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Topic: Max7219 24 Leds (Read 617 times) previous topic - next topic

Existence

Oct 02, 2011, 09:46 pm Last Edit: Oct 03, 2011, 05:03 am by Existence Reason: 1
Hi,

I have a Max7219 that I'm trying to use to individually control around 24 Leds. I'm a little unsure how I should group the Leds and determine the scan-limit and Rset values since it's not a standard 8x8 matrix. For 24 Leds would it be best to do 3 (SEG) rows of 8 (Digits)? Or something like 5 Rows of 5 Digits?

I may be reading the Datasheet wrong but the Scan-limit seems to allow you to block scanning columns of digits so in the 3x8 example would that mean I set the scan-limit to 0xX2? There seems to also be a warning in the Datasheet about using (0xX2) but I can't quite figure out what it means.

Thanks for any guidance.


Grumpy_Mike

Yes set the scan register for 3 digits only.

Quote
There seems to also be a warning in the Datasheet about using (0xX2) but I can't quite figure out what it means.

It means when running only 3 digits then keep the current per segment below 30mA when calculating the Rset value.

Existence

#2
Oct 04, 2011, 03:17 am Last Edit: Oct 04, 2011, 03:31 am by Existence Reason: 1
So when you say per Segment do you mean all the Digits on that segment together? So 10ma per Led (3x10)? or 30ma max for each led?

{edit}
After reading the Datasheet again I'm think it's per LED. I'm running 20ma 2V Leds so as per this Table I should go with a 28K resistor.

   Forward voltage
ISeg   1.5V           2.0V            2.5V            3.0V           3.5V
40mA   12.2k?   11.8k?   11.0k?   10.6k?   9.69k?
30mA   17.8k?   17.1k?   15.8k?   15.0k?   14.0k?
20mA   29.8k?   28.0k?   25.9k?   24.5k?   22.6k?
10mA   66.7k?   63.7k?   59.3k?   55.4k?   51.2k?

Grumpy_Mike

Yes that's right a segment is the same as an LED.

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