ESP Embedded Systems

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My Previous Experience

Before I started working with the ESP32 & ESP8266, my experience with embedded systems was scarce. My only usage of C, C++ and Assembly was for competitive programming and operating systems. My codes were all run on powerful machines and I never had to worry about memory management, power consumption, or real-time constraints. This is why I was flabbergasted when I first started working with the ESP8266.

ESP32 (Smooth Sailing)

The ESP32 was a breeze to work with. It has a dual-core processor, 520KB of RAM, and 4MB of flash memory. My project ESP Rock was a success and I was able to implement server hosting, AHRS analysis and a useful web interface. The ESP32 was powerful enough to handle all of this and more. It didn't differ much from my previous experience with C++ and I was able to use the STL and other libraries without any issues.

ESP8266 (Rough Waters)

The ESP8266 was a different story. It has a single-core processor, 80KB of RAM, and 4MB of flash memory. My project IOT Connect faced a lot of issues, particularly the OOM (Out of Memory) error, which is notoriously difficult to debug, providing no useful information. Turns out, the async web server I was using had memory leaks and I had to switch to the good ol' synchronous web server <ESP8266WebServer.h> to fix the issue. For some reason, the ESP8266 also had issues with non-delaying analog reads, which I had to fix by using a non-blocking delay.

// Non-blocking delay example
void func() {
  static unsigned long lastMillis = 0;
  if (millis() - lastMillis > 1000) {
    lastMillis = millis();
    // Do something
  }
}

New Learnings

First off, PROG_MEM is a thing. It's a special memory space that is used to store constant data in the flash, which is useful for storing web pages, images, and other static data. Another thing I learned is that the ESP8266 hates dynamic memory allocation, so I had to rewrite all my programs into stack-based memory allocation. Hopefully, I was already using c style arrays and structs, so it wasn't too much of a hassle, but still, without all the debugging tools I'm used to, it was a pain to find the root of the problem.

Debugging Cheatsheet

OOM (Out of Memory) Error: Use ESP.getFreeHeap() to check the free heap size. If it is decreasing over time or due to certain events, you have a memory leak, especially if it isn't being freed after the event. To find out what exactly is causing the memory leak... well, good luck :) (Yeah I'm serious)
(Runtime) Alignment Error: Compile time alignment errors are easy to fix, but runtime alignment errors are a pain to debug; sometimes I find it even more frustrating to work with than memory leaks. The best way to debug this is to look for any pointer arithmetic or casting that might be causing the issue.
Remember to .end()/.close()/.stop(): Always remember to close the file, stop the server, or end the LittleFS instance. This is a common mistake that can lead to memory leaks and runtime alignment errors. I like to introduce a garbage stack to store lambda functions that will be executed at the end of the program to ensure that everything is properly closed.


//...
#include <stack>
#include <LittleFS.h>

std::stack<std::function<void()>> garbage;

void setupLittleFS() {
  LittleFS.begin();
  garbage.push([]() {
    LittleFS.end();
  });
}

void cleanup()
{
  while (!garbage.empty()) {
    garbage.top()();
    garbage.pop();
  }
}

Use PROGMEM for static data: This is a no-brainer, but it's worth mentioning.
Use volatile for shared variables: Easy to miss and can cause a lot of headaches (like bad performance, or even worse, UB).

After Thoughts

While working with ESP8266 was a pain, it also gave me a sense of accomplishment. I was able to make a tiny little chip do so much with so little resources (squeeze hard!). It was a humbling experience and I learned a lot from it. As somebody once said, "Never judge a book by its cover," I would say, "Never judge a chip by its specs, and never think your code would work the same on it!"

Referenced Repositories

ESP Rock: My ESP32 project (cool M5StickC project!)
IOT Connect: My ESP8266 project (Moisture sensor, web server, and more!)
ESPAsyncWebServer: The async web server I used to use
ESP8266WebServer: The synchronous web server I switched to

algasami