Arduino Forum

Using Arduino => LEDs and Multiplexing => Topic started by: sircorruption on Oct 23, 2011, 11:06 pm

Title: No output/inconsistent output from MAX72XX
Post by: sircorruption on Oct 23, 2011, 11:06 pm
I'm building a project with a 16 col X 6 row matrix.  8x6 matrix per MAX72XX chip.

LEDs--
Forward voltage: 3.4v
Current: 20ma

Rset for MAX72XX--
33k Ohm

-Using LEDControl library to control the MAX72XX drivers

Used schematic from a previous project that was a 16x5 matrix that had no issues.

What I've done:

1. I started with using 2 MAX7219's and have no LEDs lighting up (but voltage in reverse polarity sitting on the LED legs).

2. Replaced the first MAX7219 in the cascade with a MAX7221 and get output to the LEDs wired to the MAX7221, but after the loop gets to a sequence where the LEDs are flashed on and off (using lc.clearDisplay for both displays then sending the bits for the display again for a few "animation frames") the output dies.

3. Replaced the second MAX7219 in the cascade with a MAX7221 (both chips MAX7221 at this point) and get no LEDs lighting again.

Currently using only USB power to test, but have had the 16X5 setup work from USB with no issues.

Has anyone else run into this sort of issue before or have any troubleshooting ideas?
Title: Re: No output/inconsistent output from MAX72XX
Post by: CrossRoads on Oct 24, 2011, 02:02 am
Anything feeling hot while this is going on?
Title: Re: No output/inconsistent output from MAX72XX
Post by: sircorruption on Oct 24, 2011, 03:43 am
No heat.

I'm building the circuit on an arduino shield and am getting the weirdest results right now.

With the shield pushed down all the way: two of the ICSP pins (1 and 6, or 1 and 2, whichever pin shares a plastic with pin 1) are touching pins on the legs of the second MAX72XX (tried with both 7221 and 7219). The pins being touching are SegE and SegDP. In this situation, SegE (or column 6) is constantly on and any data going to SegDP (col 0) is shown and then immediately the whole column goes blank.

I didn't think to check the voltage on the LEDs or the chips when this happened, and haven't been able to get it to reproduce again, but could this be a hint at one of the chips not getting enough voltage to power the LEDs?
Title: Re: No output/inconsistent output from MAX72XX
Post by: CrossRoads on Oct 24, 2011, 04:23 am
The MAX 7219/7221 switch the outputs at some high frequency, like 800 Hz. Shorting pins is gonna make weird things happen. Get that stuff fixed before proceeding.