Modular FPGA Miner Hardware Design Development

[fizzisist]

Maybe I'm missing something, but if you're going to the trouble of putting an MCU on the DIMMs (maybe daughter board is a better term), why would you need the backplanes at all? Why not just give each board a USB port?

[O_Shovah]

Each board shall have a USB connector anyway (see first post)

The motherboard will then serve as housing and the hub for the USB,SPI and JTAG connections.If you want to run a single daughterboard you are free to do that.
But i asume you dont want to use a number of USB cables equivalent to the number of boards you use as their number increases.
And in a later stage it will feature a cpu and ethernet connection for independent mining.

[Salzgitter]

Well, thats exactly the point and it was mentioned before. The whole thing is meant to be cost effective, so who would invest in a backpane that serves no purpose? I mean people who would get such a card really dont care if they have to wire them up seperatly onto a usb hub for 5 bucks or on a backpane that will be 50 bucks or more, I'm pretty sure they'll go for the cheaper alternative, it the backpace doesn't add any kind of feauture... The only reason to design a backpane, would be if it had some additional functionality, nobody is going to buy it, if its just a nice way to stack up the doughterboards.
And as far as I see it, that fuctionality would be Ethernet!

I cant imagine implementing an ethernet controller is too hard for people that can programm FPGAs, for example for Arduino (which was mentioned before and although feautures SPI, I2C etc) there are readily available schematics and libraries just for that purpose: http://www.practicalarduino.com/freetronics/EtherTen-schematic-worksheet.pdf

btw, I have been following this thread from the beginning, but since I am only in my third semester of electical engineering, I really dont have to much to say about most of the specs you're discussing here, please dont get me wrong here, I really apreciate your effort but i really think you should consider to develope a backpane that gives people a reason to buy it in the first place.

[phillipsjk]

I think one of the major benefits of a backplane is density. You can pack the modules in closer together without them getting scattered all over the place every time you bump the table.

I have little industry experience as well

Edit: By electrical, you mean circuits over 50V/20A?

[Salzgitter]

Well, when its just for density, I'd rather take a wooden board and cut some grooves ontop of it, gives me a place to put the FPGA boards for free.

Please, dont get me wrong here, I really like the whole idea of a modular assembly, but I just can't see the point in making a backpane that just serves as a mechanical mounting platform.

On other idea: if you plan to put an USB port onto every doughter card, wouldn't it be easyer, to forget about all that SPI and I2C busses and just route the individual USB conectors of the FPGA boards  to an USB hub onto the backpane, which will than connect to the host PC by only one USB port?

[fizzisist]

Well, thats exactly the point and it was mentioned before. The whole thing is meant to be cost effective, so who would invest in a backpane that serves no purpose? I mean people who would get such a card really dont care if they have to wire them up seperatly onto a usb hub for 5 bucks or on a backpane that will be 50 bucks or more, I'm pretty sure they'll go for the cheaper alternative, it the backpace doesn't add any kind of feauture... The only reason to design a backpane, would be if it had some additional functionality, nobody is going to buy it, if its just a nice way to stack up the doughterboards.
And as far as I see it, that fuctionality would be Ethernet!

Ethernet for standalone mining is definitely a good idea. I was thinking of another possibility for this, though, which would undoubtedly be cheaper than making this board and implementing the ethernet yourself: use a Pogoplug or Dockstar! These are basically a USB to Ethernet link, running a Linux operating system. If you install the Archlinux build for it (I've done this on a Dockstar and it's very easy) you can run whatever software you like on it.

As for the mechanical benefits of a backplane, I think stacking these cards up with standoffs would make for a nice, sturdy unit. For example: http://www.zerksus.com/images/overlay_stack.jpg

Stick that into an enclosure with a USB hub inside it and you've got a nice little mining box that you can set next to your PC. Want it standalone? Put a Pogoplug inside that box!

That's my feeling about it, but I suppose nothing would stop one from using the backplane/motherboard if you really wanted it. The only important question will be cost. PCB production generally costs by the area, so such a large board could get expensive.

[pusle]

I also thought the point of this was to make plugin boards for the fpga's since we don't have sockets for them.

Then the DIMM board should be only the FPGA + decoupling with a standarized pinout.

Provide lots of power pins with feedback sense for both VCC and GND + a handfull of I/O's and the configuration bus (serial or jtag).
Then all the other design choices are left up to the mainboard/backbone and the firmware you load.

There will be lots of different mainboards, but then they are all compatible with most/all DIMM fpga modules.
Add a few "Keying" pins for voltage select+ID and you can support different fpga families too  (even future ones)

oh and perhaps throw in a I2C temp sensor & voltage monitor just for kicks

[fizzisist]

Also, here's a link to the preferred FPGA with no minimum order, although there is a 7 week lead time:

http://avnetexpress.avnet.com/store/em/EMController/_/A-12942478?action=part&catalogId=500201&langId=-1&storeId=500201

[skyhigh]

A bit offtopic..  you guys using a poll to decide which one to use in prototype ..  why not build and test both ? See what works best and what could be changed to make it better.

