Electronics > Beginners
Serial connecting of max7219
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marekpasek:
Hello,
Is it possible to drive a 8Xn display (where n is a number, which exceeds 8 ) just by connecting a couple of MAX7219 in series? I have a 10-digit seven segment VFD display with common grids for every digit and common anode for coresponding segment of every digit (10-MT-20GY in particular). I would like to use four of these to display data from arduino. Since I am not a programmer I want to use MAX7219, for which there exist dozens of libraries and tons of troubleshooting articles, so every problem with it is easily solvable. I want to use all ten digits, not just eight, so I need to use two 8-digit drivers for each display. And there comes the problem. Is this possible with MAX7219? I would have to connect the common segment pin of both MAX7219 together (not literally shorten them, just use some transistor AND gate to switch the anode voltage!). Since the datasheet tells nothing about timing of pulses on common cathode pins, I do not know, what is really happening on the outputs of serially connected chips. Do serially connected chips count to eight simultaniously or one by one? Could they not interfere with each other then?

Thank you very much for any response. If you find anything unclear in the text, please ask. Writing about this problem is for some reason very difficult for me (possibly because English lacks declension and the most of grammatical forms I would use in my native language), so sorry for that.
iMo:
Marku, yes you may wire N chips in series. There are libs which support N chips in series.
Page 13.
https://www.sparkfun.com/datasheets/Components/General/COM-09622-MAX7219-MAX7221.pdf

You load data serially to all N chips (as a X*N bits long bitstream) while they share Clock and Load (low) signal. Din/Dout-->Din/Dout-->Din/Dout..
When the Load goes high all chips start to display their own actual data loaded.

Mind each 7219 chip makes its own multiplexing (8 anodes x 8 cathodes).
So you need separate Anodes and Cathodes for each 7219.
marekpasek:
Well ,that is the problem. I have 8 anodes and 10 grids, therefore a cannot use the circuit shown on the page 13 of the datasheet.
Děkuji za odpověď.
iMo:
I think a small mcu with a piece of code can do NxM multiplexing easily..
marekpasek:
Agree. Bunch of 74595s and 4017s will do the job. :D
Thank you
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