Code to demonstrate the Watch-Dog-Timer induced crashing of an ESP32 when downloading large files > 150k via WiFi
This is discussed in detail here: me-no-dev/ESPAsyncWebServer#984
In short:
The download of files > 150k Bytes via WiFi results in a crash induced by Watch-Dog-Timer (WDT) on task async_tcp (CPU 1). This happens on all types of ESP32 of the "classic" type (new versions S2, S3, C3 not yet tested). This may happen on every second download attempt!
When tested with ESP8266 (the present code is for ESP32-only but can be easily adapted to ESP8266) a WDT crash was NEVER seen!
On an ESP32 files smaller than 100k never resulted in a WDT crash. The transition from "never crashing" to "crashing" is between 100k and 150k files sizes. Worst effect was observed at a file size of 300k. Strangely, it actually got a little bit better at even larger files.
While all types of ESP32 tested have shown this behaviour, its intensity seems to depend on the type of flash being used! The first two digits of the ChipID give the manufacturer of the flash, considered here as 'flash-type'. In the example below the ChipID is d84016, thus the manufacturer is d8 (BoHong Microelectronics, me-no-dev/ESPAsyncWebServer#984 (comment)).
How to run this code:
Download zip-file for your IDE, PlatformIO or Arduino (test on Ar2.0Beta7), unzip, and open. The code is identical for both IDEs, only the file naming is different.
In file FlashCrash.h enter you proper WiFi credentials (line 14ff). Upload and run.
The default is running on a FFat file system. It can easily be changed to LittleFS or SPIFFS (file FlashCrash.h line 26-27). When you change, do not forget to adapt your partition settings in file partitions.csv!
When running, enter the ESP32's IP in a web browser, and you will see the root page like in the left side of the picture below. Click on the link Download Data by JS, and another page opens showing the downloaded content, like in the right side of the picture. This continues automatically, and every 100 downloads another line will be added to the root page (refresh to see).
The IDE also creates a log file. The one from platformio start-up is provided as platformio-monitor.log.
How this works
The link Download Data by JS will upload a small page with JavaScript content. The JS does the download, sends a message to the ESP32 when complete, and triggers the next download after a pause of 50ms. This continues indefinitely.
While the file is binary, each byte has the value of a printable ASCII character, thus is easily printable.
Note: when the ESP32 crashes, then the webbrowser goes into a timeout. JS recognizes this timeout. Unfortunately, while this takes only ~10 sec on Firefox, it takes a long ~45 sec on Chromium!
Results
In the left side of the picture you see that the first 800 downloads occured without a single crash (Boots = 0). But after that, between 1 and 5 crashes (=Boots) occurred per 100 downloads.