RISC-V Support in FPGA

I'm also sure that ARM has a crap ton of patents

That is true - but it is rather important that the parts that are processor specific are limited in scope.

Of course, the rest of the code (at least, the C or C++ code) needs to be checked and tested - there may be accidental processor specific code such as alignment errors that happen to work fine on one processor but cause bus errors on another one.

Changing architectures is not a small job, but it is not /that/ bad. In the Linux world, the great majority of code works fine on a range of

32-bit and 64-bit targets, big-endian and little-endian, with little more than a re-compile with the right gcc version (perhaps with a ./configure first). And of course, code in Java, Javascript, Python, Lua, HTML5, etc., is all independent from from the target cpu anyway.

MIPS certainly still exists, and is used in a fair number of devices. But MIPS make cores that are at least as good as ARM cores in many areas, with cores that would be ideal on microcontrollers. But the only off-the-shelf microcontrollers you can get with MIPS cores are the PIC32

- a device which was launched long before it was working, had intentionally crippled development tools, and a name designed to invoke terror in any software developer. IMHO, that greatly reduced MIPS chances of being a significant player in the microcontroller market, and I find that a great shame.

The ISA is royalty free, but as far as I know there is nothing to stop you charging for a particular implementation (either as HDL source code, pre-generated macros, silicon, or whatever).

If there's a value to the royalty-freeness it'll be in the wide usage, and the fact that manufacturers won't have to screw around with the legal issues.

If it ever came to it that ARM was losing out to RISC-V implementations, I could see them taking their considerable ARM-spertise and applying it to RISC-V-compatible cores.

--

Tim Wescott 
Wescott Design Services 
http://www.wescottdesign.com 

I'm looking for work -- see my website!

A primary motivation was to teach SystemVerilog to undergrads - rather than teach them lowest-common-denominator Verilog that's universally accepted by tools but is pretty tedious as a learning environment.

We tested it pretty extensively with Modelsim and Intel FPGA tools; we didn't have enough summer to put it through Xilinx or ASIC tools but happy to fix things if there's any issues.

Theo

I don't doubt that for a minute. It would be a business driven decision and both the cost and the benefit would be considered.

The issue is not how it would "affect business" since there should only be benefits business-wise, otherwise why switch? The issue is the cost of switching.

Yep.

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Rick C

y

At first glance I thought I'd seen some object-oriented stuff in there but it was just structs. I actually used a lot of SystemVerilog a few years ag o when I was only using Synplify, but now I write cores that have to work i n a broad range of synthesizers which sadly don't even accept many Verilog-

2005 constructs.

I wonder what is behind that. Much of VHDL-2008 is supported in most tools, at least all the good stuff. I believe the Xilinx tools don't include 2008, but I haven't tried it. Otherwise I'm told the third party vendors support it and the Lattice tools I've used do a nice job of it.

I can't understand a vendor being so behind the times.

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Rick C

Rick - yeah, it's pathetic. The synthesizable subset of SystemVerilog was actually fairly concretely defined in the SystemVerilog 3.1 draft, in 2005. We're just now - 12 years later really finding an acceptable solution for FPGA designs. To repeat myself - It's really pathetic.

Vivado seems to actually have BETTER language support for SystemVerilog than Synplify - believe it or not. But this only works so far until you hit some sort of corner case and the tool spits out a netlist which doesn't match the RTL. (We've hit too many of those issues in the past 2-3 years).

Synplify, on the other hand barfs on perfectly acceptable, synthesizable code (i.e. SystemVerilog features that already have parallels in VHDL). But Synplify has never (for us) produced a netlist which doesn't match RTL...

Regards,

Mark

Am I hearing a justification for staying with VHDL rather than learning Verilog as I've been intending for some time? My understanding is that to write test benches like what VHDL can do it is useful to have SystemVerilog. Or is this idea overblown?

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Rick C

Rick - I was speaking of Synthesizer support within FPGA tools only.

