Electron: Hardware Discussion

Hardware
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Hey Everybody!

The hardware team here is starting to get very serious about the Electron. We’re aiming to have hardware completely locked down - that’s pcbs, designs, exact individual components, everything - by mid-June.

Throughout this process we’ll be posting questions and updates here, sharing components chosen, schematics and more. We’re hoping to do this in a way that enables open and honest dialogue and feedback!. We hope you’ll participate and follow along as we get closer and closer to lockdown.

A Kickstarter campaign, and the comments and questions we get during it, always inspires some internal discussion about existing designs and causes us to look at things differently. That’s one of the beautiful things about crowdfunding and open source projects. To start out with we want to introduce conversation about 3 different things we’ve been thinking about a lot since the campaign closed, and in part because of certain questions we’ve received. Let’s get started:

###Power

The Community speaks:

The hardware team has been answering your questions and hearing your call for power. There’s a strong desire from the community to power up the Electron for all of your portable applications. We love our community and our ears are always pointed towards those great suggestions! That was a pun from “Bilbo”.

he-man

Power of Greyskull:

Unlike WiFi, 2G and 3G cellular modules are very power hungry, requiring upto 2A of peak current during transmission. This clearly exceeds USB’s limit of 500mA in most cases. When developing on the Electron connected to a computer, care must be taken not to exceed the USB’s limit while maintaining uninterrupted power to the cellular module. Most portable devices overcome this by having a battery connected at all times, like mobile phones. For a device like the Electron, it becomes essential to have the flexibility to power it either via the USB, battery or both.

LiPo battery technology is effectively today’s Power of Greyskull, and were working out how to add this to the Electron. There will be a complete battery management solution on-board the Electron, ready to seamlessly charge up the LiPo battery from USB or deliver power to the Electron’s cellular hardware. Packing this into a small area is definitely a challenge, and we accept!

LiPo Question:

See that little question mark below? Just wondering what you all prefer for a connector solution here? Many LiPo battery packs come with a 2-pin JST-PH connector installed, but they are not all the same polarity or exact style. We’re considering a special connector to ensure everything “just works” for you, and make available matching battery packs. This would also give us flexibility to make everything fit just right. How do you guys feel about this connector? Make it right for the Electron, or go with the JST flow?

enter image description here
electron-battery-mgmt.png2569×1841

###Package for the microcontroller

The current design of the Electron uses a STM32F205 microcontroller in a LQFP64 package which just about fits in the footprint. We are now toying with the idea of using wafer level chip package (WLCSP) that is almost 10 times smaller, thus saving us significant real estate on the PCB.

There are couple of design challenges in using this smaller package. WLCSP is generally not environmentally protected like the plastic LQFP counterpart. This can be overcome by adding epoxy top or a tin faraday cage like the cellular module itself. Secondly, the BGA solder pads will require higher tolerances during manufacturing and assembly, both of which can be handled by most of today’s board houses.

###Antenna Selection

Antenna selection depends on a number of parameters like gain, size, durability, etc. Some of the more common examples are telescopic, patch, bead, helical or high gain directional antenna. Cellular antennas are a little special since they need to work on multiple frequencies. For a project deployed in the field that is static, such as a weather station, one would choose a directional, high gain antenna. For projects that are mobile and portable, one would choose a PCB patch antenna. We are currently evaluating few of these types to ship along with the Electron. We would love your feedback on which antenna you would most like to see on the Electron.

It’s your turn now. What are your thoughts on power, chip package and antenna? If it was up to you what would you suggest? We’ll take all your feedback into consideration as we make final decisions.

Thanks!

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I plan on just dropping this board onto a PCB for our products which are portable solar power generators.

The Electron will basically just pull in data being sent out over I2C from a Teensy 3.1 micro controller and then send that data over 3G from locations all over the world to the cloud so it the status can be logged and graphed via Ubidots.com and then displayed in a custom web browser interface.

My question is about certifications. Will the Electron be certified so I will not have to recertify the cellular part of the device with FCC or anything else?

About power input. We will be providing the Electron with 12.8v battery power. So at a minimum make the DC input regulator capable of handling at least 15v of DC input since our batteries go up to 14.2v when fully charged.

