Got some more infornography for you today.
Here's the current two-chip rig, with chips in parallel. This is what's been running since Friday or so. I'm still guessing it's signal integrity causing issues, since with the two boards mounted directly (the shortest possible inter-chip signal path using our breakout boards) the errors dropped from 50% to 25% (based on hashrate efficiency).
And here's some new toys I built today.
The center stick is a test board of the new regulator design. I had trouble with it for a few minutes, but it was just the bootstrap pin not soldered properly. I took it up to about 7.8A without noticeable trouble (I wasn't observing output ripple though, just the average with a multimeter) so I'm already liking it better than the previous design.
The guy on the left is a new USB power meter. I haven't attached the second socket yet, as I was using it for impedance measurements. This guy takes in USB signal+power in the B socket. The currently-headered pair of pins right above the B socket allow for USB power to pass through. If that's not jumpered, the 4 pins adjacent (two +V, two GND) are the power input. The two pins past that (labeled A and G) are the current meter pair; voltage on the A pin (with respect to G, the ground) is approximately 100mV per amp of current. After calibration (linear regression on 12 data points from 50mA to 3A) the actual current is 0.99 the measured value. The second set of pins, by the USB A socket, is for measuring the output voltage at the sockets. The first socket (present) gets USB data, but the second will be power only. I set one up with two sockets because I want my regulator board pulling from USB to emulate actual source impedance conditions, and the signal socket runs the USB-UART adapter (which is also powering the PLL and IO LDOs on the breakout board).
The guy on the right is a stripped-down version of the above, except designed to socket right inbetween my regulator board's output and the BM1384 breakout board. It's only got two pin sets, one for voltage and one for current. A linear regression on its calibration (14 data points between 800mA and 10A) had up to 3% error in the low end (below 4A or so) and I wasn't really happy with it, but for some reason 0.97(Im^1.01) yields between a maximum error of 0.83% with an average error of 0.33% so I'm okay with that.
There's a CR2032 button cell socket on the other side, so this guy is a self-contained meter. Unfortunately with a Vcc of 3.0V, I can only measure up to a bit below 10A, since I'm using a 3mOhm sense resistor, DC gain of 100, and my output is divided by 3 to scale evenly to 100mV/A which means my maximum voltage range at the output pin is 0-1V.
Now that both meters are characterized and calibrated, I can hook them up to the new regulator board tomorrow with an adjustable dummy load and get some efficiency curves. If it behaves how it should, after that I'll tie in a single-chip BM1384 board, the USB-UART adapter and LED circuit and get an actual matrix of Compac W/GH performance data. Expect news tomorrow - hopefully good news, but all I can guarantee is there should be news. And also sandwiches.
If the regulator works how it should, I'll finish redesigning the Compac board and might post some sample layouts. I'll probably also put together a basic two-chip BM1384 breakout with both parallel and string IO/power options so I can better test chip cooperation for Amita and TypeZero designs. As soon as both of those boards are ironed out, we'll be sending off for prototype PCBs. If the Compac board tests well, we'll be looking to ship a few out to our selected testers for finagling. If they report back with favorable reviews, we can start looking into batching a big order.
Also, on a side note, Novak's getting really darn good at toner transfer etches. That regulator chip is a 16-pin 3x3mm QFN package, and the current sense amplifiers on the meter boards are SC70-6, which is about half the size of a SOT23 and twice as many pins.