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Topic: Another silly common cathode/anode question... (Read 834 times) previous topic - next topic

Easty

...but it's got me confused.

I am planning to use an LED matrix with the MAX7219.

Is there a difference between common cathode and common anode matrices?

Surely if you have the cathode connections and anode connections together there's no difference as long as you are driving it the right way round?

Sorry for the silly question but I'm confused!

Oracle

For a matrix, there shouldn't be.  You have the anodes along the rows and cathodes along the columns (or vice versa, but it's the same thing, just rotate it 90 degrees).

For 7-segment LEDs, there is a difference.  For a common anode display, on each digit you address the cathodes individually, but there is one "common anode" for that digit.  And it's repeated for all the digits.

The 7219 is also intended to drive 8 7-segment digits, so that's where the distinction becomes important for that chip.

Easty

Thanks Oracle. I understood the 7 segment differences, but having read the information in the playground section (http://www.arduino.cc/playground/Main/MAX72XXHardware) and the fact that RS have it down as common anode (http://uk.rs-online.com/web/search/searchBrowseAction.html?method=searchProducts&searchTerm=451-6650&x=30&y=18) I had to ask!

Sparks

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For a matrix, there shouldn't be.  You have the anodes along the rows and cathodes along the columns (or vice versa, but it's the same thing, just rotate it 90 degrees).


True if the matrix is a single color - like yours.

Oracle

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For a matrix, there shouldn't be.  You have the anodes along the rows and cathodes along the columns (or vice versa, but it's the same thing, just rotate it 90 degrees).


True if the matrix is a single color - like yours.


I was thinking of a single colour matrix.

I just looked at the playground link for the first time.  Why do they have c1 and c2 in parallel like that across the power supply.  Caps add in parallel, so a 10uF and 100nF side by side are basically 10.1uF...it's within the tolerance of the 10uF cap.  It seems a silly thing to do.  Why not just have the 10uF?

kg4wsv

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Caps add in parallel

At DC, yes.  With AC, there's a bit more going on.

In this case, they filter ripples on the output power.  Two different values filter on different frequency ranges.

-j


Oracle

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In this case, they filter ripples on the output power.  Two different values filter on different frequency ranges.



That does make sense, thanks.

It does raise the question of whether the chip is that sensitive that it's actually necessary (and I fully realize that switching 160mA of LEDs at kHz frequencies can wreak havoc on the voltage level, especially off batteries).  I would have put my normal 100nF cap across the power supply without even thinking about it before I saw that schematic.

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