Brown Out Reset (BOR) is one of the most important reliability features in modern microcontrollers. When the supply voltage drops below a safe operating level, BOR automatically resets the device to prevent unstable operation, memory corruption, and unexpected system behavior.
What is Brown Out Reset?
Brown Out Reset (BOR) is a built-in voltage-monitoring function in many microcontrollers that resets the device when the supply voltage drops below a set threshold. It prevents the microcontroller from running in an unsafe voltage range where the CPU, memory, clock, and peripherals may become unstable. BOR holds the device in reset until the voltage returns to a safe operating level, preventing operation outside the device's specified voltage range.
How Brown Out Reset Works and Configuration Parameters
BOR Operating Sequence
Brown Out Reset uses an internal voltage-monitoring circuit to continuously track the microcontroller supply voltage. During normal operation, the supply voltage remains above the BOR threshold, and the microcontroller runs normally.
When the supply voltage falls below the configured BOR threshold, the monitoring circuit detects the condition and immediately generates a reset signal. Program execution stops, peripherals return to their reset state, and the device remains in reset while the voltage is below a safe operating level.
Once the supply voltage recovers above the BOR release threshold and remains stable for the required delay period, the reset is released, and the microcontroller restarts from a known state. This process prevents the microcontroller from operating while the supply voltage is outside its guaranteed operating range.
Key BOR Configuration Parameters
| Parameter | Description |
|---|---|
| Threshold Voltage | The voltage level that triggers a BOR reset when the supply falls below it |
| Hysteresis | The difference between the trigger voltage and the release voltage prevents reset oscillation |
| Reset Delay Time | The time the voltage must remain stable before the reset is released |
| BOR Level Selection | Configurable threshold options provided by the microcontroller |
Many microcontrollers offer multiple selectable BOR levels. The available thresholds vary by device family and are typically configured via fuses, option bytes, or configuration registers. The selected threshold should be above the minimum voltage required for reliable operation while avoiding unnecessary resets due to normal voltage fluctuations.
How to Select a BOR Threshold
Step 1: Identify the Minimum Operating Voltage
Begin by reviewing the microcontroller datasheet to determine the lowest supply voltage at which reliable operation is guaranteed. This value defines the minimum safe operating limit for the CPU, memory, clock system, and peripherals. For example, if the device operates from 2.0 V to 3.6 V, then 2.0 V is the minimum voltage that must be maintained to ensure stable operation.
Step 2: Measure Worst-Case Voltage Dips
Next, evaluate how the actual power supply behaves under the most demanding operating conditions. Use an oscilloscope to monitor the voltage directly at the microcontroller while motors start, relays switch, radios transmit, processors reach peak activity, or batteries approach discharge. For example, a system with a nominal 3.3 V supply may experience temporary drops to 2.2 V during high-current events.
Step 3: Consider Supply Tolerances
The BOR threshold should also account for variations that occur in real-world operation. Factors such as regulator accuracy, battery aging, temperature changes, and manufacturing tolerances can affect the actual supply voltage. If these factors introduce a worst-case variation of ±0.1 V, sufficient margin should be included to maintain reliable operation under all conditions.
Step 4: Review Available BOR Levels
Most microcontrollers provide several selectable BOR thresholds rather than allowing an arbitrary voltage setting. Common options might include 1.8 V, 2.1 V, 2.7 V, and 4.0 V. Reviewing the available levels helps determine which threshold best matches the system's operating voltage and power-supply characteristics.
Step 5: Select the Most Appropriate Threshold
Choose a BOR threshold that remains above the minimum operating voltage while avoiding unnecessary resets during normal voltage fluctuations. In the example system, a 1.8 V threshold is below the safe operating range, 2.7 V may trigger resets during normal voltage dips, and 4.0 V exceeds the system supply voltage. A 2.1 V threshold provides the best balance between protection and operating margin, ensuring the microcontroller resets before entering an unreliable operating region without causing nuisance resets.
