Uno R4 Built-in LED Matrix Randomly Flickering

Other Hardware Science and Measurement
1

I'm working on a little project that takes the current temperature from a DHT11 temperature/humidity sensor and prints the temperature graphically to the built-in display.

It seems like every few updates, there will be one randomly flickering pixel. It gets super bright for a fraction of a second, and is a pretty unpleasant visual effect. Can anyone take a look at my code and suggest reasons why this might be happening? Or is it a hardware effect?

#include "Arduino_LED_Matrix.h" 
ArduinoLEDMatrix matrix;
#include "DHT.h"
#define DHTPIN A0
#define DHTTYPE DHT11
DHT dht(DHTPIN, DHTTYPE);

byte result[8][12] = {
  { 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 },
  { 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 },
  { 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 },
  { 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 },
  { 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 },
  { 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 },
  { 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 },
  { 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 }
};

byte zero[8][6] = {
  {0,0,0,0,0,0},
  {0,1,1,1,1,0},
  {0,1,0,0,1,0},
  {0,1,0,0,1,0},
  {0,1,0,0,1,0},
  {0,1,0,0,1,0},
  {0,1,1,1,1,0},
  {0,0,0,0,0,0}
};

byte one[8][6] = {
  {0,0,0,0,0,0},
  {0,0,1,0,0,0},
  {0,1,1,0,0,0},
  {0,0,1,0,0,0},
  {0,0,1,0,0,0},
  {0,0,1,0,0,0},
  {0,1,1,1,0,0},
  {0,0,0,0,0,0}
};

// Number 2
byte two[8][6] = {
  {0,0,0,0,0,0},
  {0,1,1,1,0,0},
  {0,0,0,1,0,0},
  {0,0,0,1,0,0},
  {0,0,1,0,0,0},
  {0,1,0,0,0,0},
  {0,1,1,1,1,0},
  {0,0,0,0,0,0}
};

// Number 3
byte three[8][6] = {
  {0,0,0,0,0,0},
  {0,1,1,1,0,0},
  {0,0,0,1,0,0},
  {0,0,1,1,0,0},
  {0,0,0,0,1,0},
  {0,0,0,0,1,0},
  {0,1,1,1,0,0},
  {0,0,0,0,0,0},
};

// Number 4
byte four[8][6] = {
  {0,0,0,0,0,0},
  {0,1,0,1,0,0},
  {0,1,0,1,0,0},
  {0,1,1,1,0,0},
  {0,0,0,1,0,0},
  {0,0,0,1,0,0},
  {0,0,0,1,0,0},
  {0,0,0,0,0,0}
};

// Number 5
byte five[8][6] = {
  {0,0,0,0,0,0},
  {0,1,1,1,1,0},
  {0,1,0,0,0,0},
  {0,1,1,1,0,0},
  {0,0,0,0,1,0},
  {0,0,0,0,1,0},
  {0,1,1,1,0,0},
  {0,0,0,0,0,0}
};

// Number 6
byte six[8][6] = {
  {0,0,0,0,0,0},
  {0,1,1,1,1,0},
  {0,1,0,0,0,0},
  {0,1,1,1,0,0},
  {0,1,0,0,1,0},
  {0,1,0,0,1,0},
  {0,1,1,1,0,0},
  {0,0,0,0,0,0}
};

// Number 7
byte seven[8][6] = {
  {0,0,0,0,0,0},
  {0,1,1,1,1,0},
  {0,0,0,0,1,0},
  {0,0,0,1,0,0},
  {0,0,1,0,0,0},
  {0,0,1,0,0,0},
  {0,0,1,0,0,0},
  {0,0,0,0,0,0}
};

// Number 8
byte eight[8][6] = {
  {0,0,0,0,0,0},
  {0,1,1,1,1,0},
  {0,1,0,0,1,0},
  {0,1,1,1,1,0},
  {0,1,0,0,1,0},
  {0,1,0,0,1,0},
  {0,1,1,1,1,0},
  {0,0,0,0,0,0}
};

