Poll

Which form factor do you want to see

Gum stick with dual USB-C ports
11 (47.8%)
Module with LGA land patterns
2 (8.7%)
Module with micro mezzanine connectors
10 (43.5%)

Total Members Voted: 23

Author Topic: [Poll] FPGA board form factor  (Read 5936 times)

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Online blueskull

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[Poll] FPGA board form factor
« on: September 06, 2019, 02:29:54 am »
I'm working on a Gowin Arora GW2A-18 FPGA module for integration and prototyping.

Which form factor do you think it's the best if you are looking for an FPGA module?

I'm aiming at development prototyping and very low volume prototype production runs.

FYI, the design features are as follow:

1. 21k 4-LUTs+16k FFs
2. 42Kb distributed SRAM+828Kb block SRAM+64Mb SDRAM
3. 48 M18 DSPs (24 DSP slices, each can be configured as dual M18 or single M36, both come with MAC/ALU options)
4. 2 PLLs+4 DLLs with on board 12MHz clock
5. 34 IOs in 4 1.2V~3.3V banks+31 IOs in 3.3V banks, most IOs are true LVDS capable
6. On board SPI FLASH+FTDI downloader+FTDI UART
7. Supports Cortex M1 (from Gowin), RiscV (from Gowin) and ZPU (from me)
8. On board PMIC accepting single 3.5V~5.5V input with input voltage sensor
9. On chip USB HS PHY IP (under development, using DLL, PLL and IO gearbox), tentative
10. A few LEDs for heart beat and debugging, maybe one or two micro buttons

As for form factor, I intentionally left out castellated hole option as I simply don't like it. I also left out HumanData's PLCC form factor as it's just to expensive.

Target total BOM is <$15 at 100 pieces batch.

Poll locked. Final form factor: DIP gum stick with micro USB-B and FPC expansion connector.
« Last Edit: September 14, 2019, 04:53:44 am by blueskull »
 

Online DaJMasta

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Re: [Poll] FPGA board form factor
« Reply #1 on: September 06, 2019, 03:10:45 am »
Are LGA modules that easy to assemble into a low volume product?  Castellations would be hand solderable and as a result don't require anything fancy (not that I'm saying you should use them), but would an LGA module need special oven profiles or handling to use properly?  Could limit viability in small production runs, even though the overall cost is probably the cheapest.

I'd think USB C would be most desirable to the hobbyist level user, but it probably counts for less in commercial or industrial development environments.  If the mezzanine connectors themselves aren't too expensive, I'd probably go that route and make a breakout/development board with the mating mezzanine connectors and the USB C.  That way you have an easy-to-integrate module with a lower base price and a carrier board for more convenient prototyping.

If you were thinking of aiming primarily at the hobby market, maybe the USB C alone would be preferable, since the combination of a module and a carrier board would likely be a fair bit more expensive.


LGA is probably the cheapest to use in production runs, but it would probably be a pain to develop on.  Maybe if you had an LGA module and a USB C carrier board you could develop on the carrier version and then integrate only the module into the product - maybe a good balance in usability and price, especially if the mezzanines are going to be expensive or really low profile is an advantage.
 

Offline james_s

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Re: [Poll] FPGA board form factor
« Reply #2 on: September 06, 2019, 03:39:39 am »
I like 0.1" pin headers for GPIO and some more specialized connectors for high speed stuff like LVDS. It really depends on what one wants to build though.
 
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Online blueskull

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Re: [Poll] FPGA board form factor
« Reply #3 on: September 06, 2019, 04:00:55 am »
Mezzanine connectors are cheap, just don't buy from a major distributor.
The problem is pitch. To keep the profile low, the pitch can not go beyond 0.5mm, which makes carrier board design a nightmare.

LGA can be dead bugged, and since I will only do top side loading, you can solder LGA as a normal larger BGA part (I can shrink the entire thing down to 20mm*20mm).

DIP is good, but high speed is a problem. I think it's more of a hobbyist-oriented form factor than a production package.

Maybe an LGA core module with a carrier board which has USB ports, mezzanine connectors for HS LVDS and DIP pins for breadboarding?
 

