Hello all. I searched the FAQ for resistor discussion, but didnt see anything and I searched g+ and gitter. Just wondering if anyone has a link or reference to a discussion of the pros/cons of different series resistor values for the WS2812 and APA102 clock/data lines. Is it 100ohm? 200ohm? 300ohm? Is the resistor for signal integrity only or is it to protect the microcontroller data lines incase of an accidental short to +Vcc? Thanks for any comments!!
-frenchy (Steve French)
http://www.voltvision.com
The use of a series resister in my opinion is twofold. Primarily as explained by others, it is used for impedance matching to minimize reflections that would impinge data signal integrity. I also view it as MC output protection. The value is determined by the length of wire used for the signal lines. Many determine it empirically by experimentation, I’ve seen commentary where a potentiometer is used. Typ values seem to be under 470 Ohms
If using CAT5 or CAT6 cable, with 1 signal per pair and the other wire connected to ground, then 100 ohms is the correct value to match the twisted pair’s characteristic impedance.
@PaulStoffregen is that true regardless of the length?
yes, impedance match is the same regardless of wire length. For CAT5 & CAT6, 100 ohms is always the right resistor. However, very long wires have all sorts of other issues besides impedance.
@Steve_French the term here is signal termination. Normally this is done at the end of a signal line, but it also can be done at the beginning. Then it’s called source termination. More on this:
http://www.ti.com/lit/an/snla034b/snla034b.pdf
I’m using a 220 Ohm 15 turn trimmer and carefully tune until problems are gone.
Best if you can monitor the signal with an Oscilloscope during this process.
Then I measure the pot and replace it with an E24 series resistor.
@Juergen_Bruegl what are you looking for when you tune with the trimpot and oscilloscope? Are you looking for oscillations and then minimizing them? Where do you probe? At the microcontroller pin? At the resistor? At the location of the first pixel? (I suppose this is the one that really matters). Do you think my old Picoscope 2203 is adequate bandwidth (5MHz)? Thx!
@PaulStoffregen thanks for your reply! So in your reply the only consideration is signal integrity. The resistor is not also for protecting the micro pin from accidental shorts to +Vcc? For a common +5v pixel, an accidental field-side short to +5v would inject a max of 5v / 100ohm = 50mA back into the micro pin. As these newer micros get faster with smaller geometries, I am guessing they wont take that much injection for too long if at all. I suppose a well designed general purpose pixel driver would account for your signal integrity resistor, but also have a protection buffer to protect the micro outputs (and convert from 3v3 to 5v)…then there are 12v and 24v pixels too!!
ps- I just backed the new Teensy3.5 and 3.6 and I recommend all people to do the same!
@Steve_French Do you use decoupling caps next to your APA102 LEDs? Do you have any advice on the right value?