Dot Matrix

Hardware

• A laptop
• An ESP8266 dev kit (Amica NodeMCU)
• A USB micro-B cable that can plug into your laptop
• A MAX7219 8x32 dot matrix display module
• 5 female-female jumper wires

Software

• VS Code
• PlatformIO VS Code extension

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A little electronics knowledge

Ohm's Law

• Voltage = Current * Resistance
• V = IR

A word on safety

• These electronics operate at low voltages (5V or 3.3V) and currents (< 1A)
• They will not shock you
• You might shock them!
  • Be careful of short circuiting exposed pins
    • i.e. keep away from metal, keep pins from touching each other
    • I = V / R
    • when R = 0, I = infinite
  • Be careful of electrostatic discharge

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What is a microcontroller?

And how is it different from the computers you're used to?

• Basically a little computer that only runs 1 program
• Tiny - can embed in almost anything
• Power efficient - can run on a battery
• Essentially instantaneous boot time
• You can interact with the physical world! 😎
  • Lights, motors, switches, sensors, etc
• You have to deal with the physical world 🙁
• Debugging can be difficult
  • Have to debug hardware as well as software
    • Connection issues
  • No stack traces
  • No debugger
• You can actually run out of memory, and you need to worry about this
• Much easier to run into performance issues

ESP8266 microcontroller

• CPU
• Memory
  • RAM - volatile
  • Flash, EEPROM - non-volatile
• Input/Output
  • 17 GPIO pins
  • SPI, I2C, I2S, UART
  • 10-bit ADC (Analog to Digital Converter)
• IEEE 802.11 b/g/n Wi-Fi

ESP8266 Dev Kit

Anatomy:

• ESP8266 microcontroller module
• Power regulator
• USB chip and connector
• Reset button
• Flash programming button
• GPIO (General Purpose Input/Output) pins
• we will be using pins:
  • GPIO14
  • GPIO13
  • GPIO15
  • GND
  • 3.3V

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Firmware

• The software that runs on a microcontroller
• Needs to be installed ('flashed') into the non-volatile flash memory of the microcontroller

Arduino

• Popular microcontroller software development platform
• Originally made for AVR microcontrollers, but has been ported to other chips, including ESP8266
• C++ library

Anatomy of an Arduino program:

• void setup() - runs once on startup
• void loop() - runs continuously after setup()

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Blink

The 'Hello world' of embedded microcontroller programming

Code

• Create new PlatformIO project
  • Name: Blinky
  • Board: NodeMCU 1.0 (ESP-12E Module)
  • Framework: Arduino

#include <Arduino.h>

void setup() {
  // put your setup code here, to run once:
  pinMode(LED_BUILTIN, OUTPUT);
}

void loop() {
  // put your main code here, to run repeatedly:
  digitalWrite(LED_BUILTIN, HIGH);
  delay(200);
  digitalWrite(LED_BUILTIN, LOW);
  delay(200);
}

• pinMode(pin, mode)
• digitalWrite(pin, value)
• delay(msec)

Connect

• Connect ESP8266 to computer via USB cable (& adapter)
  • USB cable used for communication and power
• Build firmware
• Upload firmware
• Reset button

Serial debug

#include <Arduino.h>

void setup() {
  // put your setup code here, to run once:
  Serial.begin(9600);
  Serial.println("Hello");
  
  pinMode(LED_BUILTIN, OUTPUT);
}

void loop() {
  // put your main code here, to run repeatedly:
  Serial.println("On!");
  digitalWrite(LED_BUILTIN, HIGH);

  delay(200);

  Serial.println("Off.");
  digitalWrite(LED_BUILTIN, LOW);
  
  delay(200);
}

• Serial.begin(baud_rate)
• Serial.printf()
• Serial Monitor

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MAX7219 matrix display

• The MAX7219 is an IC made for controlling 8 digit, 8 segment LED displays
• Can also be used to control 8x8 LED matrix displays
• Controlled via 3 pin SPI interface
• Can be daisy chained

Connect Matrix display to ESP8266 dev board

                        MAX7219 
         4 8x8 LED matrix modules = 32x8 matrix
		 
        <-OUT                                IN<-                  ESP8266
                                                                Amica NodeMCU
      [r0, c31]    24       16       8     [r0, c0]
 bit  row +--------+--------+--------+--------+   pin         pin
   0   0  |        |        |        |        |  label       label
   2   1  |        |        |        |        | - CLK   <->    D5  GPIO14  HSCLK
   4   2  |        |        |        |        | - CS    <->    D8  GPIO15  HCS
   8   3  |    3   |    2   |    1   |    0   | - DIN   <->    D7  GPIO13  HMOSI
  16   4  |        |        |        |        | - GND   <->   GND 
  32   5  |        |        |        |        | - VCC   <->   3V3 
  64   6  |        |        |        |        |
 128   7  |        |        |        |        |
          +--------+--------+--------+--------+
      [r7, c31]    24       16       8     [r7, c0]

Code

• Install MD_MAX72XX library

MD_MAX72XX matrix(
    MD_MAX72XX::FC16_HW,
    15, // CS
    4   // # of devices
);

• matrix.begin()
• matrix.clear()
• matrix.setPoint(row, column, state)
• matrix.setColumn(column, value)
  • uint8_t - 8 bits, 0 to 255

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Dot Matrix

• esp1/dot-matrix

Try it

• Build firmware
• Upload firmware
• Upload filesystem image
  • PlatformIO -> Project Tasks -> nodemcuv2 -> Platform -> Upload Filesystem Image
  • If you have problems, close any other processes that may be using the serial port, e.g. Serial Monitor

Initial setup

• Matrix should display dot matrix
• Open Serial Monitor
• Connect to the dot matrix wifi network with your phone
• The captive portal wifi settings page will appear
• Enter text into the text box to see it show up on the LED display
• Enter your wifi settings at the bottom of the page to have the Dot Matrix display connect to your wifi network instead of starting a captive portal

Normal operation

• Matrix should display -> wifi and it will attempt to connect to your wifi network
• When it connects, the matrix will display its IP address
• Open the IP address of the matrix in a browser
• Enter some text and see it show up on the matrix!
• Disconnect from computer, power with e.g. USB battery

Code anatomy

setup()

• server.serveStatic("/", LittleFS, "/web/")
  • data directory
• server.on(url, callback)
  • req->hasParam()
  • req->getParam()
  • req->send()

loop()

• render::update_display(&state)
• delay(state.loop_delay_msec)

Testing firmware

• Put tests in test dir
• Run PlatformIO: Test command

Updating web application code

• Put web application code in data/web/

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3D Printed Enclosure

Here are files to 3D print an enclosure for the LED matrix:

• cover.3mf
• base.3mf