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.

Since we’re using a 3.3v bus, the pullup resistor on DQ is 2.2KΩ, not 4.7K.

In addition to my modified TM1637 component, the project uses David Antliff’s components for the 1-Wire bus protocol and the DS18B20 digital thermometer. You’ll need to follow the instructions in the project README file to clone the project and recurse the three submodules.

Usage

After cloning the project, you’ll need to configure the GPIO pins in use using make menuconfig. One GPIO is used for the 1-Wire bus and two lines are used for each TM1637 display for a total of 5 GPIO pins. Be careful that you do not use pins that are in use for other purposes on your particular board. For example, I initially use GPIO12 for one of the TM1637 displays, but when connected, was unable to flash the ESP32. It appears that GPIO12 is used as a bootstrapping pin. On the particular board I used for this project, I believe that GPIO12 needs to be pulled low at reset but the TM1637 was not permitting it. Moving do another GPIO solved the problem.^[More about GPIO12 here]

After completing the hardware connections and specifying them in make menuconfig, you can then simply make flash to upload the application to the device.

All of the source code is on github.

References

• Source code for this project
• TM1637 datasheet
• DS18B20 datasheet
• My improved ESP32 TM1637 component
• esp32-owb - David Antliff’s ESP32-compatible C library for the 1-Wire bus protocol
• esp32-ds18b20 - and his ESP32 C component for the DS18B20 digital thermometer
• Source for the TM1637-based displays - This company also has presence on Aliexpress which is where I ordered these inexpensive displays.^[I prefer these displays to the 74HC595 because the TM1637 handles all of the multiplexing for you, whereas the shift-register displays require the host controller to take on constant refreshing of the display.]
• Pinout of the Doit ESP32 DEVKIT V1 - the board used in this project.

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.

MCP23017 component API

All of my custom components are on github. You can clone or download the MCP23017 component there. Once you’ve installed the MCP23017 component in the components directory of your project, you can follow the preliminaries here to start working with it.

Preliminaries

You will need to include the header file for the component in your project and specify the I2C pins that correspond to your hardware design:

#include "mcp23017.h"

#define I2C_SDA	23	// GPIO_NUM_23
#define I2C_SCL 22	// GPIO_NUM_22

In addition, you will also probably want to create a reference to the mcp23017_t structure on a global scope:

mcp23017_t mcp;

Initialization

Before we can use the component, we have to initialize it. The API provides a function for this purpose mcp23017_err_t mcp23017_init(mcp23017_t *mcp).

mcp.i2c_addr = MCP23017_DEFAULT_ADDR;
mcp.port = I2C_NUM_1;
mcp.sda_pin = I2C_SDA;
mcp.scl_pin = I2C_SCL;

mcp.sda_pullup_en = GPIO_PULLUP_ENABLE;
mcp.scl_pullup_en = GPIO_PULLUP_ENABLE;

esp_err_t ret = mcp23017_init(&mcp);
ESP_ERROR_CHECK(ret);

We simply populate the mcp23017_t struct with the data need to initialize the bus and pass its address to the initialization function, checking the return value to make sure everything initialized without error.

Reading and writing registers

The API provides two functions mcp23017_write_register and mcp23017_read_register to write and read MCP23017 registers respectively. To write to a register, you provide the address of the mcp23017_t struct, a register, a group and the value you are writing. All of the device registers are paired into A and B groups. So when you use one of these two functions you provide a register name and the specific group you’re addressing. For example to set all of the pins in group A to output mode:

mcp23017_write_register(mcp, MCP23017_IODIR, GPIOA, 0x00);

Reading is a little different because the return value of mcp23017_read_register like its write counterpart is still an error/status code. Therefore, we pass the address of the variable into which we’d like to read an eight-bit value.

uint8_t current_value;
mcp23017_err_t ret = mcp23017_read_register(mcp, reg, group, &current_value);

If you want to check out the complete demonstration project, you can find it on github.

References

• My ESP32 custom components respository
• MCP23017 datasheet
• MCP23017 ESP32 ESP-IDF demonstration project