Train All the Things: Speed bumps
Sun Mar 29, 2020

As part of getting started on my project a couple months back I took a look at what boards were supported by Tensorflow lite. Seeing an esp board I went that route since I've heard alot from the maker/hacker community and thought it would be a good opportunity to learn more. Additionally it's been quite a while since I had a project that was primarily C/C++ so that was exciting. Like any good project I ran into multiple unexpected bumps, bugs and issues. Some were minor, others were frustrating. I'm capturing some of those here for anybody else that may be starting down the path of using Tensorflow Lite and an ESP32 board.

Tensorflow speed bumps

Getting started with TF Lite is easy enough, but something I noticed as I continued to work on the project is just how little things are designed specific to the platform. Instead the examples are setup with Arduino as a default, and then work is done to make that run on X target. In the case of the ESP-EYE this looks like packing everything into an Arduino compatible loop, and handling that in a single FreeRTOS task. I get the reason for this, but it's also a bit of a headache later on as it feels like an anti pattern when addin in new task and event handlers.

Another bump you are likely to notice is that the TF Lite examples rely on functionality present in the TF 1.x branch for training, but require TF >= 2.2 for micro libs. Not the end of the world, but it means your going to manage multiple environts. If managing this using venv/virtualenv keep in mind you're going to need the esp-idf requirements in the 2.x environment, or just install in both as you may find yourself switching back and forth. In addition to python lib versions the examples note esp-idf 4.0, but you will want to use >=4.0 with this commit or you will run into compiler failures. I ended up using 4.1 eventually, but something to note.

Finally interaction with the model feels flaky. It's an example so this kind of makes sense, but I found that while the word detected was pretty accurate the new_command and some of the attributes of the keyword being provided by the model weren't matching my expectation/use. I ended up using the score value and monitoring the model to setup the conditionals for responding to commands in my application.

Overall the examples are great to have, and walking you through the train, test and load cycle is really helpful. The main thing I wish I had known was that the TF Arduino path for ESP was pretty much the same as the ESP native path with regards to utility and functionality just using the esp-idf toolchain.

ESP speed bumps

From the ESP side of things the core idf tooling is nice. I like how open it is and how much I can understand the different pieces. This helped a few times when I ran into unexpected behavior. One thing to note is if you follow the documented path of cloning esp-idf you will want to consider how you manage the release branch you use and when you merge updates. Updates are not pushed into minor/bug fix branches instead they go into the release branch targeted on merge.

Being new to the esp platform something I didn't know when I got started was that esp-idf 4.x released in February of 2020. Because of this alot of the documentation and examples such as ESP-WHO and esp-skainet are still based on 3.x which has a variety of differences and changes in things like the TCP/network stack. Because of this checking the version used in various docs, examples etc is (as usual) important. Since the TF examples reference version 4 that's where I started, but a lot of what's out there is based on v3.

One other bump somebody may run into is struct initilization in a modern toolchain when calling the underlying esp C libraries from C++. I spent some time digging around after transitioning the http request example into the TF C++ command_responder code and the compiler told me I was missing uninitilized struct fields and their order made them required.

The example code:

esp_http_client_config_t config = {
        .url = "http://httpbin.org/get",
        .event_handler = _http_event_handler,
        .user_data = local_response_buffer,
};
esp_http_client_handle_t client = esp_http_client_init(&config);
esp_err_t err = esp_http_client_perform(client);

And how I had to do it in C++:

esp_http_client_config_t* config = (esp_http_client_config_t*)calloc(sizeof(esp_http_client_config_t), 1);
config->url = URL;
config->cert_pem = unexpectedeof_casa_root_cert_pem_start;
config->event_handler = _http_event_handler;

esp_http_client_handle_t client = esp_http_client_init(config);
esp_http_client_set_method(client, HTTP_METHOD_PUT);
esp_err_t err = esp_http_client_perform(client);

I had a similar issue with wifi and you can see the solution here.

I really enjoyed my lite trip into idf. It's an interesting set of components and followed a workflow that I use and appreciate. I wrote a couple aliases that somebody might find useful:

alias adf="export ADF_PATH=$HOME/projects/esp-adf"
alias idf-refresh="rm -rf $HOME/projects/esp-idf && git clone --recursive git@github.com:espressif/esp-idf.git $HOME/projects/esp-idf && $HOME/projects/esp-idf/install.sh"
alias idf=". $HOME/projects/esp-idf/export.sh"
alias idf3="pushd $HOME/projects/esp-idf && git checkout release/v3.3 && popd && . $HOME/projects/esp-idf/export.sh"
alias idf4x="pushd $HOME/projects/esp-idf && git checkout release/v4.0 && popd && . $HOME/projects/esp-idf/export.sh"
alias idf4="pushd $HOME/projects/esp-idf && git checkout release/v4.1 && popd && . $HOME/projects/esp-idf/export.sh"
alias idf-test="idf.py --port /dev/cu.SLAB_USBtoUART flash monitor"

And I look forward to writing more about esp as I continue to use it in new projects.

Approaching the end of this project it's been a larger undertaking than I expected, but I've learned a lot. It's definitely generated a few new project ideas. The code, docs, images etc for the project can be found here and I'll be posting updates as I continue along to HackadayIO and this blog. If you have any questions or ideas reach out.


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