Some FPGA Beginner Questions

Hi Christian,

I just saw your discussion today.  I don't check the element14 FPGA page much since activity is rare.

There's some good FPGA discussion in this thread at the Raspberry Pi group, including some other development boards.

I've done a lot of design with the Xilinx Spartan-3A.  It's a nice architecture.  For learning FPGAs, there's really no need to jump to the Spartan-6 unless you need the increased capacity for a specific project.  Although the Spartan-6 ICs cost about the same as Spartan-3A, the boards are still quite a bit more expensive.

Lattice had eval boards for their small-FPGA iCE40 series with promotional pricing of US$19 for a short time, but I think they're back to the standard US$39, so for that price you're better off spending a little more for a 200K gate Spartan-3A, e.g., the US$55 XESS XuLA-200.  I haven't tried that board myself, but the specs are attractive.

All the major FPGAs can synthesize from VHDL or Verilog.  JMO/YMMV: If you like to type and love the Ada language, go with VHDL.  If you like C and prefer more concise notations, go with Verilog.  My preference of the two is Verilog, but neither is really a good fit for FPGA design.  According to my recollection, VHDL was conceived for specifying and simulating behavior of VLSI designs.  Verilog was conceived for generating test vectors and expected behavior.  Neither was conceived as a language for synthesizing logic, so you have to be inventive to get the synthesizer to produce the logic you really want.  The best language I've used for that was Altera's AHDL, but I don't think it's an option any more.

FPGAs are very powerful and a lot of fun.  You get to design digital chips with zero cost for making logic errors.  Have lots of fun and please report problems and progress.

Hi John,

thanks for your info as well!

I don't check the element14 FPGA page much since activity is rare.

That's good to know. Might I ask which page is a good alternative with some more activity? So far I'v only seen another german site and it's sister over @ http://embdev.net .

Right now I pretty much narrowd down my choice to two of the Digilent boards, the Nexys 3 and the Atlys. My final considerations are:

• I'm not really interested in video processing, so the Atlys' HDMI ports don't do anything for me.
• The Atyls seems quite a lot beefier. Do I need that? Probably not. Do I want it? Absolutely
• The Nexys 3 comes with four pmod extension slots for all kinds of stuff. I don't really have a plan to use more than one but I like to have them as it seems a very nice way to add stuff to the board further down the line. The Atyls only has one pmod slot.
• I will probably get the display vmod for christmas, which is awesome   It also means that I don't really care about the VGA and HDMI ports for video output, I'll just use the small FPGA screen and be amazed by that

So in short: the nand2tetris project can be seen to run on a Cyclone II, so a Spartan 6 should be able to handle it. I would very much like to own a powerful Atyls board, but when I'm honest with myself, I probably won't need its power anyway - even if I extend the project to more stuff. In a perfect world I'd like to have a small OS running on the FPGA, and the Spartan 6 board should be able to handle that (plus, there's quite a road ahead till that milestone will be reached).

YourVerilog/VHDL perspective is very welcome! I'm just now toying around with Xilinx' ISE to get a small Hello World to run and the Verilog/VHDL question is indeed what I have to figure out next. I already started with VHDL but now I guess I'll just try a few simple logic stuff with Verilog as well.

Cheers!

Hi John,

thanks for your info as well!

I don't check the element14 FPGA page much since activity is rare.

That's good to know. Might I ask which page is a good alternative with some more activity? So far I'v only seen another german site and it's sister over @ http://embdev.net .

Right now I pretty much narrowd down my choice to two of the Digilent boards, the Nexys 3 and the Atlys. My final considerations are:

• I'm not really interested in video processing, so the Atlys' HDMI ports don't do anything for me.
• The Atyls seems quite a lot beefier. Do I need that? Probably not. Do I want it? Absolutely
• The Nexys 3 comes with four pmod extension slots for all kinds of stuff. I don't really have a plan to use more than one but I like to have them as it seems a very nice way to add stuff to the board further down the line. The Atyls only has one pmod slot.
• I will probably get the display vmod for christmas, which is awesome   It also means that I don't really care about the VGA and HDMI ports for video output, I'll just use the small FPGA screen and be amazed by that

So in short: the nand2tetris project can be seen to run on a Cyclone II, so a Spartan 6 should be able to handle it. I would very much like to own a powerful Atyls board, but when I'm honest with myself, I probably won't need its power anyway - even if I extend the project to more stuff. In a perfect world I'd like to have a small OS running on the FPGA, and the Spartan 6 board should be able to handle that (plus, there's quite a road ahead till that milestone will be reached).

YourVerilog/VHDL perspective is very welcome! I'm just now toying around with Xilinx' ISE to get a small Hello World to run and the Verilog/VHDL question is indeed what I have to figure out next. I already started with VHDL but now I guess I'll just try a few simple logic stuff with Verilog as well.