Common Causes of Brown-Out Conditions
| Category | Typical Causes | Effect |
|---|---|---|
| Power Supply Problems | Battery discharge, battery aging, insufficient current capability, regulator instability, poor transient response, input fluctuations, excessive ripple or noise | Supply voltage falls below the BOR threshold |
| Load-Induced Voltage Drops | Motors, relays, solenoids, wireless transmitters, LED drivers, DC-DC converters, and large capacitor banks | Temporary current surges create voltage dips |
| Power Distribution Problems | Long cables, connector resistance, wiring losses, narrow PCB traces, shared power paths | Voltage is lost between the source and the microcontroller |
Voltage-Drop Case Study
A wireless sensor operates from a lithium battery and normally runs at 3.0 V. During radio transmission, the transmitter draws a short burst of current that temporarily reduces the supply voltage.
| Condition | Supply Voltage |
|---|---|
| Normal operation | 3.0 V |
| During radio transmission | 2.4 V |
| BOR threshold | 2.7 V |
When the voltage drops to 2.4 V, BOR detects that the supply has fallen below the configured threshold and immediately resets the microcontroller. After the battery voltage recovers above the release threshold, the device restarts normally.
Without BOR, the microcontroller could continue operating below its specified voltage range, potentially causing system faults and data integrity problems.
Brown-Out Reset vs POR, Watchdog Timer, and External Voltage Supervisor
| Aspect | Brown Out Reset (BOR) | Power-On Reset (POR) | Watchdog Timer (WDT) | External Voltage Supervisor |
|---|---|---|---|---|
| Purpose | Protects against low-voltage operation | Ensures proper startup after power is applied | Detects firmware lockups | Provides dedicated voltage monitoring |
| Monitored Parameter | Supply voltage | Power-up condition | Program execution | Supply voltage |
| Typical Trigger | Voltage falls below BOR threshold | Initial power application | Software fails to service the watchdog | Voltage exceeds programmed limits |
| Primary Benefit | Prevents unstable operation and memory corruption | Starts the device from a known state | Recovers from software faults | Provides precise voltage supervision and reset control |
Troubleshooting Brown Out Problems and Common Design Mistakes
| Problems / Issues | Cause | Fixes |
|---|---|---|
| Random resets | Supply voltage drops below the BOR threshold. | Measure voltage at the microcontroller pins, improve regulator current capacity, add proper decoupling and bulk capacitors, and check high-current loads. |
| Device freezes | The microcontroller operates below its minimum safe voltage. | Enable BOR, select a suitable BOR threshold, and make sure the supply voltage stays within the datasheet operating range. |
| Communication errors | Peripherals malfunction during voltage dips. | Stabilize the supply rail, separate noisy loads from sensitive circuits, improve grounding, and check signal integrity during load changes. |
| Corrupted data | Memory writes are interrupted by low voltage. | Use BOR protection, validate stored data with checksums, avoid writing during unstable power, and use safe data-update methods. |
| Boot loops | Voltage repeatedly fluctuates around the BOR threshold. | Add hysteresis, increase reset delay, improve power-supply stability, and check for startup inrush current. |
| Intermittent operation | Marginal power supply stability or wiring resistance. | Inspect connectors, shorten power paths, use wider traces, check cable voltage drop, and verify regulator transient response. |
| BOR disabled | The microcontroller may continue running in an unstable voltage range. | Enable BOR in firmware or configuration bits. |
| BOR threshold set too low | The device may execute code below the minimum operating voltage. | Choose a BOR level above the minimum safe operating voltage. |
| BOR threshold set too high | Normal voltage dips may cause unnecessary resets. | Select a threshold that protects the device without triggering during expected load transients. |
| Ignoring hysteresis | The device may repeatedly reset near the threshold voltage. | Use BOR hysteresis or an external voltage supervisor with stable reset behavior. |
| Reset delay too short | The device may restart before regulators and clocks stabilize. | Increase reset delay or add a power-good/reset supervisor circuit. |
| Reset cause not checked | Root-cause troubleshooting becomes difficult. | Read reset status registers after startup and log BOR events when possible. |
| Same BOR level used for all designs | The threshold may not match the actual power system. | Select the BOR threshold based on the microcontroller voltage range, regulator tolerance, load transients, and battery behavior. |
When to Use an External Voltage Supervisor Instead of Internal BOR
Internal BOR is suitable when the MCU's built-in threshold matches the system voltage and only one main supply rail needs basic low-voltage protection.
