Teensy 3.5/3.6 Ethernet Wish List (PoE)


I was catching up on the KS comments and saw a couple pleas for PoE support and various questions/comments on the design.

I'd very much like to see 802.3af support. This means choosing a more expensive set of magnetics (either integrated jack or discrete). Certainly if I was dreaming big I'd say 802.3at (25~30 W), but most projects will easily run within the power budget of 802.3af PoE (12.95 W, yeah I know it's 15.4, but some turns to heat in the cable). Rather than reinvent the wheel I'm happy if support is provided by optionally adding a SilverTel module as suggested in the first KS comment about PoE.

The SilverTel module used on the Ardunio boards is from the Ag9000 series (http://silvertel.com/products/poe-power-over-ethernet/26-poe-modules/162-ag9000.html). The Ag-9120-S in fact, which outputs 12V by default but I believe the Arduino uses the adjustment pins to lower the voltage to 9V, perfect for the regulator on board to be powered. For Teensy I'd suggest the Ag9050-S instead and bump the output up with adjustment resistor to 5.25V, perfect for feeding 3.3V LDO and 5V for USB host or 5V peripherals (WS2812 LEDs anybody?).

The SilverTel module is super easy to use, completely passive from the microcontroller's point of view and handles the 802.3af PD (powered device) signature enabling power to be provided from the PSE (power sourcing equipment, i.e., PoE capable switch). It does require PoE compatible magnetics, center-taps on the line side of the transformers. Like the Arduino board and shield the PoE module can be made optional if the magnetics are in place with no changes needed to the rest of the environment.

<RANT>I know it's easy to argue that using a passive PoE cable adapter, splice wires in, etc should be sufficient. This is a poor solution and leads to damaged equipment when wiring mistakes are made, cables used with XYZ project are grabbed to connect a laptop, etc. The overall project appearance becomes sloppy, although some boards have had passive PoE built in - hopefully with jumper selection. Some will argue cost and that PoE switch is not a hobbyist item. Again, doing something the hacky way works fine until a mistake is made. Not all applications will be hobbyist projects. PSE cost shouldn't be a factor. Today a PoE NetGear 1gig 8port (4 w/PoE) switch (GS108PE) is $107 on Amazon, IIRC the GS108 was more when gigabit entered the home/soho market. Used enterprise switches (especially 100mb) are also affordable in the second hand market. Injectors are available sub-$20, smaller/slower switches are sub-$50. In qty 1 the magnetics are probably the bigger cost looking for a jack with PoE magnetics at digikey several years ago was depressing (think $10-$15 premium). Having 802.3af PoE capability doesn't preclude someone from using a passive adapter set.</RANT>

A true risk with PoE is square footage, looking at the photos on KS, perhaps with an integrated magnetics jack there may be just enough space to drop the module in, but I wouldn't be surprised to see the Ethernet shield end up longer or wider than the Teensy itself. This brings up a rabbit hole of the PoE capable board and Ethernet only board. At this point the non PoE board seems more attractive to design first and leave the PoE board to not get designed. Great opportunity for another builder, but probably a low quantity option, especially from a 3rd party.

Just pulled up the app note for the LAN8720A qualified/suggested magnetics, http://ww1.microchip.com/downloads/cn/AppNotes/cn562793.pdf, the only qualified PoE jack doesn't immediately appear available with 1-2 minutes of googling. So that's the other risk - finding a compatible magnetics.

Sorry for the long message, but I wanted to capture as much information initially. Can't wait to see Teensy 3.5 and 3.6. Love everything PJRC does for their customers and the community more generally. I hope I'm able to find time to test out Ethernet capability as development progresses, I stepped up my support to get in on the early Ethernet module after my initial thought of waiting for the module to be generally available.
+1, but I wouldn't integrate the PoE on the board itself. From a PJRC point of view, this only adds complexity and costs for a feature not that many users need. I would just use a PoE compatible magjack and provide the option to use an additional PoE extractor board. Like you said, it probably won't fit on one T3.6 sized board anyway.
Maybe Halo TG110-RP55N5LF could work? Looks like four 10 nF capacitors (100V rated) also need to be in series with the 75 ohm resistors?