Simulation support depends entirely on your vendor, and is an entirely different beast. We've been happy with Modelsim for all our SystemVerilog simulations - for many years. Can't comment much on other simulation vendors, and their support. I've not used VCS, or NCSIM (or whatever they're now called) in many years. Never tried Xilinx "free" simulators, but for "free" I'd expect you'd get what you pay for.

I'll not wade any deeper into language wars - use what you're most comfortable with. Doesn't hurt to have experience with both.

Regards,

Mark

s

2. > We're just now - 12 years later really finding an acceptable solution for

In my case, I mostly write for Vivado, but I have to write code which will also work for some ASIC synthesis tools which don't like anything too moder n. I'm not sure why; I just know I have to keep to a low common denominato r.

Anyway, and this is a different topic altogether, I've reverted to writing very low-level code for Vivado. I've given up the dream of parameterizable HDL. I do a lot of Galois Field arithmetic and I put all my parameterizat ion in Matlab and generate Verilog include files (mostly long parameters) f rom that. The Verilog then looks about as understandable as assembly and I hate doing it but I have to. It's the same thing I was doing over ten yea rs ago with Perl but now do with Matlab. Often Vivado will synthesize the high-level version with functions and nested loops, but it is an order of m agnitude slower (synthesis time) than the very low-level version. And some times it doesn't synthesize how I like. I've just given up on high-level s ynthesizable code.

(continuing a bit OT...)

Kevin,

That's unfortunate. We've been very successful with writing parameterizable code - even before SystemVerilog. Heck even before Verilog-2001. Things like N-Tap FIRs, Two-D FIRs. FFTs, Video Blenders, etc... All with configurable settings - bit widths, rounding/truncation options/etc.. I think in a previous job I had a parametizable Galois Field Multiplier too.

I'm not sure what trouble you had with the tools. It takes a bit more up front work, but pays off quite a bit in the end. We really had no choice, given the number of FPGAs we do, along with how many engineers support them. Lot's of shared code was the only way to go.

If you've got something you like, then I suggest keeping it. But for others, I think writing parameterizable HDL isn't too much trouble - and is made even easier with SystemVerilog. And higher level too.

Regards,

Mark

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I've just been burned too many times. I know better now. The last time I made the mistake I was just making a simple PN generator (LFSR). The only complication was that it was highly parallel--I think I had to generate may be 512 bits per cycle, so it ends up being a big matrix multiplication over GF(2). First I made the high-level version where you could set a paramete rs for the width and taps and so on. It took forever for Vivado to crank o n it. This is just a few lines of code, mind you, and is just a bunch of X ORs. Then I had Matlab generate an include file with the matrix packed int o a long parameter which essentially sets up XOR taps. That was, I think, ~20x faster, which translated into hours of synthesis time. The synthesize d circuit was also better for various reasons. This is just one example. I also still have to instantiate primitives frequently for various reasons. The level of abstraction doesn't seem like it's changed much in 15 years if you really need performance. This doesn't really have anything to do wi th the SystemVerilog constructs. I'm just talking about high-level code in general. If I were allowed, I would still use modports, structs, enums, e tc.

Ah, we did find something similar in Vivado. For use is was a large parallel CRC - which is pretty much functionally identical to your LFSR (big XOR trees).

We had code that calculated, basically a shift table to calculate the CRC of a long word. The RTL code worked fine for ISE. But when we hit Vivado, it'd pause 10 minutes or so over each instance (we had lots) which significantly hit our build times.

So, I changed this code to "almost" self-modifying code. The code would by default calculate the shift matrix using our "normal" RTL, which looked something like: assign H_n_o = h_pow_n( H_zero, NUM_ZEROS_MINUS_ONE ); where H_zero was an "matrix" of constants, and NUM_ZEROS_MINUS_ONE a static parameter. The end result is a matrix of constants as well, but "dynamically" calculated. (Here "dynamically" means once at elaboration time, since all inputs to the function were static).