Those are the only questions I have so far.

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Why not consider the UFBGA package ?

It is fully encapsulated, has an easier ball pitch, and while larger than the WLCSP results in a smaller practical footprint than the TQFP because you don’t have to fan out from the pads (the TQFP is a 10x10 package, but the effective footprint before you can actually route traces is more like 13x13.)

If you want to do WLCSP, use heat cured black epoxy - it’s a tried and trusted production flow.

Industry standard 2mm JST connector gets my vote for the battery hookup.

Make sure the battery is optional if the user can supply a stiff enough external power source.

What are your current thoughts on pin numbering ?

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Having a U.FL connector on the board with a U.FL-SMA adapter of some kind would be great. I’ll be using my own antenna setup, which has an SMA connector. The rest of the system components have F-Type, MCX and SMA connectors, so I, already, have multiple adapters.

For LiPo connector, as long as there are clear docs and, maybe, an adaptor, I don’t have a preference. I’m planning to power this from a 5V source.

Would be great to have some kind of enclosure design, even if it’s something I can download from Thingiverse and 3D-print.

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I think we should stay with the JST connector and just list the brands of batteries that are supported and which need to be switched or adapted. The availability of batteries at the local BatteriesPlus or walmart is probably a necessity in delivered products.

It is already a “de-facto” standard. DVORAK anyone?

Maybe some adapters to switch polarity or use special packs?

1A usb 2.0 is popular and USB specs out to 5A (but different connection to get it)
How long are spikes, would an ultra cap on board and the standard 5V USB in suffice?

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Something I would like to see, and something that may be beneficial for your design is a soft start.

Essentially, when the unit powers up, it initializes the firmware, and keeps the WiFi module off, and the cell module off. It takes a base reading of the power/voltage then attempts to power the other units on. With an on board super capacitor you could even keep the firmware module alive while you attempt the power up, and prevent the process from looping. This could also protect your hardware from drop outs.

Another idea is to have separate power sources for the Photon and the cell module. Basically follow the pin-out for the Photon and Core, but power the cell module via an optional or additional power pin with a separate regulator. Cell modems can be power hungry, so the ability to control the modem would be in the best interest since most of the time they are boosting power just to try and get a clear signal. If you had the controller specifically wait to transmit and receive and sleep the cell module, your battery life/power usage would be very minimal.

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From the kickstarter FAQ:

The Electron will be FCC/CE/IC certified. We will also be pursuing PTCRB certification to reduce or eliminate the need for any further certification of your final products using the Electron. We are still researching PTCRB so our certification plans are not yet finalized, but our goal is to make it easy to build a product with the Electron at the lowest cost possible.
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Power is not my field but a battery management IC similar to the MCP73871 seems like a smart choice, but you need more current than it can handle at peak due to its internal pass transistor. I am not sure what else has all those features but uses an external MOSFET. I think it does have all the features you want, just not the current capacity.

I think JST is the best choice for the battery connector.

The wafer level chip package is meant for really high volume applications (millions per month). You can use it of course but your yield will be lower since it cannot be reworked or reballed that I know of, and you will want to be very sure to bring out any test pins for your bed of nails tester so you can do any failure analysis. As @AndyW said, black heat-set epoxy is the standard covering and so totally bullet-proof you can’t really get under it with anything but boiling red nitric acid or destructively milling it off. Board planarity and board material (special FR-4 with low thermal movement) really matters with the wafer scale packaging too. Folks also often use nitrogen reflow ovens to solder this type of package because it works better. It is a lot of work and the payback really shows up in high volume applications.

Antennas are fun! I think a lot of folks will want SMA approx. 60mm style antennas which are typically +2 or +3dBi. You could offer one PCB antenna and one SMA with u.fl to SMA adapter and cover a lot people.

This is really sounding like a great product!