[fizzisist]

I also thought the point of this was to make plugin boards for the fpga's since we don't have sockets for them.

Then the DIMM board should be only the FPGA + decoupling with a standarized pinout.

Provide lots of power pins with feedback sense for both VCC and GND + a handfull of I/O's and the configuration bus (serial or jtag).
Then all the other design choices are left up to the mainboard/backbone and the firmware you load.

There will be lots of different mainboards, but then they are all compatible with most/all DIMM fpga modules.
Add a few "Keying" pins for voltage select+ID and you can support different fpga families too  (even future ones)

oh and perhaps throw in a I2C temp sensor & voltage monitor just for kicks

Well, then that's a different design philosophy. I would argue against that idea, because the cost of the motherboard would be prohibitive to someone who is curious about FPGA mining, but not sure they want to invest enough to buy many daughterboards, thus making the motherboard worth it.

The real question is how much is saved by moving all of this stuff off the daughterboard and onto the motherboard?

[Olaf.Mandel]

[...]
The real question is how much is saved by moving all of this stuff off the daughterboard and onto the motherboard?

Going from N hybrid DIMMs plus one backplane to N standalone cards, the savings are:

• Nx DIMM connector: < N x 3EUR
• 1x Extra connectors and chips: <20EUR
• 1x Board cost: ~30EUR
• 1x Population cost: ~50EUR

So the board solution would cost approx 120EUR more, compared to a total cost of 2xNx(114EUR+50EUR???), e.g. 1640EUR. Actually, it's a bit better because I neglected the cost of the USB hub and the many cables for a stand-alone solution. So approx 7% of the total is due to the board. That is probably acceptable for more flexibility.

Going from N single-purpose DIMMs plus one backplane to N standalone cards: Same as above, but you save a bit more for the DIMM+backplane solution, because you don't need the USB interface on each board. That may even add up to cover maybe half the cost of the backplane.

For everything I wrote above: it's difficult to make precise calls on the money aspects without having designed the system and costed the manufacturing costs (the numbers I gave above may even be on the low side, if we don't reach small series count: prototypes cost more).

[fizzisist]

For everything I wrote above: it's difficult to make precise calls on the money aspects without having designed the system and costed the manufacturing costs (the numbers I gave above may even be on the low side, if we don't reach small series count: prototypes cost more).

You hit the nail on the head here: the cost of this project will be inversely proportional to the number of orders we can get. The cost of PCB production and assembly, as well as the components, is very strongly affected by economies of scale. Anything we can do to increase the number of orders will have a huge effect on reducing the price. I believe that reducing the cost to get started in FPGA mining will encourage many more people to get into it. If the daughterboards can't run without the motherboards, the motherboard cost will deter many people, therefore bringing up the cost.

Of course, the real question is how much interest is there? How many people will pledge to buy? How many will provide the money up front? I for one am willing to invest the amount of money for a 10 FPGA system, possibly even more so that the cost per FPGA is lower. Maybe something like Kickstarter will help to gauge this?

[O_Shovah]

No offence to anyone,but to make this clear:

I don't tihnk we should change or rediscuss decicions allready made or we will end up running in circels.


The Daughterboards are intended to be hybrid capable wich means they will be abled to operate standalone via USB or as slave in the motherboard.

So there are some points

- Minimising the design of the daughterboard would require a complete redesign of the daughterboard if we add a cpu and ethernet on a later motherboard version later.

- As the daughterboard is stand alone capable you dont need to buy the motherboard if you don't want it.

- According to my rough calculation each daughterboard should end up at a price of 300 Euro,so a additional price of maximum 140 Euro should be bearable if you buy more than one daughterboard.


I Would really appreciate it if we could return to ther design discussion regarding the BUS system to be used.

[fizzisist]

O_Shovah, I'm sorry to derail the thread. I think Salzgitter had a great idea:

One other idea: if you plan to put an USB port onto every doughter card, wouldn't it be easyer, to forget about all that SPI and I2C busses and just route the individual USB conectors of the FPGA boards  to an USB hub onto the backpane, which will than connect to the host PC by only one USB port?

If we wanted to add ethernet to the motherboard later, this would mean adding a MCU that could serve as a USB host.

USB has my vote.

[makomk]

If we wanted to add ethernet to the motherboard later, this would mean adding a MCU that could serve as a USB host.

Yeah, that could be... fun.

[pdki]

I think you all should try to keep it simple. That increases chances to produce something that is working in time and without multiple iterations for PCB production and assembly (money!). The later this thing is ready probably the higher is mining difficulty, this in the end also costs money.

