Nexys A7-100T vs Arty A7-100T

[Shivam Potdar]

Hello. I believe similar questions are present on the forum before mine. But I specifically want to know aside from the peripherals difference (RAM, SDCard, VGA etc.), is it ideal to assume the HDL designs (except the peripherals IP) would be compatible with both of them if one is supported by some project. 

For instance, lowRISC has a guide based on Nexys for its core booting Linux (https://www.lowrisc.org/docs/untether-v0.2/fpga-demo) whereas SiFive E310 has been demonstrated on Arty. 

The LiteX project supports both the boards (at least from the code base it looks so). 

Can someone experienced with this please guide me. I'm inclined towards the Nexys version as most of the peripherals I would be interested in are present on board, but I certainly don't want to have limited capabilities for certain projects compared to Arty. 

Thanks 

[zygot]

17 minutes ago, Shivam Potdar said:

is it ideal to assume the HDL designs (except the peripherals IP) would be compatible with both of them

You can assume that HDL designs can be used with little modification on different boards with the same device. Of course external clock modules and interfaces might be different. You can also assume that HDL designs can be used on different boards with FPGA devices from different families from the same vendor ( Artix, Kintex, Spartan 7 etc ) with a bit of IP re-generation and minimal source modification. For boards that have FPGA devices form different vendors HDL designs are a bit less portable as the basic building blocks like clocking, logic and block memory can be substantially different. But still HDL designs won't require tons of effort to port.

A bigger question is how you intend to use your board. Your question suggests that you want to recreate recipes for existing designs. If you are competent at VHDL and Verilog then you shouldn't have too many issues handling the inevitable surprises that will come your way. If you are hoping to avoid getting into the HDL grass then venturing outside of known platforms for your recipes will be difficult. So deciding to stray from the known depends on you confidence in your HDL skills.

I would assume that the RISC projects that you mention have implementations for specific devices and boards with a fixed set of external interfaces like Ethernet, SD card, etc. Normally, I'd direct beginners to FPGA development to the cheapest board until their skills are honed. You are starting with a more specific use, which is to explore implementing RISC type CPUs in logic, so you want a suitable set of interfaces to work with.

I haven't quite answered your question but perhaps I've helped guide the discussion in a positive direction. Digilent boards and support are mainly geared towards the Xilinx board design flow using Xilinx IP and a Xilinx MicroBlaze CPU or ZYNQ ARM AXI bus interfaces. I know that there are a few FPGA platforms designed specifically for specific RISC CPU implementations but my impression is that there isn't a wide selection to choose from. If you want to use your board with a particular soft processor as a computer that's one thing. If you just want to experiment with CPU design elements then any board with an FPGA and large external DDR memory could be fine as long as there's at least one relatively fast PC interface like USB or PCIe to interact with it. But, then you are on your own as far as building the connectivity.

[Shivam Potdar]

Thanks a lot @zygot for the quick and detailed reply. 

 

9 minutes ago, zygot said:

But still HDL designs won't require tons of effort to port.

That makes me understand that since I am talking about two boards with the exact same FPGA, this would not be very difficult with few HDL changes as you mentioned.

 

11 minutes ago, zygot said:

Your question suggests that you want to recreate recipes for existing designs.

In some sense yes. I have worked on simulation of cores, epsecially RISC-V as a GSoC student. I won't call myself an expert but I have worked with Basys2 (+Verilog) and Altera MAX-V (+VHDL) in an internship and university course. But this has been limited to a level of undergraduate digital design course. In those days, neither I had the time to use these boards for exploration or such nor was I much conversant with computer architecture.

16 minutes ago, zygot said:

I would assume that the RISC projects that you mention have implementations for specific devices and boards with a fixed set of external interfaces like Ethernet, SD card, etc.

In some cases yes, but they are flexible with respect to modifying those according to your needs. For example I can have open source DRAM controller but SD Card IP and so on.

17 minutes ago, zygot said:

I'd direct beginners to FPGA development to the cheapest board until their skills are honed

I am looking at this range (~$200) since I have worked with smaller ones and I am expecting to use this board for a long time in projects, open source contributions etc.

 

18 minutes ago, zygot said:

. I know that there are a few FPGA platforms designed specifically for specific RISC CPU implementations but my impression is that there isn't a wide selection to choose from.

Yeah I looked at Zynq boards in the same range but they offer a fairly small number of logic cells (around about half of Artix 7T), and at least at the moment I do not have any applications with ARM in mind. Even if I have to, I have an RPi 3B and I can consider interfacing Artix based FPGA with that. So in that case I believe I am doing the right thing by getting an Artix board. My understanding is although Zynq based boards have higher amount of DRAM (and more peripherals in some cases), these are meant primarily for the hard ARM cores, and the FPGA can be a configurable accelerator in some sense. Please correct me if I am wrong. 

Also I looked at OSHW boards like the ULX3S but since they are still in a nascent stage, I thought its better to go with Xilinx at the moment (SymbiFlow + Project XRay has support for 7 series boards though, using reverse engineered bitstreams if I am not wrong)