I am planning on maxing out my arduino and using both digital and PWM pins. Long story short, the wiring for the 2 do not seem to work the same, and all of the info I am finding points to using PWM pins for common anode rgb LED's. ![]() I thought, yay, this looks exactly like the common cathode RGB LED that came in my kit, I'll take 3. I got so antsy to want to control more than one RGB LED, that I went to radio shack and saw they had a "full color" led. Intensity didn't seem to vary much with the number of segments lit, which doesn't so much imply anything about the circuit (one lone segment should be sinking a solid 20mA or so, while all eight in parallel should be sinking maybe 3-4mA each, tops?), as about human visual sensitivity, which is insensitive to modest changes in brightness.I am playing with the make arduino starter kit I got in the last week like crazy. Obviously, this argument fails for fully integrated matrix drivers, which need the full area of both types! Incidentally, I once drove a 8x8 red LED matrix direct from a pair of 74LS273s, no resistors worked just fine, the pull-ups being effectively current-limited due to their kinda boosted-passive-pullup output stage. This might be external, in discrete transistors or an array, so it doesn't affect the matrix driver at least. Proportionate area is required in P-ch, for sourcing equal total currents to the anodes, of course. It's still true today, albeit to a lesser degree - in Si CMOS, the N-ch is about 2.5 times higher performance than P-ch, thus die area is saved using N-ch sinks. Going a bit further, most logic devices that were manufactured to be part of a uP/uC system in the 1980's through the 2000's were made compatible with the ubiquitous TTL as far as HIGH and LOW logic levels and the inertia for that technology trend has persisted, as well into this century. This has persisted all the way to current days. And since all of those bits of hardware had to do that then all of the software needed to conform as well. So, since this was the dominant technology for about 20 years through the late 1960's through the mid 1980's, everything that interfaced with those displays also had to handle them in a common-anode manner. That TTL output was connected to the base of said PNP. Also, since TTL had a (fairly) strong pull-down output structure, it would be able to strongly turn on a PNP digit strobe transistor connected to Vcc and that would then pull UP that common anode. Since LED displays with a common pin for anode (or cathode) would conduct much more current through the common pin than any of the individual cathodes (or anodes) and since that common pin would conduct much more current than even a LOW TTL gate could suck down that common pin was connected to a digit strobe transistor to turn the digit on. The dominant hardware technology then was TTL which was designed with a much stronger output LOW drive capability than an output HIGH drive capability. So most displays were made that way and most SW (what there was of it 50 years ago) was written to support that. Indeed, common-anode displays are more common because it was more convenient from a hardware standpoint to drive them that way. ![]() The answer to that question has some history behind it. But you'll see that on a lot of old schematics and it has become a habit to sink current into devices rather than source from the device. Most modern MCUs and logic chips have totem-pole outputs which can source as much as they can sink, so the choice now is arbitrary. So you could conceivably drive a display that is powered by more than 5 volts. It can allow a voltage higher than the chip's VCC appear on the segment drivers when they are off. The open collector configuration is convenient for other reasons. Since the 7447 can only provide a current sink for the segments, they must necessarily be common anode. ![]() It can only sink current through it's NPN output transistor which is in an "open collector" configuration. In fact if you look at the schematic of the outputs you'll see it can't source any current at all. ![]() TTL devices in this family could sink (take into an output) much more current than they could source (get out of an output). I think you can blame the 7447 LED Display Driver.
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