Or maybe there's a PoE compatible connector with magnetics integrated?

Layout-wise, looks like there's no easy option. These SilverTel modules are 2.2 inches length, but we have 2.0 inches left after the RJ45 connector takes up 0.4 inches of the board.
I ordered a AG9703-S and AG9703-2BR. No idea how I'll actually use them... guess I also need to get something that sends the power. Anyway, they'll be here in my parts box for later when working on the ethernet board.
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Obviously I need to learn more about this. I thought PoE was safe for any normal ethernet device that uses magnetics (transformer). The only problem would be some cheap hacky non-standards-compliant device connecting without a transformer (...which due to common-mode noise pickup, might not work with a long network cable, either) ?
Cheap switch: https://www.amazon.com/dp/B0152WZRBM/ref=cm_sw_r_sms_awd_x_cNWYxbJQHAPX3

Inexpensive injector: https://www.amazon.com/dp/B00B4H00EO/ref=cm_sw_r_sms_awd_x_bZWYxbT5QQ574

Cheap injector: https://www.amazon.com/dp/B001PS9E5I/ref=cm_sw_r_sms_awd_x_H1WYxbWD6HB0C

I showed the first injector because it looks like a Microsemi/PowerDsine rebrand, they're one of the biggest OE manufactures for vendors selling WAPs and other PoE PDs. I have no experience with any of these but have used the PowerDsine products in the past and many different switches.

Paul if you PM or email me and I can send you something.
I thought PoE was safe for any normal ethernet device that uses magnetics (transformer). The only problem would be some cheap hacky non-standards-compliant device connecting without a transformer

That makes sense, but you don't have to travel far before finding people complaining that PoE blew out the nic in their laptop. A 802.3af PSE won't apply power before sensing the PD signature.
I'm fairly new to this, but my understanding is the power source is supposed to apply a lower voltage test signal that checks for resistor which indicates a PoE capable device. It's not supposed to apply 48V power until it sees the proper impedance to know the device is PoE compatible.

Almost all modern ethernet designs use the Bob Smith termination circuit to reduce common mode noise, which has 75 ohm resistors from the unused pairs (for 10/100 speeds) and center taps of the data pairs to ground. If you look between the RJ45 jack and magnetics, you can see I put those 75 ohm resistors on the ethernet beta test board.


If the PoE source blindly applies 48 volts to the 2 unused data pairs, each of those 75 ohm resistors will get half the voltage. That's 7.68 watts in each resistor! Since they're rated for only 0.1 watt, they'll almost certainly burn.

PoE designs are supposed to add 10 nF capacitors in series with those 75 ohm resistors. The caps need to be able to block up to 57 volts DC, while passing high frequency noise those 75 ohm resistors are meant to suppress.
There's also an issue with the magnetics, at least according to info I've been able to find. Here's a crude drawing of my understanding of the problem (which admittedly is based on a bit of guesswork), where the hand-drawn lines represent the 48V power's DC current flow.


The "transformer" part is fine, but the the common mode chokes on the cable side see DC current flow through their windings. I'm not sure why this is a big problem, since the winding is supposedly center tapped, and the cable is supposedly a twisted pair of the same wire type. But maybe if the DC current isn't perfectly matched, the transformer sees a net DC current which can saturate its core.

What's actually different about PoE compatible magnetics is still a bit of a mystery to me. Looks like it might be just both chokes wound onto a common magnetic core? Maybe that reduces the possibility of DC imbalance? Or maybe they actually use a different material that can handle higher flux density?

Maybe someone who really knows about PoE magnetics will comment??
Opps, I just totally got that diagram wrong, drawing on the wrong side!

Here's what's actually happening:


In the transformers, equal current DC flows in opposite winding directions (into the "dot" end on 1 winding and out of the "dot" end on the other winding), so in theory is should cancel out, magnetically speaking.

But in the common mode chokes, it's flowing in the SAME direction in both windings!

