Teensy-LC ARM Stamp pin 17 driving WS2812b/Neopixels

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t3andy

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There are many discussions on this forum board concerning this special WS2812b/NeoPixel drive pin 17 from the Teensy-LC.
(Sort of confusing) :confused:

#1. Do you still need the WS2812b/NeoPixel recommended 1000 uf. cap. for the +5V supply for the WS2812b/NeoPixels?

#2. Do you still need the WS2812b/NeoPixel recommended 300-500 ohm "series" resistor on this drive pin?

#3. What is this pin 17 that mirrors the VIN input?

#4. Is there an alternate "normal buffered" pin 17?

#5. Any special drive electronics needed when using this pin for NeoPixels?
 
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t3andy said:
(Sort of confusing) :confused:
Click to expand...

What's so confusing? Can you be more specific?

#1. Do you still need the WS2812b/NeoPixel recommended 1000 uf. cap. for the +5V supply for the WS2812b/NeoPixels?
Click to expand...

That depends entirely on your your power supply and wiring. Teensy has nothing to do with that.

#2. Do you still need the WS2812b/NeoPixel recommended 300-500 ohm "series" resistor on this drive pin?
Click to expand...

If the wire between the Teensy and LEDs is long, you might need a resistor.

#3. What is this pin 17 that mirrors the VIN input?
Click to expand...

The buffer that increases the voltage is powered by VIN, so the logic high output is whatever your VIN voltage happens to be.

If you're using a weak power supply or skimpy wiring, such that you'd need to add a capacitor (question #1), then consider the VIN voltage might be changing as your power consumption changes.

#4. Is there an alternate "normal buffered" pin 17?
Click to expand...

Well, you can use the normal pin 17, which connects directly to the microcontroller, just like all the other I/O pins.

#5. Any special drive electronics needed when using this pin for NeoPixels?
Click to expand...

The intention is to allow direct connection to NeoPixel input for the vast majority of moderate size LED projects.
 
I'm not sure I understand #3 and #4. If you hook wires up to pin #17 (A3) it will deliver 3.3v with a max amperage of 20mA. You can use it for digital input or analog input as long as the voltage is 3.3v or less. If you use the pin on the back (the Vbat pin on the Teensy 3.1), that is an output only pin, that delivers 5v with a max amperage of 8mA. The pin is also a PWM pin, so you could use it for either neopixels, or driving servos. You would need to pass the VIn to the power port on the neopixel/servo, the back pin for data, and ground.

There is no way to input VIN level inputs without doing level conversion (or risking burning out your LC).

The capacitor is needed if the 5v supply has noise in it. Generally if you are only going to use batteries, it probably is not needed to have the capacitor (but it is still a good idea), but if you ever plug it into an A/C circuit, it may be needed.
 
Just trying to sort things out ...

Adafruit recommends this ...

// IMPORTANT: To reduce NeoPixel burnout risk, add 1000 uF capacitor across
// pixel power leads, add 300 - 500 Ohm resistor on first pixel's data input
// and minimize distance between Arduino and first pixel. Avoid connecting
// on a live circuit...if you must, connect GND first.

from OctoWS2811 discussion
// However, a series resistor located close to the Teensy 3.0 pin is often necessary to reduce ringing and cross-coupled signals. The series resistors help match Teensy's outputs to the characteristic impedance of your wiring. The best value to use depends on the type of wire and its spacing to nearby ground wires. Values in the range of 47 to 220 ohms are probably best.

What my main question is "do I still need all of the above" using the new Teensy-LC?
There seems a difference of opinion on the series resistor?
 
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It looks to me like the answer is there in the above posts - taking the words at face value not having touched any of it.

The Cap and Resistor are installation dependent - if you do the other things right (standard connection rules) it will connect without any damage - but ideal performance may take minor adjustment hardware as noted.

If the power isn't clean or strong the Cap will clean that up and you'll see proper function.

If the wiring is too long or corrupted by local noise the Resistor will clean that up and you'll see proper function.
 
So even using the new Teensy-LC, a large supply filter cap. is still recommend and also the series drive resistor.
I was hoping, with the new Teensy-LC that my WS2812b/NeoPixel noise problem would go away or disappear.
 
The resistor is still "recommended", but if your wires are relatively short, it makes little difference.

Of course, sites like Adafruit and PJRC will "recommend" the way that is most likely to work even if longer wires or other poor design choices are made.

Likewise, you really only need a big capacitor if you have a weak power supply. A fresh battery should work fine, but as it starts to run low and its output impedance increases, a large cap will help lower the power impedance the LEDs see during rapid changes. That will likely allow running a little longer.

Something you should understand is we're talking very generally about how thousands of different hobbyists with little knowledge of electrical engineering will build their LED projects. You should try to understand what needs to be "recommended" in the context of a company like Adafruit or PJRC trying to do the right thing for hobbyists. They are sensitive to cost, since they usually fund their projects out-of-pocket. They're building with inexpensive tools. They usually don't have fancy equipment like an oscilloscope, or the knowledge & experience to use one even if they did have it. They're not engineers with deep knowledge of circuit theory. These sorts of recommendations are written to try to help hobbyists have the best chance successfully build and troubleshoot their projects.

Many will disregard the recommendation. In fact, most probably do. But when some of them run into common problems, these recommendations give them a way to get things working. They also set that expectation up-front (and often they buy the extra stuff "just in case"), which is FAR better for hobbyists than learning after a project is already build and not working well.

Even if the recommendations don't suit your personal taste, please try to understand the large and widely varied hobbyist market we try to serve.
 
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