ESP32 Brownout Reset Error

ESP32 Brownout Reset (rst:0xf) #

If you're seeing this in your serial monitor:

rst:0xf (BROWNOUT_RST), boot:0x13 (DOWNLOAD(USB/UART0))
Brownout detector was triggered

…it means the ESP32 reset itself because the voltage dropped too low, even briefly.

This usually happens during startup, Wi-Fi init, or when peripherals like cameras begin drawing current.

Situations Where It Commonly Occurs #

Brownouts are rarely caused by faulty code or the board itself. They're almost always a power delivery issue.

Some common situations:

• The board resets right after WiFi.begin()
• Code uploads fine, but it crashes before reaching setup()
• It works on one USB port, but not another
• Flickering power LED under load
• ESP32-CAM fails at esp_camera_init() with no obvious error

Steps to Debug the Issue #

You can confirm the cause programmatically:

#include "esp_system.h"

void setup() {
  Serial.begin(115200);
  delay(1000); // let Serial stabilize
  Serial.println(esp_reset_reason());  // Returns ESP_RST_BROWNOUT if that was the cause
}

If you're running into this, here's what actually helps:

1. Power it better
A laptop USB port might only deliver 100–300mA before current limiting kicks in. The ESP32 can spike to 400–500mA during Wi-Fi startup. Try:

• A wall-powered USB adapter
• A powered USB hub
• Supplying 5V directly to the VIN pin from a regulated source

2. Change your cable
Low-quality USB cables often have high resistance. They can technically “work,” but still cause just enough voltage drop to trigger a brownout. If you're not sure, try a short, thick cable — ideally one that charges other high-draw devices.

3. Add a capacitor
A bulk capacitor near the ESP32 can absorb transient drops:

+---------+ 
| ESP32   |
|         |--- GND -------+
|         |               |
|         |--- 3.3V ------+----||---- GND
                          470–1000µF electrolytic

This is especially useful on ESP32-CAM boards or projects using Wi-Fi and sensors together.

4. Reduce startup current
Disable Wi-Fi and Bluetooth if you're not using them:

WiFi.mode(WIFI_OFF);
btStop();

If your project doesn't need full CPU speed:

setCpuFrequencyMhz(80);

If it's idle for a while:

esp_sleep_enable_timer_wakeup(10 * 1000000); // 10 seconds
esp_deep_sleep_start();

Conclusion #

One last note: if swapping the board “fixes it,” that doesn’t mean the original board was bad. It might just have a regulator with less margin. If you're powering multiple peripherals, consider isolating them from the ESP32’s supply.

Brownouts aren't mysterious — they're just a signal that your setup isn't supplying clean enough power, fast enough, when it's needed most.