Connecting of the unused MAX7219 pins to the ground?

Hi,
we use MAX7219 to control the 5x7 LED (LTP2157AE, total of 8 pieces). Since the module normally drives the 8x8 LED, we have 3 column outputs and 1 row output free (see appendix).

We used MaxMAtrix Library to display the texts on the prototype display (consisting of 2 LEDs) and it never worked properly. Only after we connected the unused pins 10, 5, 6 and 22 to ground it worked. It has been working long and stable!

Has anyone here gained the experience? Is it not that too big a burden for MAX7219? What voltage is there on the output pins and what resistance do the outputs have when they are not being used?

Which solution would be the most optimal solution here? Go through a resistance to the mass? If so, what resistance should we choose, to which we connect all the 4 outputs in parallel?

Thank you
Afriend2You

Also here

180f8d6ef3cf0bca0748fb20bedfbf9459a6a730.jpg

While the MAX7219 does seem to be reasonably immune to abuse of this sort, it is clearly not appropriate to do this.

The fault is most likely in your wiring layout and the failure to have a bypass capacitor on each IC.

Yes, the value of the “9.53K” resistor is completely absurd - but not a problem as such.

You should not connect unused pins to anything.

Only after we connected the unused pins 10, 5, 6 and 22 to ground it worked.

Then you had something else wrong.

What's wrong with 9.53K? https://www.digikey.com/products/en/resistors/through-hole-resistors/53?k=resistor&k=&pkeyword=resistor&pv2085=u9.53+kOhms&sf=1&FV=ffe00035&quantity=&ColumnSort=0&page=1&pageSize=25

CrossRoads: What's wrong with 9.53K? https://www.digikey.com/products/en/resistors/through-hole-resistors/53?k=resistor&k=&pkeyword=resistor&pv2085=u9.53+kOhms&sf=1&FV=ffe00035&quantity=&ColumnSort=0&page=1&pageSize=25

Well, I was just thinking, 9.76 might have been better ... :roll_eyes:

Hi Paul__B,
according to the attachments 9,76 would lower the Iseg und Vled to approx. 35 mA and 3,2 V appropriately and 9,53 give us 42 mA and 5 V approx.
Does it make a significant difference?

db452ba5db41dbac17a8195a4c2c7f532956f11d.png

d0edf5149655d665915f7d2c7c25634a6bfc2d95.png

afriend2you:
according to the attachments 9,76 would lower the Iseg und Vled to approx. 35 mA and 3,2 V appropriately and 9,53 give us 42 mA and 5 V approx.
Does it make a significant difference?

No, that is exactly the point, it makes absolutely no difference whatsoever!

The appropriate value is 10k - or anything resembling it or higher. 11k, 12k, even 15k would barely make a noticeable difference in brightness.


The point is that unused pins should not be connected to ground - or anything other than more LEDs if you needed them - which you might if you wished to attach additional indicators to your display.

If you have any unreliability, you need to correct the problem, which is most likely to be a failure to provide bypass capacitors to each MAX7219 and any other logic chips used, and to keep all the wiring laced tightly together so that it forms no open loops.