PMOD vs FMC vs. third option - Page 1
Forum section: electronics / general

author: PDXjason
author_rank: Contributor
timestamp: August 03, 2017, 03:46:51 pm
content: We've spoken to a LOT of customers over the years and, through our own experience, we've seen a real need for something that settles into the space between the PMOD and FMC standards for cost and performance.Would there be interest in something like this as an open standard?

author: rstofer
author_rank: Super Contributor
timestamp: August 03, 2017, 05:18:28 pm
content: Strictly thinking about FPGA boards that I own...I always liked the 50 pin headers like those used on the Digilent Spartan 3 Starter Board. It's easy to install a matching header on an add-on board. In addition, there are enough sockets (3) on the board to dedicate one to the logic analyzer (pre Vivado days).One of the things I don't care for is the idea that I bought a fairly large FPGA (on a board) but I get just a few IO lines coming out through PMODs. Adding something like a Compact Flash to a project tends to require a lot of signals. It's nice when they all come out through a single header. Sure, I could use an SD card (and one may be provided) but the CF interface is so much simpler.Some of the least expensive FPGA boards have the most IO lines. That's easy to do when there are no on-board features. Still, there is a place for the 'Stamp' format.OTOH, if you want to add a simple interface, say SPI, the PMOD is perfect!So I don't think there is an easy answer and I'll just continue to use what is offered.

author: PDXjason
author_rank: Contributor
timestamp: August 03, 2017, 05:37:46 pm
content: rstofer, do you ever use FMC?

author: Scrts
author_rank: Frequent Contributor
timestamp: August 03, 2017, 05:43:24 pm
content: Quote from: rstofer on August 03, 2017, 05:18:28 pmI hated that right angle connector on S3E Starter Kit. I also did not like the HSMC connector on Altera devkits. Either 1.27mm or 1mm spacing pinheads are OK, unless you need high speed impedance matched diff pairs - then you have to go with those extra expensive connectors.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: August 03, 2017, 06:26:56 pm
content: I think it's mostly about finding a sane connector and basing a standard around it.By sane I mean - easily available ( in stock at Digikey et al), sensible cost in 1x, hand-solderable, but dense enough to not be stupidly big (like 0.1" headers). Availability in both TH and SMD would be a bonus, as well as straight & right-angle options. Been around a while so not likely to disappear any time soon. Routable on a 2-layer PCB for less critical applications would be nice.Anything specifically designed for high speed is likely to be expensive - maybe something as simple as a 1.27mm header with plenty of grounds? A nice thing about that is it's easily scaleable to different widths.

author: rstofer
author_rank: Super Contributor
timestamp: August 03, 2017, 06:50:52 pm
content: Quote from: PDXjason on August 03, 2017, 05:37:46 pmNot yet! The first example board I found at Digilent that uses the FMC is on the pricey side for my purposes. I have used boards with the FX-2 connector. I like the 100 pin FX-2 format and, for end stacking boards, it tends to be correctly oriented - it's a right angle connector.The Digilent example using an FMC seems to have the openings up. I'm guessing that the daughter card just sits over the top with a mating connector on the bottom. ETA: That means the system board and add-on board are not co-planar. I like co-planar and that's why I like the 50 pin or FX-2 connector.https://reference.digilentinc.com/_media/reference/programmable-logic/zedboard/zedboard_ug.pdfMy guess is that Digilent uses the FMC connector for its higher speed capability. As most of my projects run at 50 MHz, I haven't had a problem with the older FX-2 or even the 50 pin header.ETA: Please consider my comments as those of a hobbyist. I'm long retired and I won't be buying many more FPGA boards. Although I did just buy a Nexys4 DDR board for a possible expansion of a prior project.

author: nctnico
author_rank: Super Contributor
timestamp: August 03, 2017, 07:11:54 pm
content: If you have a choice then don't use FMC. These are BGA style connectors which need to be reflowed and some assemblers don't like them very much.

author: PDXjason
author_rank: Contributor
timestamp: August 03, 2017, 07:40:19 pm
content: What about something affordable, but offering standard and transceiver connectors? Higher pin-count than PMOD.You know, just wondering.

author: nctnico
author_rank: Super Contributor
timestamp: August 03, 2017, 07:52:37 pm
content: Check Mouser or digikey and look for SMT board to board connectors. 0.8mm pitch is easy to solder by hand and there are versions with a large ground connection in the middle (like Mictor). If not then you can put 1 ground between each pair.

author: daveshah
author_rank: Unknown
timestamp: August 04, 2017, 06:57:25 am
content: In my opinion FMC isn't nearly as bad to work with as any kind of BGA IC. The 1.27mm pitch and alignment pegs mean it's easy to assemble if you have access to any kind of reflow oven, and I've found you can break out plenty of signals on a 4 layer board using standard design rules.Still a lot harder than standard header pins though...

author: chickenHeadKnob
author_rank: Super Contributor
timestamp: August 04, 2017, 12:41:15 pm
content: Disclosure, I am not a customer nor affiliated with Trenz Electronic in any way. I have however returned to their website often and thought I will buy one of their in-house designed (Trenz branded) modules when I have time for a certain project - HaHa. They feature downward facing samtec "razor-beam" hermaphrodite connectors :http://suddendocs.samtec.com/catalog_english/lshm_dv.pdfTrenz sells the connectors as well:https://shop.trenz-electronic.de/en/Products/Connectors/The physical style of their module boards greatly appeals. I wish all fpga dev boards were similar. Interestingly you can get Zynq dev boards from different manufacturers MYIR, ZED board, Paralella, Krtcl - SnickerDoodle with downward facing connectors but no Altera SOC equivalent competiter.

author: PDXjason
author_rank: Contributor
timestamp: August 10, 2017, 09:18:30 pm
content: On Monday, August 14, we will be releasing a new open standard for peripheral connectivity for FPGA carrier boards.