This post will guide you common issues and mistakes that cause Fatal Exception and wdt reset. Fatal exception comes at execution time. program compiles well logically looks correct but at Running suddenly these fatal exceptions come. They are difficult to find out. But I have kept a record of few causes of these fatal exception. That I am sharing here.

First we will simulate these errors with test code and correct it.

Example 1: LED Blink Wdt Reset

LED Blink Program Giving Wdt Reset Cause Error

wdt reset
load 0x4010f000, len 1384, room 16 
tail 8
chksum 0x2d
csum 0x2d
v09f0c112
~ld
⸮

Now we will look into program that caused wdt reset

The program is simple LED Blink but Why this wdt reset error ? can you guess….

This error is caused simply because #define LED 9. This IO 9 of ESP8266 is on bottom side these IOs cannot be used. Its My First Lesson.

Lesson 1: Make sure you are using correct IO numbers.

Example 2: External Interrupt Causing Fatal Exception(9)

The error in serial monitor.

Exception (9):
epc1=0x40100f50 epc2=0x00000000 epc3=0x00000000 excvaddr=0x00000003 depc=0x00000000

ctx: sys 
sp: 3ffffc70 end: 3fffffb0 offset: 01a0

>>>stack>>>
3ffffe10: 40103bfc 00000017 3ffe843c 00000001 
3ffffe20: 40101180 00d877c3 3ffee2c4 00000000 
3ffffe30: 3ffe88f0 3ffee2c4 3ffee378 40201e95 
3ffffe40: 00000000 00d3b30f 40201098 000003e8 
3ffffe50: 3ffee2c4 80017004 00000001 40201ed4 
3ffffe60: 0000007f 00000000 3ffee378 40201c05 
3ffffe70: 40106b4c 00080000 00000000 40106b9a

Lets Look at the program that caused fatal exception(9)

Look closely to the Serial monitor You can See “You Pressed Flash Button” message that we are sending from interrupt handler.

Behavior of program:

LED Blinking works perfectly No Wdt Reset or any Fatal Exception until we send external interrupt. Right. Whats the problem then? Interrupt is called we can see message also.

Where is our second message “I am after 1 Sec” Yes you are right some thing after first serial out. i.e. delay(1000); This line causes fatal exception(9). Then Why? Logically every thing is correct then this fatal exception(9) from a delay????

Yes its bug in ESP8266 libraries. that are get used while compilation.

Lesson No. 2: Never Use Delays in External Interrupt Handler

How to Solve this Fatal Exception(9)? I want to use delay in interrupt

To solve this issue I use delayMicroseconds in interrupt handler this will not create any problem.

Modified Program with Same Functionality Without Fatal Exception(9) Error

#define InterruptButton 0  //On board Flash Button as interrupt
#define LED 2  //on Board Blue LED

void setup() {
  Serial.begin(115200);
  pinMode(LED,OUTPUT);  
  pinMode(InterruptButton,INPUT_PULLUP);

  attachInterrupt(digitalPinToInterrupt(InterruptButton),myInterruptHandler,CHANGE);
}

void loop() {
  // LED Blinking repeatedly:
    digitalWrite(LED,HIGH);
    delay(1000);
    digitalWrite(LED,LOW);
    delay(1000);
}

void myInterruptHandler()
{
  Serial.println("You Pressed Flash Button");
  delayMicroseconds(1000000);  //Simple 1 Sec Delay
  Serial.println("I am after 1 Sec");
}

Example 3: ESP as Web Server and AP

This program Looks perfect. But causing  No errors or Doing nothing.

After Uploading This program.

1. We must see HomeServer wifi network.
2. But there is no such network created by this code.

Why No WiFi Access point ?

Compilation is ok, No Errors or Exceptions in serial monitor then Why missing HotSpot?

The problem is in Analog = String(analogRead(A0));

void loop() { 
  server.handleClient();
  Analog = String(analogRead(A0)); 
}

By commenting Analog read line and re-uploading the program you will immediately see the HomeServer access point

/*
 * Copyright (c) 2015, circuits4you.com
 * All rights reserved.
/* Create a WiFi access point and provide a web server on it. */

#include <ESP8266WiFi.h>
#include <WiFiClient.h> 
#include <ESP8266WebServer.h>

/* Set these to your desired credentials. */
const char *ssid = "HomeServer";

ESP8266WebServer server(80);
String Analog;
//=======================================================================
//                    handles main page 192.168.4.1
//=======================================================================
/* Just a little test message.  Go to http://192.168.4.1 in a web browser
 * connected to this access point to see it.
 */
void handleRoot() {
  String s = "hello from esp8266!";
  s = s +" ADC:" + Analog;
  digitalWrite(2, HIGH);
  server.send(200, "text/plain", s);
  digitalWrite(2, LOW);
}

//=======================================================================
//                    Power on setup
//=======================================================================

void setup() {
  delay(1000);
  /* You can remove the password parameter if you want the AP to be open. */
  WiFi.softAP(ssid); //AP without password Open Network

  IPAddress myIP = WiFi.softAPIP();
  server.on("/", handleRoot);
 
  server.begin();
}

//=======================================================================
//                    Main Program Loop
//=======================================================================
void loop() {
  server.handleClient();
  Analog = String(analogRead(A0));
}
//=======================================================================

Lesson No.3: Give Enough Time to server.handleClient();

To solve this problem update loop() with below code

void loop() {
  server.handleClient();
  Analog = String(analogRead(A0));
  delay(1000);
}

You can use analogRead in hadleRoot() subroutine also.

Example No 4: ESP8266 Trapped in Infinite Loop

This type of error occurs when there is any loop that holding the program. Such as polling of a IO line. Waiting for some event to occur. Such as Wait until Motion Detected. Motion sensor related programs.

Soft WDT reset

ctx: cont 
sp: 3ffefeb0 end: 3fff0090 offset: 01b0

>>>stack>>>
3fff0060: 3fffdad0 00000000 3ffef060 40201ef0 
3fff0070: feefeffe feefeffe feefeffe 40205584 
3fff0080: feefeffe feefeffe 3ffef070 40100718 
<<<stack<<<

ets Jan 8 2013,rst cause:2, boot mode:(1,6)


ets Jan 8 2013,rst cause:4, boot mode:(1,6)

Lesson 4: Never Use IO polling in program holding state

Secret Function yield() to get rid of many problems

The yield() function is also implemented inside the ESP8266 libraries:

Yielding

This is one of the most critical differences between the ESP8266 and a more classical Arduino microcontroller. The ESP8266 runs a lot of utility functions in the background – keeping WiFi connected, managing the TCP/IP stack, and performing other duties. Blocking these functions from running can cause the ESP8266 to crash (fatal exceptions) and reset itself. To avoid these mysterious resets, avoid long, blocking loops in your sketch.

The amazing creators of the ESP8266 Arduino libraries also implemented a yield() function, which calls on the background functions to allow them to do their things.

How to use yield() Function?

Example 1: Causing Hang and WDT reset

while(digitalRead(Key)==HIGH); //This will hold the program until button event is occurs

To solve this issue use yield() function in this way

while(digitalRead(Key)==HIGH)
{
  yield(); //This will prevent wdt reset
}

Now Let’s See List of Exception Codes of ESP8266

Infos from Xtensa Instruction Set Architecture (ISA) Reference Manual