Use an external voltage supervisor when the system needs more accurate reset thresholds, extended reset delay, stronger noise immunity, or multi-rail monitoring. This is common in industrial, automotive, medical, communication, and data-logging systems where unstable power may cause data loss, unsafe outputs, or field failures.
An external supervisor is also useful when regulators, clocks, sensors, or communication ICs need extra time to stabilize before the MCU starts. It can hold the system in reset until the required voltage rails are valid.
In general, use internal BOR for straightforward MCU protection. Use an external voltage supervisor when reset timing, voltage accuracy, multi-rail supervision, or system reliability requirements are strict.
Should You Enable BOR? Applications and Trade-Offs
BOR should usually be enabled when reduced supply voltage could cause unstable code execution, corrupted data, communication faults, or unsafe output behavior. It is especially useful in battery-powered sensors, IoT devices, industrial controllers, automotive electronics, data loggers, and systems with motors, relays, solenoids, or wireless transmitters.
In battery-powered products, BOR prevents the MCU from running below its safe voltage range when the battery is old, cold, nearly discharged, or stressed by a radio transmission burst. In industrial and automotive systems, BOR helps the controller recover from voltage dips caused by motor startup, relay switching, long cables, engine cranking, or load transients.
The main trade-off is threshold selection. A BOR threshold that is too high may reduce usable battery capacity or cause nuisance resets during normal load changes. A threshold that is too low may allow the MCU to run in an unreliable voltage range.
If BOR causes frequent resets, do not simply disable it. Measure the supply voltage at the MCU pins, check the regulator transient response, review decoupling and PCB power paths, and adjust the BOR threshold only after confirming the real voltage behavior. For high-reliability systems or multi-rail designs, use an external voltage supervisor when the internal BOR threshold is not accurate or flexible enough.
Conclusion
Brown Out Reset protects microcontrollers by forcing a controlled reset whenever supply voltage falls below a safe operating level. Selecting the correct BOR threshold, designing a stable power system, and implementing proper hardware and firmware safeguards can significantly improve system reliability. In most embedded applications, enabling and correctly configuring BOR is a fundamental design practice.
Frequently Asked Questions [FAQ]
Q1. How do you determine whether a Brown Out Reset threshold is set too high or too low?
A BOR threshold is too low if the microcontroller can still execute code below its guaranteed operating voltage. It is too high if normal voltage dips during events such as motor startup or radio transmission cause unnecessary resets. The best threshold protects while allowing normal operation during expected supply fluctuations.
Q2. Why can a microcontroller still experience reset problems even when BOR is enabled?
BOR only detects low-voltage conditions; it does not eliminate the underlying power problem. Issues such as weak regulators, insufficient decoupling capacitors, excessive PCB voltage drops, or large transient loads can still cause repeated resets. Identifying and correcting the power-source issue is often required.
Q3. When should an external voltage supervisor be used instead of relying solely on BOR?
An external voltage supervisor is beneficial when an application requires more accurate voltage monitoring, adjustable reset timing, better noise immunity, or monitoring of multiple power rails. This is common in industrial, automotive, and other high-reliability systems where tighter voltage control is required.
Q4. How can engineers verify that brown-out conditions are actually causing system failures?
The most effective approach is to monitor the supply voltage directly at the microcontroller using an oscilloscope while the system is under load. Checking reset status registers after restart can also confirm whether a BOR event occurred. Combining voltage measurements with reset-cause information provides a reliable diagnosis.
Q5. Does enabling BOR reduce battery life in battery-powered devices?
BOR circuitry consumes very little power, so its direct impact on battery life is typically small. The larger effect comes from the selected threshold; a threshold set to a higher level may cause the system to reset earlier and leave some battery capacity unused. Designers must balance maximum battery utilization with reliable operation.