Offline Berni

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Re: [Poll] FPGA board form factor
« Reply #4 on: September 06, 2019, 05:45:51 am »
To be honest i don't really see the point in having a carrier board for such a FPGA chip.

From a quick look over the Gowin GW2A series its available in QFN and TQFP packages and if you want lots of pins also available in the nice big 1mm pitch BGAs that are reasonably easy to solder by hand. So if you are designing a board for this module to fit onto you might as well just solder the chip directly to it. The chips also look reasonable in terms of external support circuitry. Just needs 3.3V and 1.1Vcore and a SPI flash and on a small FPGA like this the Vcore could even be done with a simple LDO.

Yes 0.1in headers are the most friendly for prototyping but usually existing dev boards out there already offer plenty of those. What i have went to for my recent Lattice board is 0.5mm FPC connectors. This allows you to connect other boards to it via a flexible cables for convenience and a FPC connector is pretty easy to solder. These FPC cables can perform very well at high speeds if you put a ton of grounds on the pinout. No need to bother with fancy high speed cables, all you have to do is alternate signal and ground traces and this forms a nice transmission line, works well for LVDS too (In that case you go GND D+ D- GND D+ D- GND ...). Works with 1.27mm pitch ones too but not quite as well (I had 600Mbit MIPI running just fine over one of these tho).
 
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Online blueskull

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Re: [Poll] FPGA board form factor
« Reply #5 on: September 06, 2019, 06:29:08 am »
So DIP+mezzanine/FPC?
 

Offline jhpadjustable

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Re: [Poll] FPGA board form factor
« Reply #6 on: September 06, 2019, 07:31:06 am »
There are FPC to DIP breakout boards all over Aliexpress. I'd imagine them suitable for extending I/O to the experimenter.

If you keep enough important signals at the edge and at roughly 1.27mm pitch, and place higher-density signals and returns near the center, a hobbyist could build a simple footprint with long-toed pads and nothing in the middle, then hand-solder the module to their board as if it were a large, wide-pitch QFN without wettable flanks.
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Offline chickenHeadKnob

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Re: [Poll] FPGA board form factor
« Reply #7 on: September 06, 2019, 07:53:31 am »
I chose "gumstik" only because I assumed this means card edge fingers down one side, and then thought maybe you use this term differently. Mezzanine boards with different connectors are a non-starter. If more than one connector the mezzanine board should  only be one connector height/type, I suggest samtec.

I also agree with Berni, the low pincount devices are easy to solder and I would just include the footprint on my own PCB.

I haven't been following the GOWIN threads closely, how fast can the RISC-V core implementation be clocked?
 

Online blueskull

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Re: [Poll] FPGA board form factor
« Reply #8 on: September 07, 2019, 12:34:57 am »
From the feedback I received, here's my next game plan:

0.6" wide 0.1" DIP with 2*12 pins for power, ground and 16 low speed IOs.
0.5mm FPC with 32 pins for 4 low speed IOs and 8 pairs of high speed IOs.

DIP pinout (all IOs are 3.3V):

VEXT    VSYS
V3V3    PBTN
RETN    RETN
LS01    LS02
LS03    LS04
LS05    LS06
LS07    LS08
LS09    LS10
LS11    LS12
LS13    LS14
LS15    LS16
RETN    RETN

FPC pinout (LS17, LS18, HS1~HS4 are VIO1, LS19, LS20, HS5~HS8 are VIO2):

VIO1    LS17    LS18    RETN    HS1P    HS1N    RETN    HS2P    HS2N    RETN    HS3P    HS3N    RETN    HS4P    HS4N    RETN
VIO2    LS19    LS20    RETN    HS5P    HS5N    RETN    HS6P    HS6N    RETN    HS7P    HS7N    RETN    HS8P    HS8N    RETN

Total board size is 18mm*36mm, and is wide DIP24 socket compatible.



One push button is provided on board, plus being routed to DIP header. Depending on when and how long it was pushed, it could be FPGA IO, FPGA reset, USB FPGA/FTDI mux switch or power button.

Four LEDs are provided, with amber LED serving as power LED or blink indicator for push button timer, and RGB LEDs for FPGA.

Any suggestions?
 

