Over the air (OTA) Updates

In this tutorial we will learn how to update the firmware of the ESP32 over the air (OTA). This mechanism is used by jag firmware update or by Artemis whenever a new firmware is sent to the device.


We assume that you have set up your development environment as described in the IDE tutorial.

We also assume that you have flashed your device with Jaguar and that you are familiar with running Toit programs on it. If not, have a look at the Hello world tutorial.

The OTA program will use HTTP to download the new firmware. While not necessary, you can have a look at the HTTP tutorial first.

A new firmware consists of updating a Toit firmware envelope with the desired containers and to build a binary image out of it. We will provide the necessary steps to create a new firmware, but you might need to update your setup to have the required tools (toit.compile and firmware) accessible. See the containers tutorial for how to set up your environment, and for more information on how containers and envelopes work.


We are going to download the new firmware through HTTP. As such, we will use the http package. To install it, run the following command:

jag pkg install github.com/toitlang/pkg-http@v2

See the packages tutorial for more information on Toit's package management system.

You can probably just write jag pkg install http, but the full ID together with the version is more explicit, and will make sure you get the right package.


The ESP32 has two partitions for storing firmware. This allows us to update the firmware without the risk of bricking the device. The firmware is always running from one of the partitions, while the other one is used for updating the firmware. When the update is complete, the device will reboot and run the new firmware. To avoid getting stuck with a bad firmware, the new firmware then needs to "validate" the update. Otherwise, the device runs the old firmware again once it reboots.

In this tutorial we will thus have two programs: one that downloads the new firmware and writes it to the other partition, and one that is run with the new firmware and validates the update.

The validator

The validator is a simple program that prints a message and then validates the update. Without that step the device would boot into the old firmware after a reboot. (Feel free to leave the validation out if you want to test the recovery mechanism.)

Typically, a validating program would check some important properties, like access to a server, before validating the update. If the check fails, the program would eventually just reboot the device, which would then run the old firmware again.

Create a file validate.toit with the following content:

import system.firmware

  print "hello after update"

  if firmware.is-validation-pending:
    if firmware.validate:
      print "firmware update validated"
      print "firmware update failed to validate"

This program imports the system.firmware library, which provides the is-validation-pending and validate functions. The first function returns true if the device booted into a new firmware, but has not yet validated the update. The second function validates the update and returns true if the validation was successful.

Preparing the new firmware

A new firmware needs to be extracted from Toit's firmware envelopes. Each SDK version comes with a set of envelopes the user can choose from. See the release assets for the latest SDK release and its envelopes.

Download the envelope you want to use. For this tutorial we will use firmware-esp32.gz. Gunzip it and extract the file:

gunzip firmware-esp32.gz

You should now have a file called firmware-esp32.

Compile the validate program to a snapshot and add it to the envelope:

toit.compile -w validate.snapshot validate.toit
firmware -e firmware-esp32 container install validate validate.snapshot

Optionally, add other containers to the envelope.

Run the firmware tool again to extract a binary image from the envelope:

firmware -e firmware-esp32 extract --format=binary -o ota.bin


For reference, here is a Makefile that creates an ota.bin file. It uses a different name for the envelope file (firmware.envelope), but otherwise follows the same steps.

The paths are set up for a Linux system with the Toit SDK installed in /opt/toit-sdk. You might need to adjust the paths to match your setup.

