8 digit 7 segment display issues

Question

I've been trying to follow a few tutorials on how to get the 7 segment to display simple output but none seem to work 100%.

It starts out fine. It increments the number across the screen and then writes 8s to all of them and then reverses. After a few loops however it starts to act weird. Here's a video link where it acts a bit weird (a few digits don't write correctly and fix themselves) and then at the end, it only displays half of the 8 digit display before it goes to 8s and crashes. https://youtu.be/LD_C7I_bGyc

Here's the code:

//We always have to include the library
#include "LedControl.h"

// pin 4 is connected to the DIN pin
// pin 2 is connected to the CLK pin
// pin 3 is connected to the CS pin
// 1 as we are only using 1 MAX7219
LedControl lc = LedControl(4, 2, 3, 1);

void setup(){
    // the zero refers to the MAX7219 number, it is zero for 1 chip
    lc.shutdown(0, false);// turn off power saving, enables display
    lc.setIntensity(0, 0);// sets brightness (0~15 possible values)
    lc.clearDisplay(0);// clear screen
}

void loop(){
    // numbers 7 to 0
    for (int a = 0; a < 8; a++){
        lc.setDigit(0, a, a, false);
        delay(100);
    }
    delay(1000);
    // display number 8 on all segments
    for (int a = 0; a < 8; a++){
        lc.setDigit(0, a, 8, false);
        delay(100);
    }
    delay(1000);

    for (int a = 8; a >= 0; a--){
        lc.setDigit(0, a, a, false);
        delay(100);
    }
}

Hardware: Arduino Uno: https://www.jaycar.com.au/duinotech-uno-r3-development-board/p/XC4410 8 digit display: https://www.jaycar.com.au/8-digit-7-segment-display-module/p/XC3714

At this stage it's hard to tell if it's a hardware issue or if it's setup related. If someone could please give me some insight or maybe run it if they have the same hardware that would be fantastic.

Thanks!

Answer 1

The last loop exceeds the limits of 8 Digits. You iterate 9 times (0-8).

   for (int a = 7; a >= 0; a--){
        lc.setDigit(0, a, a, false);
        delay(100);
    }

Answer 2

This was an unexpected result. @Dougie was pretty on to it. I didn't know how to adjust the timings as I was pretty sure the library took care of it, so the only thing I could think of was to redo all the wiring again. That's when I noticed that the ground wire pin into the board was half sticking out.

In the video you can see the ground pin at the top right. So it was making a connection (otherwise the whole thing wouldn't work), but after replacing the wire it runs fine. So that would have messed up the timings if the connection was unstable.