Esp32

Although FreeRTOS¹ is an indispensible tool for working on anything more than the simplest application on ESP32, there are some difficulties to master, such as multitasking. Multitasking using FreeRTOS is accomplished by creating tasks with xTaskCreate() or xTaskCreatePinnedToCore(). In both of these calls, one of the parameters is uxStackDepth which is the allocated stack size for the task. The FreeRTOS documentation on the subject is clear about the units for uxStackDepth:

In a previous post I wrote about displaying arbitrary data on a TM1637-based 4 digit LED display, highlighting an ESP-IDF component that I extended to display positive and negative floating point numbers. Now we’re going to put that component to use and display actual data from a DS18B20 temperature sensor.

The {% asset_link DS18B20.pdf “DS18B20” %} temperature sensor operates on the Dallas Semiconductor 1-Wire bus. In this application, we aren’t powering the devices using parasitic power. Instead we’re powering the device from an external supply.

While the ESP32 sports a number of GPIO pins, not all are broken out on every board, meaning that sometimes a GPIO expander is necessary. This project is a simple design to test interfacing the ESP32 to an MCP23017 via the I2C interface.

There are so many tutorials on the MCP23017 that I won’t delve in depth into how it works, but I’ll point out a few features of the custom MCP23017 component that I’m developing as part of this demonstration project. If you need to get up-to-speed developing applications using I2C within the ESP-IDF environment, this tutorial from Luca Dentella is excellent and concise.

There are three main types of 4 digit seven segment displays to be found on the market:

• Bare displays without any driver. These come in a variety of colors and with either decimal points or clock-type display with a colon dividing two sets of two digits.
• 74HC595-based displays. Usually these displays have two daisy-chained 74HC595 shift registers and rely on the host controller to fill the serial registers and handle the multiplexing. Depending on the processor speed, this ends up being a significant overhead.
• TM1637-based displays. These displays reduce the burden on the host controller because all of the multiplexing is handled on the interface chip.

This post is about the TM1637 LED displays. The {% asset_link Datasheet_TM1637.pdf TM1637 datasheet %} is terrible, but fortunately there are several libraries for Arduino that provide a little insight into how others have managed to make this work. First things first, the communication protocol for this device is not I2C despite what vendors on Aliexpress frequently claim.

Previously, I wrote about using the ESP32 to read sensor data over I2C from the Si7021 temperature and humidity monitor. Today, I’m going to briefly take you through the process of serving this data via the web.

Description

The project plan is to connect to WiFi in STA mode, collect temperature and humidity data every 5 seconds from a Si7021 sensor via the I2C bus. We will launch a web server and whenever we have a GET/ request we’ll serve a simple web page that reports the temperature and humidity. If the URL path is /h (e.g. 192.168.1.x/h) then we’ll turn on an LED connected to GPIO 4. If the path is /l (e.g. 192.168.1.x/l) then we’ll turn off the LED. In both latter cases, we’ll also serve the same page showing the temperature and humidity.

Recently I wrote about reading Si7021 temperature and humidity data using a Raspberry Pi. Now let’s try a completely different platform, the ESP32. This is essentially a project to explore using I2C on the ESP32 platform and to understand the build process.

Project layout

Since we’re developing the Si7021 interface code as a reusable component, we need to structure our project in such a way that we can easily refer to it in our main code. Here’s how I structured this project:

The Heltec WIFI Kit 32 is an interesting little module that integrates a WiFi/MCU SoC and a small OLED display on a single board. If you want to set up the Arduino IDE to work with this device and you’re on macOS, this is for you. This particular ESP32 module has a number of impressive features: 240 MHz processor speed and 4 MB of flash memory. The onboard OLED display can be conveniently used for debugging.