I'm guessing they're just using a single toroid for both chokes, winding all 4 wires onto it? Then the 2 positive side paths that flow out of the "dot" side ought to cancel the magnetic field from the two negative side paths that flow into the "dot" side.

There's also a DC path from the PoE power supply through the other 2 unused pairs. I wonder what happens if the DC current doesn't balance perfectly between them? In other words, if a little more flows in through the unused pairs and flows back out through the magnetics path?
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There are two methods to put PoE power on the line, alternative A and alternative B. The standard is pick one, so a PSE should not provide two paths for power (may be different as we get super high power devices - but 802.3af is pick one). So power is either on 1/2, 3/6 or 4/5, 7/8.

When I thought I had the time to layout a board for atmega32u4 + wiznet I was studying the Arduino schematics. Here's some bits.

Two blocks of the Arduino Ethernet Shield - shaded area is TX/RX off to wiznet, top is RJ45 connector, lower is AG9012 module.

Arduino Shield Extract.png

Here's a schematic from the datasheet for the RJ45+magnetics on the Yun, I just looked at an Arduino Ethernet board here and it's the same part number, RB1-125BHQ1A.


Here's what's actually inside the Pulse H1102 we used on the beta test. Definitely 4 separate toroids.

(click for full size)

I ordered the Halo part that claims to be PoE compatible. Will take a photo when they arrive.
Hmmm, even more mystery. When I search for RB1-125BHQ1A, I find a datasheet with this schematic. Not anything like the schematic from Arduino's forum... and doesn't look PoE compatible!


Edit: oh, this can't be right, only 8 (or 5) pins. I can see there's 10 pins on the bottom side of my Arduino Ethernet R3 Shield. Hmmm.....
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I ran into that schematic earlier, the arduino forum post I linked above has a data sheet on github link and it looks like the jack is from UDE.
@Paul: You appear to be looking at the datasheet for RB1-125BAG1A (that's also what the tooltip for your image says). Post #13 has the datasheet for RB1-125BHQ1A.
Here's what's actually inside the Pulse H1102 we used on the beta test. Definitely 4 separate toroids.

On the Raspberry Pi they used ethernet socket with magnetics/transformers to reduce cost and to reduce footprint, does that mean that this pulse chip wouldn't be needed?
(and with magnetics one gets leds in the socket :) )
I know I am coming into this conversation a little late but I have been tinkering with POE on personal projects and have now been given the task of creating a 4 port PSE at my job. From my perspective using the MagJack would make a lot of sense on the side of being simple and small. Part of the POE spec states that the PSE and PD both have to have isolated power supplies relative to the case so for the sake of space it would most likely not be worth it to put a power supply on the Ethernet board. What I did for a project of my own was to remake a Ethernet module board with a MagJack and then break out the + and - pins to a header. I then made a separate board that took the +&- voltage and had the class detection circuit. This allows for me to easily use or not use POE and I can use whatever power supply circuit I need. For the Ethernet board if you use the MagJack or independent magnetics and break out the + and - lines to a header it would allow the user to either use a premade POE power supply with detection or build their own and it would only take up the space of 2-4 .1" pins on the board. Hopefully that all made sense. I tend to have it all straight in my head but my words come out as gibberish.

Doing it in that manner would allow the user to use either 802.3af or 802.3at as long as the magnetics/MagJack are rated for the current draw.
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My T3.6 Sized POE Shield

Just to add confusion into the mix. Here is my next project for my T3.6, A T3.6 Footprint size POE 802 compliant LAN Shield.

The POE module is out mid to end NOV, but they are very small and not overly expensive ($7) per 1000 modules from SilverTel.

A few things need tidying up and I need to get a module in hand to work out how much space I actually have under the POE module. It's double loaded, But I've made it so that the bottom components fit in the gap between the MCU and USB port (Hopefully). So the Shield could in theory be loaded really close to the T3.6


@johnnyfp, you are right, confusion.

T3.6 is stating that it can do ethernet, however I have 2 questions.
- how to connect T3.6 to ethernet?
- Is there a shield available for T3.6? Eventual with PoE?
Or would I have to develop this myself?

Thx, Karel.