# MAX7219 Entering Test Mode and Displaying Random Characters

I'm using MAX7219 with 6 common cathode 38mm (each segment has 2 red LEDs except DP) 7-segments controlled by Arduino Mega2560. I have connected MAX7219 and the board as stated in MAX7219's datasheet (DIN to MOSI[51th pin], CLK to SCK[52th pin], LOAD to 50th I/O pin). I'm using LedControl library as numerous sources suggested. Arduino board is supplied by a voltage regulator(10A max) that outputs 7.5V to the board's Vin and GND pins. The connection scheme is below.

Moreover, I have a SGR 56391 Wiper motor connected to Arduino compatible relay module as shown in below. This wiper motor draws about 400mA when I don't apply any external force, and draws 2.5A when I apply external force stopping its rotation.

The relay on the relay modules are srd-05vdc-sl-c and as far as I understand, I should not supply the relay module by Arduino Mega's 5V and GND pins (I had been doing so then I supplied the relay module from the 5V 3A regulator).

I have bunch of other inputs and devices but they are practically disabled because I'm using the test code given below.

THE PROBLEM is; when I upload the code and start the system, the display starts from 000000 to 999999 because of the for loop I have written in setup(), the motor works fine but the display most of the time becomes 888888 with full brightness as soon as motor stops or starts(not while moving). Sometimes, it takes more than 1 stopping or starting of motor for the display to become 888888 with full brightness. And sometimes, the display completely goes off. The other times (rarely) I saw random segments of random digits going on or off when the motor stops or starts. I have read through the MAX7219 datasheet again and found out that there is a mode called test mode which makes all the segments and digits on (i.e. 888888) with full brightness, but I have no code associated with test mode. I have also two videos showing the problem, the links are here and here.

So, I think the data from the board to MAX7219 IC (data is transmitted from 51th[MOSI] pin of the board to DIN pin of the IC) is corrupted for some reason but I don't know why and how and what to do about it. Maybe, I should just use BTS 7960B motor driver, I don't know.

Oh, and also I have seen no problem at all when I connect a much smaller DC motor instead of the one I'm using. But I don't know what are the technical specifications of that motor, I only have seen that it has a plastic gearwheel on the back and a small circuitry with bunch of electrical components.

I have put a lot of time and effort finding out the problem and trying to find out the solution, yet I have nothing so I need help. What do you guys think?


const byte raiseBarrier = 22;
const byte lowerBarrier = 23;
const byte Max7219_CS = 50;
const byte Max7219_DIN = 51;
const byte Max7219_CLK = 52;

LedControl MAX7219 = LedControl(Max7219_DIN, Max7219_CLK, Max7219_CS, 1);

void setup() {
pinMode(lowerBarrier, OUTPUT);
pinMode(raiseBarrier, OUTPUT);

digitalWrite(lowerBarrier, HIGH);
digitalWrite(raiseBarrier, HIGH);

MAX7219.shutdown(0, false);
MAX7219.setScanLimit(0, 6);
MAX7219.setIntensity(0, 10);
MAX7219.clearDisplay(0);

for(int i=0; i<10; i++){
for(int j=0; j<6; j++){
MAX7219.setDigit(0, j, i, false);
}
delay(250);
}
}

void loop() {
digitalWrite(raiseBarrier, LOW);
delay(200);
digitalWrite(raiseBarrier, HIGH);
delay(1000);

digitalWrite(lowerBarrier, LOW);
delay(200);
digitalWrite(lowerBarrier, HIGH);
delay(1000);
}

• count 0 to 9 on only one digit then delay for a second or two before exiting setup() – jsotola Jul 12 at 16:02

You have an EMI crosstalk problem from the abrupt break of motor inductance with current and made worse with any contact bounce on start or stop.

There are a few standard arc suppression methods.

1. Add clamping diode pair on each motor contact to prevent high voltage arc during open circuit. Use 1N400x to each rail, so they are normally reverse biased. This prevents over/under voltage spikes V=LdI/dt where dt is almost instant.
2. Then use twisted pair power cable at right angles to the control cable to minimize crosstalk.
3. RC snubber to choose low Q filter 3A rated parts (optional)

Stopping the motor with BEMF with a short circuit across contacts will generate the locked rotor current of 2.5A until stopped and reversing the voltage will rise quickly to a 5A surge since the apparent motor voltage from BEMF will be double until steady state 400 mA if no load.

Normally this might done with a (>5x rated) 10A Nch H bridge with the top pair controlling direction and the bottom with PWM to create a boost voltage for top Vgs and also control current + acceleration with PWM rate.

• I have applied all three. It seems the diodes has stopped back EMF messing up the circuit. Although, you said that 1N400X would be enough, I have seen that their current rate is 1 A, so I have used another diode name of which I don't remember but its rate was 3A. Thanks a lot for the help, appriciated it. – Ekrem Kılıç Jul 14 at 11:19
• Great! I expected Diodes are the strongest reduction but 1N400x 1A is Average , Peak Forward Surge Current 30A 8.3ms, half sine – Tony Stewart Sunnyskyguy EE75 Jul 14 at 13:15
• I also should add that data lines DIN, CLK, LOAD/CS should be put on PCB in a way that other lines and these 3 lines cannot affect each other. This is needed, since MAX7219 uses SPI and it is 4 MHz. – Ekrem Kılıç Sep 3 at 10:04