Online blueskull

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Re: [Poll] FPGA board form factor
« Reply #9 on: September 07, 2019, 08:03:07 am »
Update:

1. Changed USB-C to USB-uB for size concerns.
2. Added USB mux to allow single USB port to operate with FTDI and FPGA.
3. Added FTDI mux indicator and TX/RX indicator.
4. Moved button to center.
5. Added more LEDs to form a nibble.
6. Removed crystals and replaced with one always on 12MHz MEMS oscillator.

The following diagram did not show resistors, but those 0201 ones should squeeze well with every possible space.



The board looks routable with 4L HDI process.
 

Offline SiliconWizard

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Re: [Poll] FPGA board form factor
« Reply #10 on: September 07, 2019, 03:01:47 pm »
I'd vote for castellations as well (but that can be a bit costly to manufacture), otherwise the LGA thing. The version with 2.54mm TH and an FPC connector is nice for simple prototyping stuff, not so much for integrating with another PCB frankly. And I'd like to see more IOs broken out as well.

Of course you could offer both versions, which would address two different sets of needs.
 

Offline Kilrah

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Re: [Poll] FPGA board form factor
« Reply #11 on: September 07, 2019, 07:02:19 pm »
Keep the holes, AND add rows of castellations with more IO next to them. Those who want breadboard use the TH IO, those who want SMD use the castellated IOs, those who want loads of IOs find a way to use both that suits them.
 

Online blueskull

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Re: [Poll] FPGA board form factor
« Reply #12 on: September 07, 2019, 09:43:59 pm »
Keep the holes, AND add rows of castellations with more IO next to them. Those who want breadboard use the TH IO, those who want SMD use the castellated IOs, those who want loads of IOs find a way to use both that suits them.

I'd love to, but as you can see from the picture, I'm really running out of routing space.

I probably can only have sufficient access to half of the pins (with 0.2mm/0.4mm vias and 0.1mm tracks) if I don't use HDI.

Even with HDI, without laser drills and plugged vias, I don't think I can fan out more pins.

Those QFNs are 0.4mm pitch ones and fanning out them can be hard.

I once did a half fanout on a 0.5mm QFN88 (ADSP-BF706), and that was hard enough.
 

Online coromonadalix

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Re: [Poll] FPGA board form factor
« Reply #13 on: September 08, 2019, 03:21:25 am »

quote : Total board size is 18mm*36mm, and is wide DIP24 socket compatible.             Thats fine with me,  bread board compatible  etc ...   :-+

We had to design a special board at my job  to replace a xilinx xc9572  with an atf1504,  the board has castellation, and i can tell you for us its a nightmare to solder, sometimes the pads bridge together, we had to use a finer solder paste ... the castellation is not perfect on the board edges, sometimes i have to use a filer to clean it a little, its depends where your boards are made and the quality they can do / provide you.

For me its a no no

 

Online blueskull

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Re: [Poll] FPGA board form factor
« Reply #14 on: September 08, 2019, 04:15:21 am »
the board has castellation, and i can tell you for us its a nightmare to solder

That's why I hate them. Even commercially made half vias can be tricky to solder, especially on small (no extension) pads and without a stencil.

Said from my experience with ESP32-WROOM and SIM7000G.
 

Offline pigrew

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Re: [Poll] FPGA board form factor
« Reply #15 on: September 08, 2019, 05:12:34 am »
If there is a spare FPGA pin, it would be good to be able to disable the oscillator (controlled by the FPGA) for low-power modes.

Power supply is always a tricky thing. It'd be nice if one can power the board from either 3.3V or 5V. Does it work if I connect 3.3V to the VEXT pin, or do I need to connect it directly to the V3V3 pin?

I've started to really rather USB-C over micro-USB due to their robustness. USB-C is about 9x8mm, whereas microB is ~6x7. Could enough of the port be hanging off the edge of the PCB that they are about the same?

How do you plan to do the USB? Bit-banging? Or is there a PHY? Is the differential transceiver in the FPGA good enough to serve as a PHY? What about enable-disable of the USB pullup? Is it full speed or high speed? Would something like USB3322 fit? On the other hand, I have not had any use cases where I would need high-speed.

I don't have the datasheet open, but are there clock input pins that are broken out?

