ESP32 Module Recommendations
Flash Memory
Due to the growing size of the firmware binaries, an ESP32 module with at least 8 MB is required to be able to use OTA updates. You may run OpenDTU-OnBattery on ESP32 boards with only 4 MB of flash memory, but you will have to update the firmware on these boards using the USB connection, i.e., physical access is required.
ESP32 vs. ESP32-S3
You should prefer an ESP32-S3 (note the "S3"). The ESP32-S3 has more GPIOs (not all may be pinned out) and a slightly more efficient core. More importantly, it has native USB support, freeing one hardware UART.
Make sure that the board, to which the ESP32-S3 is soldered, actually uses the native USB capability, such that the third hardware UART becomes available. Look out for a USB to UART transceiver. If there is one on the board, the chip is possibly not connected natively to the USB port. There are boards with two USB connectors, where one does connect natively to the ESP32. In that case, the board is fine.
ESP32-S3 Reserved Pins
Modules with 8 MB of PSRAM or 16 MB or more of flash memory use an octal SPI interface. On these modules, pins GPIO 35, 36, and 37, which are usually wired to the DevKit board's pin header, are in use for the internal communication between ESP32-S3 and SPI flash/PSRAM memory. Thus these pins are not available for external use, even though they are wired to the DevKit board's pin header.
| Name | Flash | PSRAM | Antenna |
|---|---|---|---|
| ESP32-S3-WROOM-1U-N16R8 | 16 MB | 8 MB | external |
| ESP32-S3-WROOM-1-N16R8 | 16 MB | 8 MB | PCB Trace |
| ESP32-S3-WROOM-1U-N16R2 | 16 MB | 2 MB | external |
| ESP32-S3-WROOM-1-N16R2 | 16 MB | 2 MB | PCB Trace |
| ESP32-S3-WROOM-1U-N16 | 16 MB | - | external |
| ESP32-S3-WROOM-1-N16 | 16 MB | - | PCB Trace |
| ESP32-S3-WROOM-1U-N8R8 | 8 MB | 8 MB | external |
| ESP32-S3-WROOM-1-N8R8 | 8 MB | 8 MB | PCB Trace |
| ESP32-S3-WROOM-1U-N8R2 | 8 MB | 2 MB | external |
| ESP32-S3-WROOM-1-N8R2 | 8 MB | 2 MB | PCB Trace |
| ESP32-S3-WROOM-1U-N8 | 8 MB | - | external |
| ESP32-S3-WROOM-1-N8 | 8 MB | - | PCB Trace |
You will not be able to leverage 16 MB of flash memory with OpenDTU-OnBattery any time soon. However, given the price difference, you might want to go for a version with 16 MB, allowing the module/board to be reused in a different project eventually.
