how to flash ESP32-S2 mini

[gaborfri]

PROBLEM DESCRIPTION

How does the flashing of an EST32-s2 mini work?
"No serial data received" appears in the EMS-ESP-Flasher.
if I try it via cmdline, after several attempts I get the message that the bootloader is not S2-compatible.
I was able to get the only successful flash with a tasmota-factory-bin.

do i have to compile it first or is there a special s2 image

gabor

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[MichaelDvP]

There is no prcompiled bin, you have to comile yourself (there is a pio env for S2-mini).
But you would not get happy wih S2, it is single core and has less RAM. Spend a few euros and get a S3 with PSRAM.

[math322]

For my S2 board i have to press Reset + Button 1 and release Reset to bring the board into flash mode. Also i need to add the 2 lines to the S2-mini env:

board_upload.use_1200bps_touch = false
board_upload.wait_for_upload_port = false

And the S2 (as example ESP32-S2FN4R2) has 2MB PSRAM.

In the command line i use the following parameters:

esptool.py --port /dev/ttyACM0 write_flash 0x1000 bootloader.bin 0x8000 partitions.bin 0x10000 firmware.bin

[gaborfri]

it works.

1. edit the platform.ini, add the information from math into the block of [env:lolin_s2_mini]
2. compile the sources running "pio run -e lolin_s2_mini -t upload"

and now I am able to see the new wifi and can connect.
thanks for your help

[gaborfri]

now after successful flash, i have the problem: how to connect the pins.
3V and GND is easy, but which pin i need to use for RX and TX on the esp-s2?

in the configuration i found the pins rx=23 and tx=5, but there is no pin 23.
i have only 1-18 and 21 and 33-40.

gabor

[proddy]

you can pick any pin, as long as it's connected to the circuit board in the correct way. https://emsesp.github.io/docs/Configuring/#board-profile

[MichaelDvP]

The predefined S2-Mini profile uses
LED: 15
Dallas: 7
TX: 12
RX: 11
Button: 0

[gaborfri]

Thank you very much, with the pins I can now see the appropriate outputs in the frontend.
but I don't get any messages in the MQTT explorer.
when I look at the system log I unfortunately see "[mqtt] Publish failed: low memory"

Looks like I need a different ESP32 after all. Which one would you recommend?

[MichaelDvP]

You can try to reduce or skip the check here

EMS-ESP32/src/mqtt.cpp

Line 595 in 1db4a33

if (ESP.getFreeHeap() < 60 * 1024 || ESP.getMaxAllocHeap() < 40 * 1024) {

This is mainly for the esp32 without PSRAM. Never tried what are usefull values for the S2 chip.

If you don't need ethernet the esp32S3 is best, pick a module with more than 4 Mb flash and min 2M PSRAM. The Liligo T7-S3 is a good choice. https://www.lilygo.cc/products/t7-s3

[MartinP1]

Seems, that this is adopted meanwhile

if (heap_caps_get_free_size(MALLOC_CAP_8BIT) < 60 * 1024) { // checks free Heap+PSRAM

[gaborfri]

the funy part is, the esp32 s2 mini has 4mb flash an 2mb psram
https://www.wemos.cc/en/latest/s2/s2_mini.html
and the system status shows me the following information:
Heap (Free / Max Alloc) 63 KB / 39 KB
PSRAM (Size / Free) 2.045 KB / 2.034 KB
Flash Chip (Size / Speed) 4.096 KB / 80 MHz
i think i need to edit the 40*1024 and then it should work, lets try

[MichaelDvP]

You can set both values very low or remove the check. It is to keep esp32 without PSRAM responsive to web. ESP32 have 512kB SRAM, S2 only 320kB, but the large web chunks are allocated in PSRAM if present.
In normal operation the limit is not reached (or should not), only if there is a very weak wifi connection and high mqtt publish rate the queue consumes more and more memory and reaches the limit, Sadly the users with weak connection and high rate don't want to change anything on their side and keeps complaining about "freezing" web connection, so we use this way.

[math322]

I have set both values to 10 * 1024. Maybe we should move to heap_caps_get_free_size(MALLOC_CAP_8BIT) to get the full free heap size. But i don't know if this function works on Chips without PSRAM.