Then we just added code to dump each unknown table entry sort-of like: if( ( POLY_WIDTH == 8 ) && ( NUM_ZEROS_MINUS_ONE == 7 ) && ( POLYNOMIAL == 'h2f ) ) assign H_n_o = 'hd4eaf52e175ffba9; ... else // no table entry - use default RTL calc assign H_n_o = h_pow_n( H_zero, NUM_ZEROS_MINUS_ONE );

We "closed" the loop by hand. If the "table" entry didn't exist, the tool would use the RTL definition, and spit out the pre-calculated entry. All done in verilog. We insert that new table entry into our source code by hand, and continue - next time the build would be quicker.

This *workaround* was a bit kludge, but was the rare (only really) exception for us in our parameterized code. Normally the tools just handled things fine. And again to be clear the only thing we were working around was long synthesis times. The quality of results was fine in either case.

Maybe the code you were creating the pendulum swings the other way and it was more the norm, rather than the exception to see things like this.

Interesting topic, I'm glad to hear of your (and others) experiences.

Regards,

Mark

I use Vivado to do GF multiplications that wide using purely behavioural VHDL. BTW, A straightforward behavioural implementation will *not* give good results with a wide bus. I believe the problem is that most tools (in particular Vivado) do a poor job of synthesising xor trees with a massive fanin (e.g. >> 100 bits). The optimisers have a poor complexity (I guess at least O(N^2), but it might be exponential) wrt the size of the function.

You can use all sorts of mathematical tricks to make it work without need to go "low level". For example, to deal with large fanin, partition your 512 bit input into N slices of 512/N bits each. Use N multipliers, one for each slice, put a keep (or equivalent) attribute on the outputs, then xor the outputs together. This gives the same result, uses about the same number of LUTs, but gives the optimiser in the tool a chance to do a good job.

I use the same GF multiplier code in ISE and Quartus, too (but not on buses that wide).

The entire flow is in VHDL and works in any LRM-compliant tool. It's parameterised, too, so I don't need to rewrite for a different bus width.

I've been using similar approaches in VHDL since the turn of the century and have never been burned.

YMMV.

Regards, Allan

Hi all,

As a follow-up in the RISC-V thread.

This was on eenews Europe today:

As a small follow-up question: Does anybody have any idea how to get the hifive boards in Europe?

For the last thing I ordered in the US (a pandaboard), I had to pay VAT (ok, that's normal), but also a handling-fee for the shipping-company and the customs-service to get the thing shipped in. In the end, these additional costs where more then the VAT itself.

Cheerio! Kr. Bonne.

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I looked up my notes for the LFSR I was referring to and one instance of th e more-abstract version took 16 min to synthesize and the less-abstract ver sion took less than a minute. (And we needed many instances.) When I try to do something at a higher level it ends up like your experience: I have to do a lot of experiments to see what works and then tweak things endlessl y. It eats up a lot of time.

,

I used to do big GF matrix multiplications in which you could set parameter s for the field size and field generator poly, etc. Vivado just gets bogge d down. Now I just expand that into a GF(2) matrix in Matlab and dump it t o a parameter and all Vivado has to know how to do is XOR.

I also have problems with the wide XORs. Multiplication by a big GF(2) mat rix means a wide XOR for each column. Vivado tries to share LUTs with comm on subexpressions across the columns. Too much sharing. That sounds like a good thing, but it's not smart enough to know how much it's impacting tim ing. You save LUTs, but you end up with a routing mess and too many levels of logic and you don't come close to meeting timing at all. So then I hav e to make a generate loop and put subsections of the matrix in separate mod ules and use directives to prevent optimizing across boundaries. (KEEPs do n't work.) It's all a pain. But then I end up with something a little big ger but which meets timing.

I really wish there were a way to use the carry chains for wide XORs.