1.2" 7 segment display flicker

by jefflane on Thu Mar 21, 2013 1:32 pm

Very visible flicker on green 1.2" 7 segment display chained to another 1.2 " 7 segment and two 8x8 matrix displays. The red 7 segment has minimal flicker. Flicker is pronounced while displaying info on the 8x8 matrix displays. Each of the four displays are uniquely addressable.
The displays are connected to an Arduino UNO .
Any suggestions on how to minimize this flicker?
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Re: 1.2" 7 segment display flicker

by adafruit_support_mike on Thu Mar 21, 2013 3:40 pm

Are you using independent displays or the I2C backpack versions?
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Re: 1.2" 7 segment display flicker

by jefflane on Thu Mar 21, 2013 10:59 pm

The 7 segment displays and 8x8 matrix displays are all I2C backpack versions.
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Re: 1.2" 7 segment display flicker

by adafruit_support_mike on Fri Mar 22, 2013 10:23 am

Okay, that gives us a few things to look at.

First, and simplest, it's possible that the displays are drawing more current than the supply can deliver. An individual LED only draws about 20mA, but they add up. In this case, you've got 64 LEDs in the each of the 8x8 displays, plus 32 LEDs in each of the 4x7-segment displays, for a total of 192 devices that can draw power. Your peak draw would be 3.84 amps plus whatever it takes to keep the support circuits running. If we assume 50% of the LEDs are off at any given moment, the nominal load would still be around 2A.

The second option is closely related to that, but is more of a wiring issue than a supply issue. Every wire and connector has some small (or not so small) amount of resistance. When you run currents in the Amp range through those resistances, you get voltage.. for the 2A nominal value mentioned above, 1 ohm of resistance in your wiring will produce 2v of voltage drop.

If you have the power connections wired in series, each display sees the combined voltage drop of all the ones between it and the power supply. If the display that's flickering is the one farthest out in the chain, that's probably where the trouble is.

To avoid those problems, use a 'star' topology for your power connections: give each display its own set of wires directly to the power supply.

If that doesn't work (or if you've already done all that and are still getting flicker), we'll probably need to look at the I2C connections.
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Re: 1.2" 7 segment display flicker

by adafruit_support_bill on Fri Mar 22, 2013 10:50 am

If we assume 50% of the LEDs are off at any given moment

Since these are multiplexed, it should be less than that. I believe the 8x8's are multiplexed by row and the 7-segs are by digit.
I suspect you may be seeing the multiplex refresh.
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Re: 1.2" 7 segment display flicker

by jefflane on Sun Mar 24, 2013 9:45 am

The four displays are powered via Uno's 5 V pin in a star topology.
Any other thoughts as to how to minimize the multiplex refresh flicker?
Haven't tried it yet, would an external 5 V power supply help?
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Re: 1.2" 7 segment display flicker

by adafruit_support_bill on Sun Mar 24, 2013 11:21 am

If it is the multiplex refresh, it would be independent of power. It is the rate at which the driver updates them.

When you say flicker, do you mean occasional dimming, or a continuous flicker like you might get from a flourescent tube?
The former would be indicative of a power problem and an external power source would help. The latter would indicate that it is the multiplex refresh you are seeing. That is a function of how the displays are driven.
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Re: 1.2" 7 segment display flicker

by PIC18FORSTUDENTS on Thu Apr 25, 2013 12:32 am

7-SEGMENT DISPLAY EXAMPLE WITH COMPLETE CODE USING PIC18F4520 MICROCONTROLLER , MPLABIDE

http://pic18forstudents.co.uk/
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Re: 1.2" 7 segment display flicker

by adafruit_support_mike on Thu Apr 25, 2013 8:19 am

There's no need to shout.

In fact, if you can't follow up with a human-speech reply saying something useful about that link, I'm going to assume you're a link spammer and kill the post.
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