As far as I can tell USB is a much more complex protocol than SPI or I2C. Thats why FTDI can sell chips. Implementing USB inside the FPGA costs LE cells that could otherwise be used for mining. In addition there is a host needed to connect multiple slaves. For mining no bandwidth is needed. SPI, I2C, JTAG all are capable of pushing enough bits for mining.

My proposal:

• make the motherboard simple, put only the FPGA + decoupling + one of (SPI, I2C, JTAG), put an direct JTAG connector on the board for debugging the FPGA.
  
• let the motherboard supply one of (SPI, I2C, JTAG) + programmable power supply + reference clock. Power needs cooling and you normally don't want to have this on the daughterboard. Connect the motherboard to a standard PC power supply.
  
• The simplest motherboard would then just put an FTDI USB connector on the motherboard and you are ready to go. Easy debugging. Anybody who feels up to the task can then replace it in a second step with a microcontroller and ethernet.
  

We build a similar board (not for mining) at work and you will find out that it is a pretty complex task. You will succeed much faster if you try to keep complexity as low as possible.

And just as a reminder: to my knowledge there is at least one startup working on an ASIC, announced to be ready in October. If this is true, an FPGA board only suitable for mining will be worthless.

[fizzisist]

I think you all should try to keep it simple. That increases chances to produce something that is working in time and without multiple iterations for PCB production and assembly (money!). The later this thing is ready probably the higher is mining difficulty, this in the end also costs money.

I couldn't agree more. This is why I don't see the advantage of doing all the work to implement a bus protocol, when USB functionality costs nothing more than an FTDI chip ($3-4). If we're discussing overall strategy here, I think the priority should be on developing the FPGA daughter board. Right now, we're spending this time trying to design the perfect backplane, that will have sorts of cool features and allow for all different types of daughterboards. That's a very difficult task. It's very easy to get bogged down in this design and delay the overall development.

On the other hand, as O_Shovah said, we really shouldn't go back now and rediscuss the issues that have already been decided. It would be better to just try to move ahead with the design.

Is anyone working on the daughterboard schematic and or layout? We could probably get started now and add the bus protocol to it later.

[phillipsjk]

And just as a reminder: to my knowledge there is at least one startup working on an ASIC, announced to be ready in October. If this is true, an FPGA board only suitable for mining will be worthless.

I was sort of hoping that if the motherboard is compelling enough, people would build ASIC boards that are pin-compatible

Edit: the FPGA prototypes would be a "proof of concept".

[pdki]

This is why I don't see the advantage of doing all the work to implement a bus protocol, when USB functionality costs nothing more than an FTDI chip ($3-4).

True, but if you put an FTDI USB chip on every daughterboard (is this what you propose?), how do you connect it to the FPGA? Here you still need I2C, SPI or JTAG. Then you can also put it directly on the backplane and connect the daughterboards only using i.e. I2C. USB on the daughterboard will probably also require more careful routing and special impedance matched connectors because it is a high speed bus.

[TheSeven]

I think you all should try to keep it simple. That increases chances to produce something that is working in time and without multiple iterations for PCB production and assembly (money!). The later this thing is ready probably the higher is mining difficulty, this in the end also costs money.

I couldn't agree more. This is why I don't see the advantage of doing all the work to implement a bus protocol, when USB functionality costs nothing more than an FTDI chip ($3-4). If we're discussing overall strategy here, I think the priority should be on developing the FPGA daughter board. Right now, we're spending this time trying to design the perfect backplane, that will have sorts of cool features and allow for all different types of daughterboards. That's a very difficult task. It's very easy to get bogged down in this design and delay the overall development.

Yes, we should keep the early tasks as simple as possible. But we shouldn't sacrifice too much flexibility for that.
That's why we want to build hybrid cards, so that we can first focus on the cards and run them standalone, but at a later point are able to make a nice backplane for them.
The difference between standalone or hybrid cards is really just a DIMM connector and a couple PCB traces. Possibly a better USB bridge or MCU. Yes, it's a <$10 difference on a >$200 board. Do you really want to sacrifice the possibility of building a backplane for it later for these few bucks?

And regarding that backplane-only card idea, with a trivial one-slot FTDI backplane, I have two concerns:

• While this reduces cost for the board by a bit, it adds cost for a second board and a DIMM connector.
• It makes things less flexible. USB on the DIMM would allow future cards to change internal protocols as neccessary.

Nevertheless I would still route the SPI and JTAG interfaces to the DIMM as well. It just can't hurt.
If we're going for an MCU instead of an FTDI anyway, tristating won't be an issue. Just provide a dedicated tristating pin on the DIMM, which has a pulldown on the card, and gets pulled high on the backplane. If the MCU has the neccessary resources, adding an additional SPI link between the backplane and the MCU can't hurt as well. Oh, and having the MCU's JTAG pins on the DIMM can't hurt either. That way you can do MCU firmware upgrades from the backplane. And yes, we should go for an unbrickable MCU.