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  • DC/DC: I usually use the L7981A (Digikey ref: 497-10722-1-ND) thanks to its 3A output current.
  • Battery connector: It would be nice to follow the commonly used JST pinout for those kind of batteries : https://www.sparkfun.com/products/8483
  • STM32 package: If you plan to choose the WLCSP64 package, I would recommend you to cover it with some kind of glue. The GPRS/3G version of Spark core will be suited for outdoor use-cases. As far as I know, WLCSP64 are much more sensitive against environmental parameters (especially humidity). Furthermore, it would be nice to avoid the Flash gate of the Raspberry :wink: https://www.raspberrypi.org/xenon-death-flash-a-free-physics-lesson/
  • Antenna: Often, SMA is a great choice

I look forward to play with this new Core :slight_smile:

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Just make sure that the connectors are easily available across the world. I have some LiPos lying around where I could replace the connector if really necessary.

I'd also tend to favor JST though.

Regarding hardware in general, for me the power consumption is the most important thing. I plan on using the electron to report sensor data from a remote location, where I want to power it with a solar panel. So there needs to be a good way to put it in sleep mode immediately after doing a sensor measurement and sending the data via cellular.

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Hi,
My vote goes to the Industry standard 2mm JST connector .
And I would like to suggest to move the usb connector a bit to the left, and squeeze the JST connector in at the right of the USB connector. If possible off coure.

About the power supply, the split power option is interesting, with one diode between the USB power and the power input to the GSM module you can create a splitter that accepts external power for the GSM module if available. Without this power the GSM module would be powered by USB. Pinwise, sacrifice one of the 3v3 pins.

About an enclosure, try to align all connectors to one surface. That makes it so much easier to enclose the electron. It is easier said than done off course…
Good luck, looking forward to the final design.

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Like a few who already posted, I plan to use this remote, solar w/a battery.

JST for the battery.

Would love to have that high gain directional but as long as it’s got a standard connector for me to replace it with something OTS I’m fine.

No comment on the chip packaging. Not my wheelhouse, much smarter heads in the game here on that one. Just need it to be pretty bulletproof where the environmental comes in.

Also really like FlaredElectronics’ idea of the option for a separate power source for the cell module. Being able to independently control that apart from the photon would really give excellent control over power needs.

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I would prefer industry standard JST connector, perhaps make sure to indicate what pin is + and - in the silkscreen to make it easy to identify in case of an inverted cable.

I don’t won’t be doing any rework or resoldering so I don’t actually mind what chip packaging is used, I do care about environment damage, so whatever is used I think it’s important to keep this one safe. I’m sure some people do mind having access to the pin.

Antenna? Whatever I can use for mobility :stuck_out_tongue:

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Another vote for a JST connector with clearly marked polarity. For prototyping compatibility with the Sparkfun and Adafruit LiPo JST pinouts would be handy. A low sleep current would also be useful for solar powered nodes.

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The USB Power Delivery specification allows up to 5A at 20V, but it probably isn’t well enough supported yet to put in the Electron :

http://www.anandtech.com/show/8539/usb-power-delivery-v20-and-billboard-device-class-v10-specifications-finalized

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Great suggestions thus far!

To JST or not to JST: 2mm JST connector does indeed seem like becoming a maker standard. A JST connector along with two through hole solder pads? @Iv4n @mawrob good call on clear markings

Package: Currently the STM32F205 is only available in LQFP or WLCSP, unless we decide to upgrade to 100 pin package or a different microcontroller. @AndyW
We haven’t worked with using epoxy coating before during manufacturing, so that should be interesting.

USB: Legacy standard for USB is @500mA current draw. Being able to set the maximum current limit from the USB would be ideal, since you can’t really predict which computer a user might plug and develop the Electron on. @Rockvole @jshanab

Antenna: It seems like an uFL to a SMA adaptor cable would provide the most flexibility in antenna selection.

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My bad - you are of course correct, it is the '207 that is available in the UFBGA package. Sorry for the noise.

Looks like you’re going to have to learn about epoxy dispense & cure cycles.
Good luck, there is no rework once you encap.

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Hi @mohit

I was looking at antennas on ebay after you posted and discovered there are a lot 800/1900MHz but fewer 1700/2100MHz antennas there.

I think it is going to be confusing since in the US (and please correct me if I am wrong!) the 2G Electron would need a 800/1900 antenna but the 3G would need a 1700/2100 for best performance.

Do I have that right?

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Not one of the original questions, but ... :wink:

I just stumbled over this Photon thread and would - like @Carsten4207 - also like to see an Electron with castellated edges.