I've got a common cathode, two digit seven segment display (LDD-N512RI-RA) hooked up to the GPIO pins of a microcontroller (Coldfire MCF52233 DEMO). The problem is that the brightness of the display varies with the number of segments lit. As more segments are activated, the display becomes dimmer. From what I've read, this happens if you try to use a single resistor for the common cathode instead of using one for each anode. However, I am using one for each anode.
I have very little experience with electronics and know very little about electricity, unfortunately. So until I learn more (which I want to do), I can only guess what the problem might be. In the meantime, can anyone suggest what is going wrong here?
More information: The GPIO pins provide 3.3V. The forward voltage/current of the segments is 2.2V/10mA. I'm using 100 Ohm 1/2W resistors for each of the segments. I've got the common cathodes hooked up to GPIO pins as well, and I set the pins to the cathodes to low to activate the displays (I'm not sure if that is the correct way to go about it.). I am alternating between displaying the left/right digit using a high frequency timer interrupt, which I realize may not be the best way to do it. I am already using the timer interrupt, though, so I hoped that I could tack on some simple digit alternating logic without a problem. The board is powered via a USB line to my computer. I'm happy to provide any other relevant information, just let me know exactly what is relevant.