Cheers!

Christian Müller wrote:

 

John Beetem wrote:

 

I don't check the element14 FPGA page much since activity is rare.

That's good to know. Might I ask which page is a good alternative with some more activity? So far I'v only seen another german site and it's sister over @ http://embdev.net .

Good question.  I took a quick look at embdev.net and it seems to have a good list of topics.

For something as amazingly useful as the FPGA, you'd think there would be a lot more use and activity, yet at the present time FPGA design seems to be a specialized art.  Perhaps this is because they've been expensive in the past, and even at present when you can buy an entire 1GHz 32-bit ARM SoC for the price of a modest FPGA it's hard to justify the cost of FPGAs, especially in high-volume consumer devices where every cent matters.  Perhaps it's because there's a steep learning curve for VHDL or Verilog, and for vendor tools.  Perhaps it's because they "don't teach Logic in these schools", to quote a famous Professor.

I'm hopeful that the Rasberry Pi movement will help get more people -- especially students -- introduced to FPGAs.  So I eagerly read anything at element14 or raspberrypi.org that has to do with FPGAs.  You can easily find discussion threads about FPGAs at raspberrypi.org by searching for "FPGA" in the Google box on the front page: all the major threads show up on the first page of results.  If there's enough interest and therefore volumes, the price of FPGA development boards will fall enough so that FPGAs can really get some traction.

If you read the element14 thread I mentioned above and some of the raspberrypi.org threads, you'll see me complain repeatedly about my main FPGA bugbear, which is that none of the vendors publish their bit-stream format so you can create your own design tools.  Until this happens, there is no reason to expect FPGA design to get any easier.  Xilinx has good tools when measured by the quality of the results, but it takes a lot of experience and effort to write your logic so as to get those results.

Yeah, I was wondering about how relatively small the FPGA community seems to be. Or to be more precise, how thinnly spread out over various forums. On the other hand, most threads I've been reading consisted of 90% useful information.. so that's really nice, and something rarely seen on online forums. So quality over quantity it seems

What exactly is the Rasberry PI movement you're referring to? I've read it on several occasions, but never got caught in any discussion about it yet.

As for the vendor's and their software.. I haven't even started with FPGAs yet and I'm already annoyed by their shenanigans. Almost every thread talks about Altera vs Xilinx sooner or later, so you would expect their software to be shining examples of amazing usability in order to boot out the competition. But.. well. Not so much. Yesterday I found out about Xilinx' WebTalk "feature" and that's just an absolute show stopper for me. Who thought this was a good idea and approved it? And who thought it would then be a good idea to praise this abomination as a dandy feature the user should be thankful for?! Urgh.

Christian Müller wrote:

 

Yeah, I was wondering about how relatively small the FPGA community seems to be. Or to be more precise, how thinnly spread out over various forums. On the other hand, most threads I've been reading consisted of 90% useful information.. so that's really nice, and something rarely seen on online forums. So quality over quantity it seems

You get a lot of passive-agressive illogical comments and arrogance on many non-FPGA forums.  That kind of thinking gets you absolutely nowhere when doing FPGA design.

What exactly is the Rasberry PI movement you're referring to? I've read it on several occasions, but never got caught in any discussion about it yet.

Raspberry Pi is small ARM board with 512MB RAM, USB ports, Ethernet, and HDMI output that is capable of running GNU/Linux and other operating systems for US$35 (with a non-Ethernet version coming soon for US$25).  Since it's so much cheaper than anything comparable, it's captured the imagination of a large community and they've sold approx 500K of these little boards.  The main RasPi purpose is to be a platform for learning programming so students can play without worrying about killing the family PC.  I think it has potential to be a good platform for teaching logic design.

As for the vendor's and their software.. I haven't even started with FPGAs yet and I'm already annoyed by their shenanigans. Almost every thread talks about Altera vs Xilinx sooner or later, so you would expect their software to be shining examples of amazing usability in order to boot out the competition. But.. well. Not so much. Yesterday I found out about Xilinx' WebTalk "feature" and that's just an absolute show stopper for me. Who thought this was a good idea and approved it? And who thought it would then be a good idea to praise this abomination as a dandy feature the user should be thankful for?! Urgh.

WebTalk hasn't bothered me.  But then I generate my FPGA bitstreams on a Windows machine that's not connected to the Internet, because Windows always freezes up if I do.  I understand the purpose of WebTalk: one of the hardest things in designing an FPGA architecture is deciding how much chip area to use for various functions and how much for routing.  If you don't provide enough routing resources, you can't get good utlization of logic elements.  If you put in more multipliers or block RAMs than people actually use, you're wasting area that could be used for something else or for making the chip smaller and cheaper.  WebTalk compiles statistics of how customers actually utilize chips.  Xilinx may find the results useful, but I suspect that they'll discover that designs are diverse and it's hard to get any generally-useful result.  But that would be a result as well.

