ${PICO_SDK_INIT_CMAKE_FILE})
+    message(FATAL_ERROR "Directory '${PICO_SDK_PATH}' does not appear to contain the Raspberry Pi Pico SDK")
+endif ()
+
+set(PICO_SDK_PATH ${PICO_SDK_PATH} CACHE PATH "Path to the Raspberry Pi Pico SDK" FORCE)
+
+include(${PICO_SDK_INIT_CMAKE_FILE})
diff --git a/bootloader/docker-compose.yml b/bootloader/docker-compose.yml
new file mode 100644
index 0000000..6c56ad8
--- /dev/null
+++ b/bootloader/docker-compose.yml
@@ -0,0 +1,13 @@
+# Docker Compose for the bootloader project.
+# Same structure as color_switcher — shared build script + common submodules.
+
+services:
+  pico-build:
+    build: .
+
+    volumes:
+      - .:/project
+      - ../common:/common
+      - ../build.sh:/scripts/build.sh
+
+    command: sleep infinity
diff --git a/bootloader/src/APP_BOOT/cfg/APP_BOOT_cfg.c b/bootloader/src/APP_BOOT/cfg/APP_BOOT_cfg.c
new file mode 100644
index 0000000..dbf2f5f
--- /dev/null
+++ b/bootloader/src/APP_BOOT/cfg/APP_BOOT_cfg.c
@@ -0,0 +1,9 @@
+/******************************************************************************
+ * File:        APP_BOOT_cfg.c
+ * Component:   APP_BOOT
+ * Description: Configuration definitions for the bootloader application.
+ *
+ * Layer:       Application - configuration
+ *****************************************************************************/
+
+#include "APP_BOOT_cfg.h"
diff --git a/bootloader/src/APP_BOOT/cfg/APP_BOOT_cfg.h b/bootloader/src/APP_BOOT/cfg/APP_BOOT_cfg.h
new file mode 100644
index 0000000..8b2b869
--- /dev/null
+++ b/bootloader/src/APP_BOOT/cfg/APP_BOOT_cfg.h
@@ -0,0 +1,18 @@
+/******************************************************************************
+ * File:        APP_BOOT_cfg.h
+ * Component:   APP_BOOT
+ * Description: Configuration header for the bootloader application.
+ *              Will hold flash layout addresses, timeout values,
+ *              entry trigger config, etc.
+ *
+ * Layer:       Application - configuration
+ *****************************************************************************/
+
+#ifndef APP_BOOT_CFG_H
+#define APP_BOOT_CFG_H
+
+#include "STD_TYPES.h"
+
+/* TODO: define flash layout, timeouts, entry trigger config */
+
+#endif /* APP_BOOT_CFG_H */
diff --git a/bootloader/src/APP_BOOT/inc/APP_BOOT.h b/bootloader/src/APP_BOOT/inc/APP_BOOT.h
new file mode 100644
index 0000000..8c8614e
--- /dev/null
+++ b/bootloader/src/APP_BOOT/inc/APP_BOOT.h
@@ -0,0 +1,20 @@
+/******************************************************************************
+ * File:        APP_BOOT.h
+ * Component:   APP_BOOT
+ * Description: Public interface for the bootloader application.
+ *              Handles firmware receive, flash write, integrity check,
+ *              and application jump.
+ *
+ * Layer:       Application
+ *****************************************************************************/
+
+#ifndef APP_BOOT_H
+#define APP_BOOT_H
+
+#include "STD_TYPES.h"
+
+STD_tenuResult APP_BOOT_enuInit(void);
+
+void APP_BOOT_vRunnable(void);
+
+#endif /* APP_BOOT_H */
\ No newline at end of file
diff --git a/bootloader/src/APP_BOOT/prg/APP_BOOT_prg.c b/bootloader/src/APP_BOOT/prg/APP_BOOT_prg.c
new file mode 100644
index 0000000..d7ed18f
--- /dev/null
+++ b/bootloader/src/APP_BOOT/prg/APP_BOOT_prg.c
@@ -0,0 +1,27 @@
+/******************************************************************************
+ * File:        APP_BOOT_prg.c
+ * Component:   APP_BOOT
+ * Description: Bootloader application logic — stub implementation.
+ *              See TODO.md for the design decisions needed before
+ *              implementing the actual bootloader behavior.
+ *
+ * Layer:       Application
+ *****************************************************************************/
+
+#include "APP_BOOT.h"
+#include "APP_BOOT_priv.h"
+#include "APP_BOOT_cfg.h"
+
+STD_tenuResult APP_BOOT_enuInit(void)
+{
+    STD_tenuResult enuResultLoc = STD_OK;
+
+    /* TODO: check bootloader entry trigger (button, magic value, etc.) */
+
+    return enuResultLoc;
+}
+
+void APP_BOOT_vRunnable(void)
+{
+    /* TODO: firmware receive state machine, flash write, app jump */
+}
\ No newline at end of file
diff --git a/bootloader/src/APP_BOOT/prg/APP_BOOT_priv.h b/bootloader/src/APP_BOOT/prg/APP_BOOT_priv.h
new file mode 100644
index 0000000..c5dca47
--- /dev/null
+++ b/bootloader/src/APP_BOOT/prg/APP_BOOT_priv.h
@@ -0,0 +1,15 @@
+/******************************************************************************
+ * File:        APP_BOOT_priv.h
+ * Component:   APP_BOOT
+ * Description: Private header for the bootloader application.
+ *
+ * Layer:       Application - internal use only
+ *****************************************************************************/
+
+#ifndef APP_BOOT_PRIV_H
+#define APP_BOOT_PRIV_H
+
+#include "APP_BOOT.h"
+#include "APP_BOOT_cfg.h"
+
+#endif /* APP_BOOT_PRIV_H */
\ No newline at end of file
diff --git a/bootloader/src/SYS_ECU/prg/SYS_ECU.c b/bootloader/src/SYS_ECU/prg/SYS_ECU.c
new file mode 100644
index 0000000..3166fd8
--- /dev/null
+++ b/bootloader/src/SYS_ECU/prg/SYS_ECU.c
@@ -0,0 +1,37 @@
+/******************************************************************************
+ * File:        SYS_ECU.c
+ * Component:   SYS_ECU
+ * Description: Bootloader system orchestrator. Init sequence + main loop.
+ *
+ * Layer:       System (top of the stack)
+ *****************************************************************************/
+
+#include "STD_TYPES.h"
+#include "pico/stdlib.h"
+
+#include "MCU_USB.h"
+#include "HAL_COM.h"
+#include "APP_BOOT.h"
+
+static void vInitAll(void)
+{
+    /* MCU layer */
+    MCU_USB_enuInit();
+
+    /* HAL layer */
+    HAL_COM_enuInit();
+
+    /* Application layer */
+    APP_BOOT_enuInit();
+}
+
+int main(void)
+{
+    vInitAll();
+
+    while (1)
+    {
+        APP_BOOT_vRunnable();
+        sleep_ms(10);
+    }
+}