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?
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?
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.
