4-digit 7-segment display resets when common pin set to HIGH

Question

I'm trying to make a simple stopwatch application with my Arduino Mega 2560 and a 4-digit 7-segment display, but the leftmost three digits only light up for a split second before they go out every time the counter increments up. The rightmost digit is counting normally.

After going back to basics to diagnose the problem, I've discovered that every time one of the common pins is set back to high so I can pick another digit to manipulate, the digit goes blank.

Code for my basic program, which should display "0123" on my display.

int A = 2;
int B = 3;
int C = 4;
int D = 5;
int E = 6;
int f = 7;
int G = 8;
int P = 9;

int D4 = 10;
int D3 = 11;
int D2 = 12;
int D1 = 13;

void setup() {
  pinMode(A, OUTPUT);
  pinMode(B, OUTPUT);
  pinMode(C, OUTPUT);
  pinMode(D, OUTPUT);
  pinMode(E, OUTPUT);
  pinMode(f, OUTPUT);
  pinMode(G, OUTPUT);
  pinMode(P, OUTPUT);
  pinMode(D1, OUTPUT);
  pinMode(D2, OUTPUT);
  pinMode(D3, OUTPUT);
  pinMode(D4, OUTPUT);

  digitalWrite(D1, LOW);
  digitalWrite(D2, HIGH);
  digitalWrite(D3, HIGH);
  digitalWrite(D4, HIGH);
  delay(5);

  //0
  digitalWrite(A, HIGH);
  digitalWrite(B, HIGH);
  digitalWrite(C, HIGH);
  digitalWrite(D, HIGH);
  digitalWrite(E, HIGH);
  digitalWrite(f, HIGH);
  digitalWrite(G, LOW);

  digitalWrite(D1, HIGH);
  digitalWrite(D2, LOW);
  digitalWrite(D3, HIGH);
  digitalWrite(D4, HIGH);
  delay(5);

  //1
  digitalWrite(A, LOW);
  digitalWrite(B, HIGH);
  digitalWrite(C, HIGH);
  digitalWrite(D, LOW);
  digitalWrite(E, LOW);
  digitalWrite(f, LOW);
  digitalWrite(G, LOW);

  digitalWrite(D1, HIGH);
  digitalWrite(D2, HIGH);
  digitalWrite(D3, LOW);
  digitalWrite(D4, HIGH);
  delay(5);

  //2
  digitalWrite(A, HIGH);
  digitalWrite(B, HIGH);
  digitalWrite(C, LOW);
  digitalWrite(D, HIGH);
  digitalWrite(E, HIGH);
  digitalWrite(f, LOW);
  digitalWrite(G, HIGH);

  digitalWrite(D1, HIGH);
  digitalWrite(D2, HIGH);
  digitalWrite(D3, HIGH);
  digitalWrite(D4, LOW);
  delay(5);

  //3
  digitalWrite(A, HIGH);
  digitalWrite(B, HIGH);
  digitalWrite(C, HIGH);
  digitalWrite(D, HIGH);
  digitalWrite(E, LOW);
  digitalWrite(f, LOW);
  digitalWrite(G, HIGH);

}

void loop() {
}

But all the display shows is a 3 on the rightmost digit. I followed the video in this guide and hooked my Mega up the same way. Is this still the right way to code a 4 digit display to make all the digits display different numbers, or is this normal behavior?

Answer 1

These 4-digit LED displays use multiplexing which means that (to keep the pin count down) you only display each digit for a brief time. Because of the eye's persistence of vision, if you repeatedly display the 4 digits they all appear to be lit up.

I have a video that illustrates this effect with a LED sign. There are a lot more than 4 digits, but the video gradually shows what happens if the delay between each digit is reduced. Watch it through to the end or you won't get the effect. It is about 45 seconds long.

In your case, as jsotola suggested in their answer, if you move the code which displays the digits out of setup and into loop then the code is done repeatedly, which means that the digits are now displayed repeatedly, and you should see all of them.

Since, in your original code, you had the display code in setup then it was done once, and the final digit was the only one you saw.

Answer 2

This should work (sort of). It will be flickery, but it should give you a starting place to work from.

int A = 2;
int B = 3;
int C = 4;
int D = 5;
int E = 6;
int f = 7;
int G = 8;
int P = 9;

int D4 = 10;
int D3 = 11;
int D2 = 12;
int D1 = 13;

void setup() {

  // put your setup code here, to run once:
  pinMode(A, OUTPUT);
  pinMode(B, OUTPUT);
  pinMode(C, OUTPUT);
  pinMode(D, OUTPUT);
  pinMode(E, OUTPUT);
  pinMode(f, OUTPUT);
  pinMode(G, OUTPUT);
  pinMode(P, OUTPUT);
  pinMode(D1, OUTPUT);
  pinMode(D2, OUTPUT);
  pinMode(D3, OUTPUT);
  pinMode(D4, OUTPUT);

}

void loop() {
  // put your main code here, to run repeatedly:

  digitalWrite(D1, LOW);
  digitalWrite(D2, HIGH);
  digitalWrite(D3, HIGH);
  digitalWrite(D4, HIGH);

  //0
  digitalWrite(A, HIGH);
  digitalWrite(B, HIGH);
  digitalWrite(C, HIGH);
  digitalWrite(D, HIGH);
  digitalWrite(E, HIGH);
  digitalWrite(f, HIGH);
  digitalWrite(G, LOW);

  digitalWrite(D1, HIGH);
  digitalWrite(D2, LOW);
  digitalWrite(D3, HIGH);
  digitalWrite(D4, HIGH);

  //1
  digitalWrite(A, LOW);
  digitalWrite(B, HIGH);
  digitalWrite(C, HIGH);
  digitalWrite(D, LOW);
  digitalWrite(E, LOW);
  digitalWrite(f, LOW);
  digitalWrite(G, LOW);

  digitalWrite(D1, HIGH);
  digitalWrite(D2, HIGH);
  digitalWrite(D3, LOW);
  digitalWrite(D4, HIGH);

  //2
  digitalWrite(A, HIGH);
  digitalWrite(B, HIGH);
  digitalWrite(C, LOW);
  digitalWrite(D, HIGH);
  digitalWrite(E, HIGH);
  digitalWrite(f, LOW);
  digitalWrite(G, HIGH);

  digitalWrite(D1, HIGH);
  digitalWrite(D2, HIGH);
  digitalWrite(D3, HIGH);
  digitalWrite(D4, LOW);

  //3
  digitalWrite(A, HIGH);
  digitalWrite(B, HIGH);
  digitalWrite(C, HIGH);
  digitalWrite(D, HIGH);
  digitalWrite(E, LOW);
  digitalWrite(f, LOW);
  digitalWrite(G, HIGH);
}