C

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 Si7021is an excellent little device for measuring temperature and humidity, communicating with the host controller over the I2C bus. This is a quick tutorial on using the Raspberry Pi to talk to this device. If you are unfamiliar with the conceptual framework of I2C or how to enable I2C access on the Raspberry Pi, I suggest starting here. Otherwise, let’s jump in.

You are probably working with the device mounted on a breakout board. I used this one from Adafruit. There are no surprises on the pins that it breaks out - Vin, 3v out, GND, SCL and SDA. One the 40-pin P1 header of the Raspberry Pi, SDA and SCL for I2C bus 1 occupy pins 2 and 3.

I recently wrote about using the excellent bcm2835 library to communicate with peripheral devices over the SPI bus using C. In this post, I’ll talk about using the same library to communicate over the I2C bus. Nothing particularly fancy, but you’ll need to pay careful attention to the datasheet of the device we’re using. TheTSL2561 is a sophisticated little light sensor that has a very high dynamic range and is available on a breakout board from Adafruit. I’m not going to delve into the hookup of this device as you can take a look at the Adafruit tutorial for that. Note that we’re not going to use their library. (Well, I borrowed a bunch of their #define statements for device constants.)

Several years ago I wrote about adding analog-to-digital capabilities to the Raspberry Pi. At that time, I used an ATtinyx61 series MCU to provide ADC capabilities, communicating with the RPi via an I2C interface. In retrospect it was much more complicated than necessary. What follows is an attempt to re-do that project using an MCP3008, a 10 bit ADC that communicates on the SPI bus.

MCP3008 device

The MCP3008 is an 8-channel 10-bit ADC with an SPI interface^[Datasheet can be found here.]. It has a 4 channel cousin, the MCP3004 that has similar operating characteristics. The device is capable of performing single-ended or differential measurements. For the purposes of this write-up, we’ll only concern ourselves with single-ended measurement. A few pertinent details about the MCP3008: