This includes managing hardware resources, interrupts, memory, and execution flow. In other words, the firmware itself acts as the scheduler, the memory manager, the driver framework, and sometimes even the safety supervisor.
From a system perspective, bare-metal architecture begins at power-on and ends only when power is removed. There is no hidden layer between the CPU and your code. Every clock cycle, every interrupt latency, and every byte of RAM usage is your responsibility. The absence of an OS gives you deterministic behavior and minimal overhead, but it requires disciplined architecture and deep understanding of the microcontroller hardware.
To make this clear, let us walk through the complete bare-metal architecture using layered thinking, execution flow, memory design, interrupt handling, and power management — while keeping the structure modular and scalable.
Modular Code Structure
Bare-metal systems are often resource-constrained, so the software should be highly modular to facilitate maintainability, reuse, and optimization. Even without an OS, we must think in layers. Modularization prevents register chaos and improves long-term scalability.
A professional bare-metal architecture is typically layered as follows:
+---------------------------+
| Application Layer |
+---------------------------+
| Peripheral Drivers |
+---------------------------+
| Hardware Abstraction |
| Layer (HAL) |
+---------------------------+
| Registers |
+---------------------------+
| Hardware |
+---------------------------+
This layered approach simplifies debugging, testing, and future expansion.
The key modules include:
- Hardware Abstraction Layer (HAL)
- Interfaces directly with the hardware registers
- Provides readable and portable APIs
- Reduces register-level duplication
- Makes migration between MCUs easier
- Peripheral Drivers
- Handle specific hardware components such as GPIOs, timers, I2C, SPI, UART
- Implement configuration and runtime control logic
- Use HAL to access hardware safely
- Application Layer
- Contains the main logic of the application
- Controls sensors, actuators, communication
- Should never access registers directly
Example HAL interface:
// HAL for GPIO
void gpio_init(void);
void gpio_write(uint8_t pin, uint8_t value);
Instead of writing:
GPIOA->ODR |= (1 << 5);
You encapsulate the access, which improves clarity and security.
Main Loop Design (Super Loop)
Since there is no OS to manage tasks, many bare-metal systems rely on a super loop design. The main loop repeatedly executes the program's core logic in a continuous cycle.
Initialize System
│
▼
+------------------+
| while(1) |
| ┌────────────┐ |
| | Read Input | |
| ├────────────┤ |
| | Process | |
| ├────────────┤ |
| | Output | |
| └────────────┘ |
+------------------+
Example:
int main(void) {
init_peripherals(); // Initialize hardware peripherals
while (1) {
read_sensor_data(); // Read sensors
process_actuators(); // Control actuators
handle_communication(); // Communicate data if needed
}
}
Advantages:
- Simple
- Deterministic
- Minimal overhead
- No context switching
Limitations:
- Blocking functions freeze execution
- Poor scalability
- Hard real-time constraints become difficult to maintain
- Latency depends on loop length
Interrupt-Driven Design
To handle asynchronous events efficiently (e.g., sensor readings or communication interrupts), the system relies on interrupts.
Interrupt-driven architecture:
+----------------+
| Main Loop |
+--------+-------+
^
|
+------------------+------------------+
| | |
Timer ISR UART ISR GPIO ISR
An interrupt-driven design allows the processor to respond quickly without continuous polling, improving system responsiveness and power efficiency.
Example:
void EXTI0_IRQHandler(void) {
if (button_pressed()) {
toggle_led(); // Action triggered by interrupt
}
EXTI->PR |= (1 << 0); // Clear the interrupt flag
}
Best practices:
- Keep ISR short
- Avoid heavy processing inside ISR
- Use flags or queues
- Clear interrupt flag properly
This approach improves:
- Latency
- Determinism
- CPU utilization
State Machine Design
For more complex applications, a state machine design improves code organization and predictability.
Example states:
- Idle
- Processing
- Error handling
ASCII representation:
+-----------+
| IDLE |
+-----+-----+
|
EVENT_START
|
▼
+-----------+
|PROCESSING |
+-----+-----+
|
EVENT_ERROR
▼
+-----------+
| ERROR |
+-----------+
Code example:
typedef enum {
STATE_IDLE,
STATE_PROCESSING,
STATE_ERROR
} system_state_t;
system_state_t current_state = STATE_IDLE;
void update_state(event_t event) {
switch (current_state) {
case STATE_IDLE:
if (event == EVENT_START)
current_state = STATE_PROCESSING;
break;
case STATE_PROCESSING:
if (event == EVENT_ERROR)
current_state = STATE_ERROR;
break;
case STATE_ERROR:
if (event == EVENT_RESET)
current_state = STATE_IDLE;
break;
}
}
Benefits:
- Clear transitions
- Easier debugging
- Deterministic logic
- Suitable for safety-critical systems
Low-Power Modes
Embedded systems often need to conserve power. Bare-metal software designs include handling low-power modes.
Flow:
Main Loop
│
Check Idle?
│
Yes
│
Enter Sleep
│
Interrupt Occurs
│
Wake Up
Example:
void enter_sleep_mode(void) {
__WFI(); // Wait for interrupt
}
int main(void) {
setup();
while (1) {
if (is_idle()) {
enter_sleep_mode();
}
}
}
Advantages:
- Reduced power consumption
- Extended battery life
- Lower thermal load
Error Handling and Debugging
Bare-metal systems require robust error-handling mechanisms.
Common methods include:
- Watchdog timers
- Reset system on freeze
- Fault handlers
- Handle bus faults, memory faults
- Debugging interfaces
- SWD
- JTAG
Example fault handler:
void HardFault_Handler(void) {
system_reset();
}
Debug tools allow:
- Breakpoint debugging
- Memory inspection
- Register tracing
- Timing analysis