@Christian,

There is a univeral law (and it really is universal) sometimes referred to as the first law of Thermodynamics but better expressed as " There is no such thing as a free lunch". Xilinx are quite upfront about it - if you pay a bit for the software you can disable WebTalk - if you use the "free" version on of their paybacks is they get to collect data.

If you don't like X and A then look at the smaller players' offerings - I use Lattice parts because I find the company much easier to deal with than X or A. You could also try MicroSemi (used to be Actel). Lattice have some simple FPGAs on very cheap and simple boards.

Still no free lunches but a selection of different ways to pay !

Michael Kellett

Well John and Michael...

I'd be happy to "pay a bit for software" But the only thing I can find on their site is like $2500.   So forget that.

Also, when John Beetem says "on a Windows machine that's not connected to the Internet, because Windows always freezes up if I do."

Did it cross anyone's mind that it is actually WebTalk causing that?

Also, John says they will learn that "it's hard to get any generally-useful result".  Well, two and a half years later, they are still doubling down on it.

I am trying to teach myself FPGA, as my digital class skipped over it.  So I am willing to buy the components necessary to do so, and about to spend a couple hundred for a demo board.  I was thinking of Diligents Nexys 4 until I saw the ISE statement about WebTalk.  Now I am looking at Quartus from Spartan, and getting a Cyclone III.

Unless some on this (rather old) thread can suggest another alternative that does install spyware.

-CSW

Christopher Weber wrote:

 

I am trying to teach myself FPGA, as my digital class skipped over it.  So I am willing to buy the components necessary to do so, and about to spend a couple hundred for a demo board.  I was thinking of Diligents Nexys 4 until I saw the ISE statement about WebTalk.  Now I am looking at Quartus from SpartanAltera, and getting a Cyclone III.

I'm glad to see interest in FPGAs.  For some more recent discussion of FPGAs -- particularly on the topic of getting started with FPGAs -- you might be interested in this more recent discussion: http://www.element14.com/community/thread/38256/l/barriers-to-designing-with-fpgas

There are terrific boards out there for learning about FPGAs, many below US$100 and some below US$50.  Here's part of a comment (with a couple updates) I made in the "barriers" thread:

While FPGA development boards from FPGA vendors are often multi-hundred dollars, there are plenty of independent boards under US$100, such as the LOGI-Bone and LOGI-Pi.

 

For lots of cheap FPGA development boards, take a look at Joel Williams’ list.

 

The cheapest FPGA boards I’d consider include the $38 Papilio One with Xilinx Spartan-3E 250K and the $30 Gameduino (or similar Olimex MOD-VGA) with Spartan-3A 200K.

Regarding ISE WebPack sending information back to Xilinx, my understanding is that what they send is innocuous and is only statistical information for improving the logic versus routing tradeoff in future FPGAs.  Anyway, I'm not worried.  As I said, I never hook up Windows machines to the Internet.  I really doubt it's Xilinx software that's screwing things up.  My experience with Windows is that it has so many malware attack vectors that there's no way to tell which one is killing you.  Or maybe it's just excessive virus scanning activity that's killing me.  In any case, leaving Windows off the Internet works for me.

I've had excellent results using Xilinx ISE WebPack 12.4 on Ubuntu 11.10 and 12.04.  There are a few things that don't work for me (e.g., the graphical FPGA Editor) but everything I really need works.  One of the GNU/Linux bugs is that I don't think WebPack is able to sends anything back to Xilinx.  See, not all bugs are bad -- this is a ladybug

I'm still firmly in the Xilinx camp, since I've designed so much with it.  Plus, Xilinx Spartan-3E and -6 are the only FPGAs that work with my Flavia software which implements small FPGA/CPLD designs exclusively using free-as-in-freedom software: http://www.element14.com/community/groups/fpga-group/blog/2014/07/21/flavia-the-free-logic-array

John,

Thanks for your reply.  Well, I already downloaded ISE 14.7...  I can always whip up another isolated PC to work with.  Or use a virtual machine.

If I do that, I will get the Nexys 4.  It's easily in my range $.

I've been programming PC's professionally in C++ for decades, and am getting an EE degree.  So paying for some learning resources is not out of the question, it's just making sure I get the value out of it, and Diligent has a good academic price.  Sometimes I feel a little guilty about taking advantage of student discounts, but I'm not using anything for my employer.  It's all for my own education.

I will start reading those threads.  But is there a book you can recommend to start with?  I'm used a lot of digital building blocks (the old 74xxx chips), and done digital logic and boolean equations.  Then I also bought myself a Wellon programmer to play with Atmel PLDs.  I know FPGAs have a clock, and are substantially more complex...  But that is where I am starting from.