[proddy]

ESP.getFreeHeap() is the same as heap_caps_get_free_size() so it shouldn't make a difference.

@MichaelDvP I think all calls to ESP.getFreeHeap() and ESP.getMaxAllocHeap() should eventually be replaced with EMSESP::system_.getMaxAllocMem() and EMSESP::system_.getHeapMem() so we're only making these core system calls once in the main loop.

[math322]

This is how the output of that ram functions look like (S2 mini):

INFO 175188 61740 41488 36852 - 2094863 2074523 2070155 2064372 - 37888 65536 2094080 2136263

LOG_INFO("INFO %d %d %d %d - %d %d %d %d - %d %d %d %d",
             // internal RAM
             ESP.getHeapSize(),
             ESP.getFreeHeap(),
             ESP.getMinFreeHeap(),
             ESP.getMaxAllocHeap(),
             // SPI RAM
             ESP.getPsramSize(),
             ESP.getFreePsram(),
             ESP.getMinFreePsram(),
             ESP.getMaxAllocPsram(),
             // other
             EMSESP::system_.getMaxAllocMem() * 1024,
             EMSESP::system_.getHeapMem() * 1024,
             emsesp::EMSESP::system_.PSram() * 1024,
             heap_caps_get_free_size(MALLOC_CAP_8BIT));

[kami83]

Hi, i also used math322 description. works fine. But which pins for TX/RX did you used on the Wemos S2 mini board? I found TX=12 and RX=11 is this right? BR kami

[proddy]

yes, should be fine

[MartinP1]

Hmm, it seems quite complicated....
Had success to build the binary for esp32-s2 mini using devcontainers.

Output binary is EMS-ESP-3_8_3-dev_5-ESP32S2-4MB+.bin.
Is this the only thing needed to flash?

no special partitions.bin and boot-app0.bin?

If needed, how to build the two, or where to find?

EDIT!

Got it running....

After building I found the following in folder ./build/firmware:

-rw-r--r-- 1 martin martin 1698336 Mai 31 13:38 EMS-ESP-3_8_3-dev_5-ESP32S2-4MB+.bin
-rw-r--r-- 1 martin martin      32 Mai 31 13:38 EMS-ESP-3_8_3-dev_5-ESP32S2-4MB+.md5

(that was causing my question)

After some searching found this in folder ./.pio/build/s2_4M_P

martin@martin-D2836-S1:~/github/EMS-ESP32/.pio/build/s2_4M_P$ ls -l *.bin
-rw-r--r-- 1 martin martin   12176 Mai 31 13:36 bootloader.bin
-rw-r--r-- 1 martin martin 1698336 Mai 31 13:38 firmware.bin
-rw-r--r-- 1 martin martin    3072 Mai 31 13:36 partitions.bin

this was the esptool invocation that worked for me (adopted from the one @gaborfri posted in this thread on Feb 18, 2024)

martin@martin-D2836-S1:~/github/EMS-ESP32/.pio/build/s2_4M_P$ sudo python3 /home/martin/.platformio/packages/tool-esptoolpy/esptool.py --port /dev/ttyACM0 write_flash 0x1000 bootloader.bin 0x8000 partitions.bin 0x10000 firmware.bin

The ESP32 s2 mini is already added to my home wifi network...

Blinking frequency of blue LED is reduced

Not connected yet to interface board and my buderus WLW186i

EDIT2:

Up and running now at buderus WLW 186i

Some protocol errors (caused by the weak single core ESP32S2 mini?)

2026-06-01T14:11:05.240 W 22: [telegram] Incomplete Rx: CB 00 4B 00 88 00 08 00 90 00 10 00 C4 (crc: C2)
2026-06-01T14:27:28.427 I 23: [mqtt] MQTT connected
2026-06-01T14:33:12.094 W 24: [telegram] Incomplete Rx: 88 00 08 00 A3 (crc: 34)

[MartinP1]

First experiences:

Flashed Version 3.8.3-dev.5

It seems, that frequently the web server including api stalls. I think that happens when a second client connects to the web server.

MQTT seems to survive the stall.

May be a more performant will resolve the problems. Ordered ESP32S3 with 16 MB flash and 8 MB psram.