Lecture 2: LEDs and NeoPixel

CSC 321: Embedded Sysytem

First Semester 2020/2021

Slide 1: The ESP32-WROVER-B MCU

Slide 2: CPU Pinout

Please note: The pin numbers IOxx on the ESP32 correspond to the GPIO pin numbers

Datasheet: https:// www.espressif.com /sites/default/files/documentation/esp32-wrover-b_datasheet_en.pdf

Slide 3: Meaning of pins on ESP32

Slide 4: How to program the processor

Several development tools are available: ESP_IDF: The official Espressif development tool. Includes a gcc compiler for the ESP8266 and the ESP32 Please see: https://docs.espressif.com/projects/esp-idf/en/latest The Arduino IDE Needs extensions for these processors and the different CPU boards see: https://docs.espressif.com/projects/esp-idf/en/latest MicroPython see: https://micropython-docs-esp32.readthedocs.io/en/esp32_doc

Slide 5: Flashing the code

Slide 6: esptool

• esptool is called from the Makefiles in ESP-IDF
• esptool is used when we upload code from the Arduino IDE to the processor flash
• esptool is used with Micropython IDE on uPyCraft/Thonny
  
  installs Micropython onto the processor flash

We can however also execute esptool directly.

Slide 7: How to write a Micropython program?

First we need a Micropython interpreter!

You find the sources here:

https://github.com/micropython/micropython/

In the repository you find ports for the ESP8266 and the ESP32.

In order to compile the code you need the ESP-IDF and its cross compilers

The code compiles into a binary file (firmware-combined.bin) which contains a boot loader and the interpreter.

This binary must be uploaded and flashed.

For documentation of the ESP32port of Micropython look at

http://docs.micropython.org/en/latest/esp32/tutorial/intro.html

Slide 8: How to communicate with the Micropython interpreter?

We use a serial connection passing through the micro USB connection.

As soon as we connect the processor card to the PC we see the serial port to connect for communication

Slide 9: What is REPL?

Slide 10: The communication tools: minicom

You see the command prompt and you can interact with Micropython.

But … how to upload scripts?

Slide 11: The command line tool ampy

Slide 12: IDE for Micropython: uPyCraft

Slide 13: Thonny

Slide 14: Thonny

Thonny is a complete Integrate Development Environment (IDE)

which lets you

• Access the REPL
  
  
• Create directories on the Micropython file system
  
  
• Upload scripts
  
  
• Syntax check scripts
  
  
• Run scripts
  
  
• Install Micropython on your processor board

Slide 15: Thonny

Thonny is an IDE for Python which has provisions for Micropython.

Under Tools → Options button you can select the type of

Python interpreter you intend to use.

Slide 16: Flashing Micropython

Compiling a new version of Micropython is substantially much easier with Thonny.

You still need to install esptool first. You just select your port and firmware then install.

Slide 17: IoT Hello World program

A “Hello World” program, just printing “Hello World” on the screen

does not look very exciting.

However, this is generally used to verify that the infrastructure

Compiler, linker, downloader, flash program

are working correctly

In embedded systems printing can be quite complex

and a blinking LED is used instead.

Slide 18: Switching on and off a LED

The ESP32-WROVER-B have a “user LED” connected to GPIO 19.

How do we control this LED?

• Define that the LED is connected to GPIO 19
  
  
• Program this pin as output
  
  
• Write a logic 1 to the pin to switch it on
  
  
• Write a logic 0 to the pin to switch it off
  
  
• The logic state may be inverted if the LED is active low

Slide 19: Micropython hardware functions

Slide 20: The machine.Pin class

Slide 21: Switch the LED on, version 1

Slide 22: Switch the LED on, version 2

Slide 23: The blinking LED

Now we put the code into a script and run it

Slide 24: Changing the light intensity

The LED is connected to a digital line which can only be set to 0 or Vcc.

How can we change the light intensity and dim the LED?

The light intensity depends on the average current flowing through the LED.

The answer is PWM: pulse width modulation.

Slide 25: PWM in Micropython

Slide 26: Our PWM implementation

Slide 27: The WS2812B LED

A more complex LED:

• rgb LED used in LED chains.
  
  
• each ws18b12 contains the 3 colored LEDs and a controller.
  
  
• Can be cascaded and individually addressed, depending on its position in the chain
  
  
• Needs precise timing
  
  
• To use it we pass through the neopixel library built into micropython

Slide 28: WS2812B timing

For all the details on the ws2812b look at

https://cdn-shop.adafruit.com/datasheets/WS2812B.pdf

24 bit of colour data puts 2e24 colours at your disposal

Coding of a single bit

The control word:

Slide 29: Cascading the WS2812B

Slide 30: Using the neopixel library

Slide 31: … and our code

We have a single neopixel connected to

GPIO pin 4 (ESP8266)

or

GPIO pin 21 (ESP32)

This code works on both CPUs!

-- Isaac Armah-Mensah - 2021-02-28

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