Choices of Design and Build one FPGA Project on Digilent Arty-S7

I have an opinion :

When learning FPGA, I'd advise not to go the MicroBlaze and C++ way. Because then you're back in the microcontroller world and the usual programming languages. You don't learn the digital design.

I think the soft controllers are definitely valuable in an FPGA project. But if the first step you do when learning FPGA is to select a MicroBlaze, then switch to Vitis to build C programs, you're not advancing FPGA skills.

end of opinion .

I have to agree.  For my first project I specifically went the way you suggested when in fact it was calling out for a microprocessor based approach.  It’s a lot more interesting as well.

My opinion is that you should learn the advantages and disadvantages of each approach (implementing in programmable logic versus handling via soft processor). Both are different tools and will be useful in different scenarious. In many cases, a combination of the two will be the best way to go. Some things are just better handled in Programmable logic (e.g. a Quadrature decoder), while for some other things it just makes more sense to use the soft processor (talking to I2C sensors, etc). Learn both and mix n match! 

Now that I have an Arty board too, I'll have to follow my own opinion. Not my strongest point . 

The simple answer is perhaps to pick the right tool for the task in hand.

However it can be hard to know which is the right tool unless you are familiar with each of them.

Productivity, time and cost may influence the decision.

I've been reading a book on digital design recently and after consuming the first 150 pages or so of old school 'bare metal' logic design, the author decides to allude to the reader that it is impractical to keep on designing like this as the scale of integration continues to increase. It is a lot more productive to use computer aided design to optimise the design for you using HDL. So, next chapter it is introduction to VHDL or Verilog (depending on which version of the book you bought). It wouldn't surprise me if by the end of the book the author informs the reader that it is impractical to keep on designing like this and it is more productive to synthesise directly from C++ using modern design tools (and to buy his next book... )

As with most technologies these days various layers of abstraction likely come into play. I suspect most people are only ever working mainly within a small window on the technology timeline, with most of the detail having being abstracted away in order to increase productivity and to maintain portability across platforms.

The MicroBlaze soft processor is an interesting one. I could potentially see a learning path which makes this a very useful entry point to FPGA for those already familiar and comfortable working with microcontrollers. You start with the soft processor, you then customise the soft processor to fit your needs. You then create a custom peripheral to use along with the soft processor. Finally you create multiple soft processors and multiple peripherals to create a highly customised microcontroller design all on the one FPGA device.

There are some downsides:

 Useable resourse in an FPGA costs a great deal more than it does in application level silicon designs like micro processors and micro-controllers. (That's why Xilinx have Xynq with embedded ARM processors).

The tools to develop with standard processor designs have had the benefit of thousands of man years of development.

Simple FPGA processors like MicroBlaze are quite cheap although the single source toolset leaves a lot to be desired. As soon as you customise the processor (in any significant way)  you are on your own.

Custom peripherals are all very well but the FPGA silicon environment isn't a good place for RF, analogue or power control - if logic will do all you need that's fine.

Multiple custom processors and peripherals in a single design will get very difficult (and expensive) to develop.

I've usually found the best place to be is with a standard micro-controller and a seperate FPGA. The design complexity is much easier to manage and control. The interface between the two can be a problem, I wish there  were application processors with a good fast board level interface to FPGA type devices.

MK

"I've usually found the best place to be is with a standard micro-controller and a seperate FPGA."

Why not use something like Zynq-7000 series device? You get one or more processors and programmable fabric. Since everything is inside the chip, interfacing between the two is without any external connections.

Cost, power,  complexity ! (The comment was made in a thread based on Spartan6/7 which are being pushed as "entry level" FPGAs)

The ARM cores in the Zynq are application processors not micro controllers.

They can go fast and are usually used with a big operating system and external dynamic RAM. They don't have on chip flash.

If you follow the questions on forums you'll see that a very large proportion of the questions asked about Zynq are actually Linux issues.

You can use the ARM cores on the Zynq running bare metal but you are swimming upstream - it's not waht most people are doing and its not where the support is.

All Zynqs come in BGA packages, if you use DDRAM that will too, and your pcb design gets complex (time matched and impedance controlled traces).

I've designed lots of different micro plus smallish FPGA type systems on 4 layer 0.15mm design rule pcbs with no need for impedance control and using QFN or TQFP packages.

Usually less than 64k bytes of code in the uP.

Contrast with a typical Linux  !

I'm not suggesting that sometimes a Zynq isn't just what you need, just that when you need to format shift the audio data from a high end camera and send it out over an AES interface (a real paid for example) then what you need is £2 uP and a £4 FPGA.

MK

(steering away from Spartan, over to Zynq land. a bit of a side story:)

The new Tektronix 2 Series oscilloscope uses a Zynq UltraScale. In this interview with EEVblog, Andy Tedd briefly discusses what runs in the fabric and what on the application processors.

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RE: Choices of Design and Build one FPGA Project on Digilent Arty-S7

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It's insight that's not always out there, but no surprises on what goes where..

not affiliated.

Thanks for your excellent, detailed reply. You made some really good points there!

Cheers,
Saad