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add post about ESP32 issues
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J. Fernando Sánchez
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content/post/2025-micropython-esp32.md
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content/post/2025-micropython-esp32.md
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---
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title: "Installing Micropython on the Super Mini ESP32-S3"
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description:
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date: 2025-07-20T02:30:43+02:00
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image:
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math:
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license:
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hidden: false
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comments: true
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draft: false
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---
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## Context
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Many years ago I bought a "bargain" bluetooth scale.
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I wanted a way to automatically log my weight, and the Xiaomi equivalent was more than twice as expensive at the time.
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The problem is that it came with a very basic and somewhat sketchy software that required signing up, and none of the typical apps (openscale, gadgetbridge...) supported it.
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I looked at how to reverse engineer it, but I did not have much time back then, and I wrote it off as a "future project".
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Luckily, it had a screen, so I've been using it as a regular scale.
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I did not even bother to check the body composition metrics, because I got a very depressing reading of 35% body fat.
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Flash forward to today, when I decided to finally work on it.
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In addition to trying nrfConnect, I installed the app to see the actual values..
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It turns out the composition data was completely off!!
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When a reading starts, the app needs to tell the scale some basic data about you (age, height, sex).
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Unfortunately, there is no way (that I know of) to set a default user, so when the phone is not connected, it goes back to the wildly inaccurate reading.
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So, I decided to connect one of my idle ESP32 boards to it, to interface with the scale through bluetooth (sending the right user data) and logging the results somewhere through WiFi.
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## The problem
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Installing the firmware was not too difficult, after I got the board to properly register.
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```
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❯ uv tool run esptool.py --port /dev/ttyACM4 erase-flash
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...
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Chip type: ESP32-S3 (QFN56) (revision v0.2)
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Features: Wi-Fi, BT 5 (LE), Dual Core + LP Core, 240MHz, Embedded Flash 4MB (XMC), Embedded PSRAM 2MB (AP_3v3)
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...
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❯ uv tool run esptool.py --port /dev/ttyACM4 --baud 460800 write_flash 0 ESP32_GENERIC_S3-20250415-v1.25.0.bin
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```
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But then, I kept getting this error on boot: `OSError: (-24579, 'ESP_ERR_FLASH_NOT_INITIALISED')`.
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And I could not copy any files or install any packages.
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After some research, I narrowed the possibilities to:
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* A wrong partition table, most likely due to the fact that my board only has 4MB of flash.
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* Using an unsupported type of PSRAM (less likely)
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## The solution
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At this point, I could either try to compile the firmware with a custom partition (kind of tedious), or I could use the wonderful [mp-image-tool-esp32](https://github.com/glenn20/mp-image-tool-esp32) tool to check existing firmwares and modify partition sizes.
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I confirmed my suspicion:
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```
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❯ uv tool run mp-image-tool-esp32 ~/Downloads/ESP32_GENERIC_S3-20250415-v1.25.0.bin
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Running mp-image-tool-esp32 v0.1.1 (Python 3.12.11).
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Opening /home/j/Downloads/ESP32_GENERIC_S3-20250415-v1.25.0.bin...
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Found esp32s3 firmware file (8MB flash).
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Partition table (flash size: 8MB):
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╭──────────┬──────┬─────────┬──────────┬──────────┬──────────┬───────┬───────────╮
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│ Name │ Type │ SubType │ Offset │ Size │ End │ Flags │ │
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├──────────┼──────┼─────────┼──────────┼──────────┼──────────┼───────┼───────────┤
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│ nvs │ data │ nvs │ 0x9000 │ 0x6000 │ 0xf000 │ 0x0 │ (24.0 kB) │
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│ phy_init │ data │ phy │ 0xf000 │ 0x1000 │ 0x10000 │ 0x0 │ (4.0 kB) │
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│ factory │ app │ factory │ 0x10000 │ 0x1f0000 │ 0x200000 │ 0x0 │ (1.9 MB) │
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│ vfs │ data │ fat │ 0x200000 │ 0x600000 │ 0x800000 │ 0x0 │ (6.0 MB) │
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╰──────────┴──────┴─────────┴──────────┴──────────┴──────────┴───────┴───────────╯
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Micropython app fills 79.3% of factory partition (410 kB free)
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```
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```
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❯ uv tool run mp-image-tool-esp32 -f 4MB --resize vfs=2MB ESP32_GENERIC_S3-20250415-v1.25.0.bin
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❯ uv tool run esptool.py --port /dev/ttyACM4 --baud 460800 write_flash 0 ESP32_GENERIC_S3-20250415-v1.25.0-4MB-resize=vfs=2097152.bin
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```
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After that, I was able to install new packages with `mpremote` and `mip`:
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```
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uv tool run mpremote mip install aioble
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```
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We can check that we are fully utilizing the 2MB of PSRAM:
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```
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❯ uv tool run mpremote exec 'import gc;print(gc.mem_free(), "bytes")'
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2061456 bytes
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```
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If you see much less than that, you are probably using the wrong variant (try `SPIRAM_OCT`).
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