Since it's being through hole, are there any components on the bottom? Maybe add a footprint for SDRAM (or something else?) on the bottom that the user could add, which wouldn't be part of the standard build?

Is one able to both program the flash (for non-volatile usage) and the SRAM of the FPGA (for debugging)?
 

Offline ali_asadzadeh

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Re: [Poll] FPGA board form factor
« Reply #16 on: September 08, 2019, 05:20:01 am »
which PCB software are you using? I hope you use Altium! ;D
I'm a Digital Expert from 8-bits to 64-bits
 

Online blueskull

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Re: [Poll] FPGA board form factor
« Reply #17 on: September 08, 2019, 05:31:34 am »
1. If there is a spare FPGA pin, it would be good to be able to disable the oscillator (controlled by the FPGA) for low-power modes.

2. Power supply is always a tricky thing. It'd be nice if one can power the board from either 3.3V or 5V. Does it work if I connect 3.3V to the VEXT pin, or do I need to connect it directly to the V3V3 pin?

3. I've started to really rather USB-C over micro-USB due to their robustness. USB-C is about 9x8mm, whereas microB is ~6x7. Could enough of the port be hanging off the edge of the PCB that they are about the same?

4. How do you plan to do the USB? Bit-banging? Or is there a PHY? Is the differential transceiver in the FPGA good enough to serve as a PHY? What about enable-disable of the USB pullup? Is it full speed or high speed? Would something like USB3322 fit? On the other hand, I have not had any use cases where I would need high-speed.

5. I don't have the datasheet open, but are there clock input pins that are broken out?

6. Since it's being through hole, are there any components on the bottom? Maybe add a footprint for SDRAM (or something else?) on the bottom that the user could add, which wouldn't be part of the standard build?

7. Is one able to both program the flash (for non-volatile usage) and the SRAM of the FPGA (for debugging)?

1. The PMIC controls the OSC. If FTDI USB MUX is enabled and not suspended, then OSC is on. If not, the OSC can be enabled over I2C.

2. VEXT goes to PMIC. The FPGA can operate at 3.15V min, and the PMIC has a drop out 0f 100mV, thus, the system can happily operate at 3.3V +-1%. 3.3V pins can NOT be fed directly as that will cause current to flow back through body diode of PMIC and potentially damage it. Also, other rails (1.0V, VIO1, VIO2) are generated from VSYS, not V3V3. VEXT and VUSB are multiplexed to VSYS (with VEXT having higher priority).

3. The space is at extremely premium, so is the thickness. I'd like to add USB-C, but space really doesn't allow me.

4. USB it bit banged. GW2A has LVDS transceivers, and bias is generated externally and they can operate at a very wide VCM range, thus allowing MIPI emulation (with LVCMOS pins for LS tx/rx and termination switching). MIPI with 3.3V LSIO is compatible with USB HS. With LVDS in high-Z, the two LVCMOS pins serve as USB FS/LS transceiver. PU/PD are done with another two IO pins with corresponding resistors.

5. Clock pins are shared with LVDS pins. The FPC connector has some differential pairs that are also GCLK capable. Also, at least one LSIO is single ended GCLK capable.

6. Routing such a high pin count, high loading factor board without HDI is already hard enough. I don't want to add complexities. FYI, my usual stack up is top-parts, local interconnection, mid 1-power/ground, mid 2-remote interconnection, bottom-ground. This stack up doesn't allow bottom side loading. Anyway, this chip comes with copackaged 64Mb 3.3V SDRAM.

7. Yes. Gowin tools support programming to SRAM or (a selected range of) flash.
 

Online blueskull

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Re: [Poll] FPGA board form factor
« Reply #18 on: September 08, 2019, 05:32:07 am »
which PCB software are you using? I hope you use Altium! ;D

I paid totally more than $12k on it over the years. Why would I not use it?
« Last Edit: September 08, 2019, 05:33:40 am by blueskull »
 