-CSW

Scott Weber wrote:

 

I will start reading those threads.  But is there a book you can recommend to start with?  I'm used a lot of digital building blocks (the old 74xxx chips), and done digital logic and boolean equations.  Then I also bought myself a Wellon programmer to play with Atmel PLDs.  I know FPGAs have a clock, and are substantially more complex...  But that is where I am starting from.

I don't have a recent book recommendation.  I did a Google search for "FPGA book recommendations verilog" and the results look promising.  Or substitute VHDL.

People usually design FPGAs using VHDL or Verilog.  There's a lot of discussion as to which language is better, and more discussions about which language is worse.  I prefer Verilog, because its syntax is based on C and thus produces concise source code.  VHDL is based on Ada and produces wordy source code.  Chacun a son goût.  As a C programmer, you'll probably find Verilog more familiar.  I've heard one professional FPGA manager say that he prefers is designers use VHDL to emphasize that there's a big difference between writing FPGA source code versus CPU source code.  He found that C programmers would write Verilog as if they were writing a CPU program  and end up with bad FPGA designs.  I see his point, but disagree with him.

That said, VHDL and Verilog were both created as simulation languages, with synthesis added as an afterthought.  This means it's really easy to write VHDL or Verilog that simulates fine, but cannot be synthesized or produces a poor result.  I find this a defect of both languages.  I talk about this in an element14 discussion, with suggestions on how to write Verilog for synthesis: http://www.element14.com/community/thread/37205/l/connecting-verilog-to-synthesis

John, Okay...  Thanks for your help so far.  I have heard more about VHDL, and since I code in C++, I don't want something too similar, that would attempt to cross over, where it really shouldn't.  So, no Verilog for me...

Now this:

I started looking that this tutorial:

http://hep.uchicago.edu/~tangjian/SVT_sub/FTK_ATLAS/AUX/vhdl-tutorial.pdf

I'm good with much if it up to page 9, but there is an inconsistency.

On page 5, is shows a code that starts with :

     architecture behav of reg4 is

But then I page 7, it shows a code segment that has:

     architecture basic of d_ff is

So, they are defining the architecture of the component, or element, or whatever it's called...  (I guess "entity" is the correct term)

But what is the difference between architecture described as "behavior" and architecture described as "basic".  The tutorial text doesn't explain the difference, nor define why it is being used differently.

--------

Next, I set up a virtual machine, loaded XP, and installed the 6gig ISE.  How wonderful that it crashed my IE ver 6 right out of the box, preventing me from even getting a license file...  I managed to get around that.

I created some simple entity using their template, and filled it in with 4 inputs, 7 outputs, making a simple hex to 7-seg digit decoder.  Not a problem. Nice simple intro type of project.  Errors and warnings all cleaned up.

But then is launched some "PlanAhead" thing, I have no idea what it is, or why.  But it froze up for several minutes, grinding everything to a halt.  At some point, and had a graphic of what appeared to be a BGA pinout map, maybe for assigning pins, because they were not defined in the original template file generated by the ISE wizard.  But I couldn't figure out what was going on.  Nor have I figured out how to specify pins in the code, in the first place.  Doing it in the PLD was rather straight forward.

I would also expect to see a menu option to "Program Device".  like I see in MicroChip IDE, or in WinCupl from Atmel.  But I can't find anything like that.  Maybe because I don't have a device yet.

Then it is doing all kinds of things I can't understand. Like when I double clicked on "Edit Constraints" in the left side.  because I want to see what is in it.  A popup appears telling me it's "Another editor is already editing..."   Really?  Where?

What I really want is the basics...  Like WinCupl which lets me write PLD code, and send it to the device.  To begin with, just writing an notepad, and having a "compiler" generate code, which a tool could send to the FPGA would be nice as a starter tutorial.  So I don't have to deal with half a dozen other tools that are doing things which I don't know what yet.  Like PlanAhead, IMpact, SmartXplorer...

So that is where I am, feeling rather lost.  Any advice?

-Scott

I'm going to offer some advice which has worked well for me over the last 10 years.

Forget Xilinx and Altera and download the Lattice toolset - not as capable as the full Xilinx kit but good enough to take you along way and much simpler. I use the paid for version every day (along with other tools) and it works very well for me.

Xilinx make some wonderful big FPGAs but Lattice have a better offering (IMO) at the low end.

There is nothing as simple as WinCupl for FPGA.

There are cheap Lattice dev kits for the Ice, Mach and XP2 ranges - pick the part you might want to play with and download the right tools - (ask if not obvious which)  - then get the tools working and only bother to buy the kit if you're happy.

MK