We did exactly this on building controllers (STM32 plus an ESP32 modem). Hermetry doesn’t care which chip ends up running the image, it only talks to the device that’s logged in, so the ESP32 is the one that downloads and reports.
When you assign a package, the device gets these shared attributes in one message on v1/attributes/set: fw_title, fw_version, fw_size, fw_checksum, fw_checksum_algorithm (SHA256), fw_url and fw_sig if the package has a detached signature. fw_url is a short-lived HTTPS address, valid for 15 minutes, and the download can resume with an HTTP Range request.
The device reports progress as client attributes: fw_state goes DOWNLOADING, DOWNLOADED, VERIFIED, UPDATING, UPDATED (or FAILED with fw_error), fw_progress 0 to 100 while downloading, and after the reboot current_fw_title and current_fw_version. UPDATED alone isn’t accepted, it wants to see the new running version.
Our flow on the ESP32:
1. fw_* arrives -> check fw_title == "irr-stm32"
2. GET fw_url (Range resume) into ESP32 flash, report fw_progress
3. SHA-256 + Ed25519 check against fw_sig -> fw_state VERIFIED
4. stream image over UART to the STM32 bootloader into bank 2
5. STM32 verifies again, swaps banks, reboots -> fw_state UPDATING
6. STM32 reports its version to ESP32 -> current_fw_versionYou have a 1 MB F407, which is single bank, so for 380 KB I’d use the upper 512 KB as the download slot and a small bootloader that copies it over after checking it. External flash works too, but then the bootloader needs the SPI driver.
On the profile’s Firmware tab turn on firmware updates (off by default), and leave Allow unsigned images off outside the test bench. Updates over LTE are off by default too. Set Hardware models on the package so an ESP32 image never ends up in the STM32 slot.