Offline OwO

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Re: [Poll] FPGA board form factor
« Reply #19 on: September 08, 2019, 06:53:17 am »
Since this is very space constrained I think it's a good idea to not adhere to a fixed stackup. It is possible to have an effectively solid ground even if every ground plane looks cut into pieces if you have vias stitching them in in strategic places. I would highly recommend placing passives on the bottom side of the board because it would provide better decoupling as well. I would personally also put the spi flash right under the FPGA. At 18*36mm this doesn't look very difficult to route at all, so I can volunteer to do the PCB layout but I can't do altium (gEDA only).
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Offline OwO

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Re: [Poll] FPGA board form factor
« Reply #20 on: September 08, 2019, 07:07:50 am »
When I design boards I never dedicate layers to roles; all layers are treated equally. Every layer starts off with a solid ground fill, and components are placed on either side in clusters to minimize distances. From then on routing is very ad-hoc but there are a few rules; high speed signals require a solid ground plane but only right under the signals. It doesn't matter if the ground is broken elsewhere so I route low speed signals on the ground plane at will provided it doesn't cross any high speed traces. There are also a few circumstances where you *can* run a trace under a component, for example if that component is a decoupling cap because its body is considered RF ground. Typically most passives on the bottom side are for decoupling so I generally prefer to route signals on in2. There is, of course, a ground fill on the bottom side but it need not be solid because all the "breaks" caused by decoupling caps are not considered breaks.
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Online blueskull

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Re: [Poll] FPGA board form factor
« Reply #21 on: September 08, 2019, 07:09:34 am »
Since this is very space constrained I think it's a good idea to not adhere to a fixed stackup. It is possible to have an effectively solid ground even if every ground plane looks cut into pieces if you have vias stitching them in in strategic places. I would highly recommend placing passives on the bottom side of the board because it would provide better decoupling as well. I would personally also put the spi flash right under the FPGA. At 18*36mm this doesn't look very difficult to route at all, so I can volunteer to do the PCB layout but I can't do altium (gEDA only).

Thanks for the offering! I'll sure let you know if I can't do it (which may very well happen!).

As for this particular stack up and single sided loading, I do have three concerns:

1. It leaves a pure solid ground bottom layer with only few vias connecting to other nets. This reduces risk of shorting if the core module is not spaced from the top surface of the carrier board.
2. It leaves no parts to be bumped into. MLCCs can crack, and short catastrophically, and I've seen a few occasions.
3. It allows for LGA-style termination on non-soldered DIP pads.
 

Online blueskull

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Re: [Poll] FPGA board form factor
« Reply #22 on: September 13, 2019, 09:07:25 am »
Draft of schematic is attached here: https://1drv.ms/u/s!AkXnLOXZOgKpmCRACVRCCtURDY_A?e=dHZj0R

FTDI EEPROM is removed as I found it's not needed for Gowin programmer to talk to debugger. My Trenz board came with empty EEPROM.

Comments?
 

Offline OwO

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Re: [Poll] FPGA board form factor
« Reply #23 on: September 13, 2019, 09:50:14 am »
The BOM doesn't look very optimized, but these things stand out:

1. Really seriously consider alternatives to the FT2232. It's a big cost adder and even if you need true usb-fifo-gpio functionality a cheaper micro can usually do the job for under $1.
2. Please avoid SiTime MEMS oscillators. They cost more than surface mount TCXOs and the performance is way inferior to cheap smd powered crystal oscillators, which are easy to find on LCSC.
3. The choice of connectors seem on the expensive side; I would suggest starting your search on LCSC rather than digikey/mouser because you will often find low cost good quality connectors from Japanese or Taiwanese manufacturers that you won't find on digikey.

As it is the BOM cost is higher than the NanoVNA V2  ;)

Also maybe add a pdf schematic because I can't open those files. Better idea, switch to kicad/geda or at least easyeda  ;)
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Re: [Poll] FPGA board form factor
« Reply #24 on: September 13, 2019, 10:17:09 am »
Just noticed the tantalum caps  :palm:
47uF isn't a very large capacitance for standard MLCC at 1.0V. That tantalum has an ESR of 300mOhm which is kind of shitty (a 22uF MLCC will have < 100mOhm). I would recommend 2 x 22uF 0603 MLCC caps instead; you probably don't want to use just a single cap for the entire 1.0V rail (I would go for 4 caps, one for each "side" of the FPGA, but 2 most likely will work).
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