Not long ago, I was the happy recipient of a Silego GreenPAK4 Development Kit. I wasn’t familiar with Silego or their products until they offered me up the kit for review. Silego describes these products as a “one-time programmable mixed-signal matrix.” The chips have a variety of digital and analog I/O and a bank of programmable logic.
I tend to think in terms of microcontrollers. I do have some FPGA experience, but my head largely stays in the MCU space. These are definitely not microcontrollers, but they’re not quite an FPGA or SoC (system-on-chip). They’re more a collection of glue logic and assorted analog peripherals. The chip is configurable like an FPGA or SoC, but it’s one time programmable. That helps with production but can make configuration more difficult.
These parts have been written up a few times recently. You can get a more information in the EETimes article “Silego Poised to Announce GPAK4 Mixed-Signal FPGA.” In his excellent Embedded.com article, “First impressions on Silego's GreenPAKs,” Aubrey Kagan takes us on a test case replacing an old pulse width modulation (PWM) controller.
The chip is tiny and it’s a concept that not a lot of people are familiar with. That leads to the question: Is it practical to use? A lot of great ideas come and go, but if they’re too difficult to design with, or awkward to use, they can easily die without fanfare.
GPAK with dime
For ease of introduction to the part, Silego built a nice development board system. It also serves as the device programmer and emulator during development. The two-part development kit consists of a motherboard and a daughter board with a QFN chip holder socket. The daughter boards change with the size of the chip variant.
The documentation instructs you to use tweezers to place one of the chips in the socket. 2mm x 3mm is pretty small, but fortunately, between my day job at manufacturer, Screaming Circuits, and my own design work, I’ve gotten pretty comfortable with small parts. If you have a small tip vacuum pen, I’d use that instead of tweezers.
The development kit and the actual chip are a study in contrast. As you saw in the photograph above, the parts themselves are 2 mm x 3 mm for the variant in the photo. The development board, on the other hand, as shown below, is 2 ½” x 3 ¾” and generously stocked with 0.1” pitch headers and large test points.
Devkit with socket
It takes power and PC communications through a USB mini-B. I would prefer micro-B — now standard in the mobile device world, but that’s a pretty minor point. All of the accessible pins on the development board including the test points appear to be protected from mishandling. That’s becoming a rarity these days. The test points are even loop-type, to allow for a more solid connection.
Test point closeup
Without a good development board, this would be a difficult chip to prototype with. Fortunately, Silego did a thorough job with the board. Other than loading the chip into the big socket, you can go through an entire development to production cycle without touching the chip.
In terms of PC board design, fabrication, and assembly, you won’t run into any difficulties with service providers. Well, you shouldn’t. If a board house or assembly shop can’t deal with 0.4 mm pitch parts these days, you should be looking for a new supplier.
The land pads are 0.18 mm wide, which is 7 mils. The gap between pads is 0.22 mm, which is 8.7 mils. Almost all board shops can fabricate a PC board to 6 mil trace and space, which can easily accommodate that specification. Assembly might be a bit more of a challenge. Most of the top-tier manufacturers can handle the part. If they don’t, they should at some point as this isn’t even the smallest part around. At Screaming Circuits, we’ve seen 0.3 mm pitch parts, and I’ve read about plans for 0.25 mm pitch parts. Not all budget assemblers can deal with parts at 0.4 mm pitch though. If they can’t handle 0402 passive parts, they likely won’t do a good job with this part.
The part doesn’t require any special technique for the CAD footprint as is commonly the case with QFN parts. You will, however, likely need to create your own footprint. As you can see in the first photo of this blog, the pads are unequal length. Silego has the exact dimensions required for the footprint in the data sheet. As a manufacturer, I appreciate the level of detail. Use their dimensions, and you shouldn’t have any problems.
The software was easy to install, and the tutorials are helpful. However, I’m still getting up to speed in that area, so I’ll leave that to another blog. In summary: It’s an intriguing part that I think will be very useful. It has a good development, emulation, and programming environment. It’s not a hand-solderable part, but it should be very useable with most board fab shops and assembly service providers.