TOIT_SDK := /opt/toit-sdk
TOIT_COMPILE := $(TOIT_SDK)/bin/toit.compile
TOIT_FIRMWARE := $(TOIT_SDK)/tools/firmware
VERSION := $(shell $(TOIT_SDK)/bin/toit.lsp version)
ENVELOPE_URL := https://github.com/toitlang/toit/releases/download/$(VERSION)/firmware-esp32.gz

.PHONY: all
all: ota.bin

# Always repuild the firmware envelope since we modify it.
.PHONY: firmware.envelope
firmware.envelope: firmware.envelope.gz
    gunzip -c firmware.envelope.gz > firmware.envelope

    curl -L -o $@ $(ENVELOPE_URL)

%.snapshot: %.toit
    $(TOIT_COMPILE) -w $@ $<

ota.bin: validate.snapshot firmware.envelope
    $(TOIT_FIRMWARE) -e firmware.envelope container install validate validate.snapshot
    $(TOIT_FIRMWARE) -e firmware.envelope extract --format=binary -o ota.bin

.PHONY: serve
serve: ota.bin
    python3 -m http.server 8000

.PHONY: version
    @echo $(VERSION)

Serving the new firmware

For this tutorial the new firmware needs to be served over HTTP. You can use any HTTP server you want, including the one presented in the HTTP file server tutorial, but for simplicity we will use Python's built-in HTTP server.

python3 -m http.server

This will serve the current directory on port 8000. Make sure the ota.bin file is in the current directory.

Now find the IP address of your desktop computer. You can use one of the following commands to find your LAN address:

  • Linux: ip -j route get 1
  • Macos: route -n get default
  • Windows: ipconfig

Installing the firmware

We will now write a program that downloads the new firmware and writes it to the other partition. Create a file ota.toit with the following content. Don't forget to replace <YOUR_IP> with the IP address of your desktop computer, as found in the previous step.

import http
import io
import net
import system.firmware

UPDATE-URL := "http://<YOUR_IP>:8000/ota.bin"

install-firmware reader/io.Reader -> none:
  firmware-size := reader.content-size
  print "installing firmware with $firmware-size bytes"
  written-size := 0
  writer := firmware.FirmwareWriter 0 firmware-size
    last := null
    while data := reader.read:
      written-size += data.size
      writer.write data
      percent := (written-size * 100) / firmware-size
      if percent != last:
        print "installing firmware with $firmware-size bytes ($percent%)"
        last = percent
    print "installed firmware; ready to update on chip reset"

  network := net.open
  client := http.Client network
    response := client.get --uri=UPDATE-URL
    install-firmware response.body

Note that the program expects the server to return a Content-Length header with the size of the firmware. Given this header, the body field of the response will have the content-size field set, giving us access to the size of the firmware. The program could also obtain the firmware size in other ways.

Flashing the firmware consists of three steps:

  1. Create a FirmwareWriter with the size of the firmware.
  2. Write the firmware to the writer.
  3. Commit the writer.

Finally, when the firmware is flashed, we call firmware.upgrade to reboot the device and run the new firmware.

Running the program

Typically, this program would be executed without Jaguar, as part of a container that was installed together with the firmware.

However, as long as Jaguar is not writing to the flash at the same time, (un)installing containers or a new firmware, it is safe to run the program with Jaguar. After a reboot the new firmware will then be running without the Jaguar service. You would need to reflash the device through Jaguar to get the Jaguar service back.

jag run ota.toit


Depending on how you have configured the network on your device, you might need to bake the WiFi credentials into the OTA image. This can be done by adding a config file with the credentials to the extraction step.

WIFI_CREDENTIALS := {"wifi.ssid":"<my-ssid>","wifi.password":"<my-password>"}

ota.bin: validate.snapshot firmware.envelope
    $(TOIT_FIRMWARE) -e firmware.envelope container install validate validate.snapshot
    $(TOIT_FIRMWARE) -e firmware.envelope property set wifi '$(WIFI_CREDENTIALS)'
    $(TOIT_FIRMWARE) -e firmware.envelope extract --format=binary -o ota.bin

We also recommend to only validate a new image once a network connection has been established. Otherwise, the device might be able to boot, but not connect to the network, and thus become unreachable.


We have seen how to update the firmware of the ESP32 over the air.

The program we wrote is very simple, but it can be extended to check for updates periodically, and to fetch the new firmware from a public server (like a GitHub release page).