My GA-970-D3 with Athlon draws < 25W. My Jetway mini-ITX about 20W.
Using a Raspberry-Pi is more for the novelty of it and just finding a use for one when they finally are available.
To really save power... here's my current project: a PIC18F86J65 (cost $3), a RJ45 with MAGS ($1), a 25MHz crystal ($.50) and a few passive parts. The only real power draw here is driving the ethernet signals. And a simple PIC miner program that relays work to the FPGA cluster of up to 128 devices.
PCBs done, just waiting for fab. Overhead cost per FPGA < $21 qty 1, <$16 qty 25.
@ArtForz - I'd be interested in comments on PCB/Design if you were willing to look. I can post PNGs here.
I would love to see your design on this - have you considered making it PoE powered?
I did consider POE but it was too much hassle. And since there is power from the FPGA board I chose the economy approach. The first FPGA board provides 3.3V power to the Ethernet PIC via it's I2C connector. Simple.
I did the design in Eagle on Linux. I don't mind sharing the design files but since they use a custom library I'm not sure it's the easiest for a quick view. So I've exported a rather large PNG of the schematic, and then converted a couple Gerber views to PNG for easy viewing.
Some Notes:
This is based on an Icarus compatible FPGA design. I tried pretty hard to synthesize and place my own version of the Verilog so I could optimize everything into the FPGA (no on board MPU) but kept on having failures (maybe due to some Ubuntu issues or whatnot). Finally I decided to just adapt to the serial input on the Icarus design. The Ztex uses the CSBGA and I can't see myself working with 0.65 pitch layout on a 2 sided board. That's even more crazy than just attempting a 2 sided board in the first place.
Power is from a good quality PSU. It uses 12V, 5V, and 3.3V input. 5V is used as bias for FAN2108 DC converter, 12V -> 1.2V, and 3.3V is used directly for 1 FPGA bank and dropped to 2.5V for VCCAUX and other banks (as required by Icarus design).
Each (5cm x 5cm) FPGA board can act as master for up to 3 slaves. They tile together like square dominoes, up to 127 boards (limited by I2C address, seems like enough to me). There is a PIC16LF1503 on board that queues work/results and acts as multiplexer for CFG init and work data. It co-ordinates data from the Ethernet PIC and passes it off to slaves. It talks via bitbanged serial to FPGA. This chip was chosen as it has a CLC (configurable logic block) inside allowing routing high speed CFG data via itself without external parts. It talks to master and slaves using I2C.
There is fan control, temperature monitoring on each board. It uses the PWM of the PIC and internal temp sensor. Not the most accurate but likely good enough for this.
The Ethernet PIC is master of the whole cluster. It gets work, feeds it to the first FPGA slave, and requests work back. It has a 64Mb Flash chip on board to store FPGA CFG. I'm building these parts as modular units so that I can use them for other purposes.
So, here is PNG version of schematic and PNG version of PCB layouts with screenprint overlay.
This is just the FPGA board now. I can post the EthernetPIC/Flash board soon.
Miner SchematicTop LayerBottom LayerI'll draw up a quick overview diagram so it makes more sense how these connect together in a cluster. I'm going to send the Gerbers off for making 10 boards once I'm more confident this is a final layout. As you can see this is my 4th revision (miner4)!