// Number 9
byte nine[8][6] = {
  {0,0,0,0,0,0},
  {0,1,1,1,1,0},
  {0,1,0,0,1,0},
  {0,1,1,1,1,0},
  {0,0,0,0,1,0},
  {0,0,0,0,1,0},
  {0,1,1,1,0,0},
  {0,0,0,0,0,0}
};


byte blank[8][6] = {
  {0,0,0,0,0,0},
  {0,0,0,0,0,0},
  {0,0,0,0,0,0},
  {0,0,0,0,0,0},
  {0,0,0,0,0,0},
  {0,0,0,0,0,0},
  {0,0,0,0,0,0},
  {0,0,0,0,0,0}
};

// Subroutine to take two 8x6 bitmaps and combine them into an 8x12 frame
void combineDigits(byte result[8][12], byte digit1[8][6], byte digit2[8][6]) {
    for (int i = 0; i < 8; ++i) {
        for (int j = 0; j < 6; ++j) {
            result[i][j] = digit1[i][j];
        }
    }

    for (int i = 0; i < 8; ++i) {
        for (int j = 0; j < 6; ++j) {
            result[i][j + 6] = digit2[i][j];
        }
    }
}


// Define an arbitrary lastTemperature
int lastTemperature = 70;
// Subroutine to take a temperature and show it on the matrix
void displayTemperature(int temperature) {
  // Don't do anything unless the temperature has changed
  if (lastTemperature != temperature){
    // Separate the temperature digits
    int digit1 = temperature / 10;
    int digit2 = temperature % 10;

    // Define arrays for the digits
    byte digit1Array[8][6];
    byte digit2Array[8][6];

    // Array to index what digit corresponds to what array
    byte (*digitArrays[])[6] = {zero, one, two, three, four, five, six, seven, eight, nine};

    // Ensure digit1 and digit2 are within the valid range
    if (digit1 >= 0 && digit1 <= 9 && digit2 >= 0 && digit2 <= 9) {
      // Copy the digit arrays to digit1Array and digit2Array
      byte digit1Array[8][6];
      byte digit2Array[8][6];

      for (int i = 0; i < 8; ++i) {
          for (int j = 0; j < 6; ++j) {
              digit1Array[i][j] = digitArrays[digit1][i][j];
              digit2Array[i][j] = digitArrays[digit2][i][j];
          }
      }

    combineDigits(result, digit1Array, digit2Array);
    matrix.renderBitmap(result, 8, 12);
    }
  }
  lastTemperature = temperature;
}
  
void setup() {
    Serial.begin(9600);
    Serial.println(F("DHTxx test!"));
    dht.begin();
    matrix.begin();
}

void loop() {
    float h = dht.readHumidity();
    int t = dht.readTemperature(true); // The true parameter passed is to read as Fahrenheit, the default is Celsius

    // Display the temperature on the matrix
    displayTemperature(t);
    Serial.println(t);
    delay(2000);
}
2

I am not very familiar with the UNO R4 but I am inclined to think the problem is your code is being interrupted while writing the pixels. I am not sure if this is a good idea but try disabling the interrupts while writing the display. This could mess up tasks such as timers etc. This will allow you to test if this is correct.

3

Disabling interrupts made the R4 LED matrix not work at all... weird.

Here's a video of what I'm describing:

4

There is an interaction between the DHT library and the matrix display

Don't call dht.readHumidity() or dht.readTemperture() and set t to a random number and the problem does not occur.

However, if you do this

    int t = dht.readTemperature(true);  // The true parameter passed is to read as Fahrenheit, the default is Celsius
    t = random(100);

then the problem occurs. There must be a clash of resources or timing

6

That sounds very plausible

7

It sounds to me then, that this problem isn't easily alleviated without delving into the code for the DHT library? Any possible workarounds to try? Thanks Delta_G and UKHeliBob for your input.

10

Hello! Same issue here using a dht22..

11

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