Hello guys, now I read almost 200 pages of the book "The Designers Guide to VHDL", by Peter Ashenden. The thing is that everyone has been recommending the book, and I want to improve my skills in VHDL. I'm actually stunned... the book is CRAP by all means. Before reading that book, I read the book "Circuit Design with VHDL 2004" by Pedroni. The problem is that I found that Ashenden's book like lives in the dream world of simulation. It talks along the whole book about the command "after" within its examples, which is NEVER synthesisable (and many other non-synthesisable commands with total verbosity), and in the mean time speculates about the latency in hardware which is a "serious issue" as the book says when talking about delta delays, while it's never realistic nor practical, because most of the commands it uses are not synthesisable. On the other hand, a book like Circuit Design with VHDL, teaches all the commands, and mentions beside each command whether a command is sythesisable, and so, with such a book (which is btw only 315 pages without appendices), I could learn VHDL in less than 2 months and made many simple circuits and implemented them on my Spartan 3AN Starter Kit. And I'm now gonna read the new version of that book "Circuit Design and Simulation with VHDL 2010", which has more practical examples and which will make me dive deeper in VHDL with no non-synthesisable wonders. Now what I don't understand, why does everyone recommend that huge book that talks about wonders that cannot be synthesised in hardware, and that forces one to read more than 500 pages before even starting to talk about the synthesis process. Did I go with VHDL on the wrong foot? Am I on the wrong path? is simulation really THAT useful? Is being brief or verbose better nowadays? what would you guys say against these two books and against my judgement? could you tell me some of your experiences? please advise! I'm really getting lost and don't know what book to take anymore. I need to learn more VHDL. I'm, btw, not an engineer. I'm a PhD student physicist who needs to use FPGA's for an experiment. Thank you for any efforts :-)
> is simulation really THAT useful? Yes. At first my designs have to run in simulator. At second I can test it on real hardware. To check what's goning wrong is much easyer to use the simulator, than a logic analyzer... Duke
> is simulation really THAT useful? Have a look back to ancient times with "only" ASICs: you didn't have to possibility to "try out wether its working or not." When you start a ASIC you want to be and you must be sure that its almost finished (and hopefully only some minor problems may occur). The design (and all of its components around) has to be simulated thoroughly. And then you say: Yes, VHDL is simulation and simulation is VHDL. > is simulation really THAT useful? And alos for FPGA Designs the only answer to this is: Yes. The simulator is for a FPGA designer the same thing like the debugger for the software programmer. And: do you expect to get a litle bit complex piece of software running without a debugger? > is simulation really THAT useful? First book of Pedroni was "Circuit Design with VHDL" Now comes "Circuit Design and Simulation with VHDL" Think about that... > On the other hand, a book like Circuit Design with VHDL ... > mentions beside each command whether a command is sythesisable You can NEVER say: this exact VHDL statement isn't synthesizeable! The synthezizer may not be capable to handle a description today, but with the next software realease the synthesizer can. And so you will ALWAYS have to look in the synthesizer handbook/guide- line to know what statements a synthesizer is able to cope with. > I need to learn more VHDL. No. You want to learn more FPGA. If you want to learn VHDL then take Ashenden. But only 5% of VHDL (at maximum) is synthesizeable on hardware (ASIC/FPGA). The "wait for" statement is the most simple statement not synthesizeable (yet). There are thousands of other constructs you can use very efficient in a simulation. Just take a lok at the attributes 'stable or 'last_event or 'delayed... > "The Designers Guide to VHDL" > why does everyone recommend that huge book > that talks about wonders that cannot be synthesised in hardware Keep in mind: the name of the book isn't "Designers Guide to FPGA". Don't throw it away. You will be happy to own it in 2 years...
Thank you for your replies, guys. The problem with simulation is that not everything can be simulated. For example, a few weeks ago I coded an SPI for an ADC (LTC1407) with an analog amplifier with digital interface (LTC6912). How would I ever simulate such components? these components need some kind of a special interface for simulation that's provided by the manufacturing company, so that when I write a code for it, I would see its response on my simulator, right? such a thing doesn't exist at all. At least as far as I've seen (does every component has a simulation interface for testing? I doubt it!). Which makes simulations only useful if I'm building code from scratch. If I'm using any external components (which is almost always the case) like ADCs, DACs, amplifiers, network interfaces, ... etc. Then simulation is totally useless. Right? Fair enough, Lothar. Your arguments make a lot sense. So you're saying, that what I need for the mean time is learn more about FPGAs. This sounds fair and talks to my complaint that Ashenden's book is very verbose in simulation, or respectively VHDL. So how do I proceed? now I know almost all the basic commands in VHDL. So what should I read/do? Is there a book that you recommend to learn about FPGAs conceptually? or should I just take the manual of XST and use/read it in some way? please advise!
> If I'm using any external components (which is almost > always the case) like ADCs, DACs, amplifiers, network > interfaces, ... etc. Then simulation is totally > useless. Right? No! You could write some behavioral modules and connect them within your testbench to the design. When creating such functional models you could use not-synthesizable VHDL statements as described in Ashendens book :o)
Thank you for your answer. berndl wrote: >> If I'm using any external components (which is almost >> always the case) like ADCs, DACs, amplifiers, network >> interfaces, ... etc. Then simulation is totally >> useless. Right? > > No! You could write some behavioral modules and connect them within your > testbench to the design. > When creating such functional models you could use not-synthesizable > VHDL statements as described in Ashendens book :o) But this is somewhat a circular problem. If I already know how the devices operate exactly, why would I want to simulate them in the first place? It took me almost two weeks to know how to communicate correctly to that ADC with its pre-amplifier without losing any bits. How would a simulation help in such a thing?
> It took me almost two weeks to know how to communicate correctly > to that ADC with its pre-amplifier without losing any bits. > How would a simulation help in such a thing? First, simple testbench: You can easily compare your timing diagrams with the ones required in the ADC data sheet. That is manual work with eyes and pencil. Second, more sophisticated testbench: Let the testbench check the timings you can read in the data sheet of the ADC. Third, final testbench: Read the input for the ADC from a file or generate it by random. Check the timing on the interface to the ADC. And finally compare the converted value with the generated ADC input value. > So you're saying, that what I need for the mean time is learn more about > FPGAs. This sounds fair and talks to my complaint that Ashenden's book is > very verbose in simulation, or respectively VHDL. So how do I proceed? Take some easy excercises and play with them. Look at the RTL schematics generated out of your code. See what happens, when you change a minor part of your design. Understand why that happend. > now I know almost all the basic commands in VHDL. So what should I do? Start with a flashing light http://www.lothar-miller.de/s9y/archives/80-Hello-World!.html Switch over to a chasing light http://www.lothar-miller.de/s9y/archives/61-Lauflicht.html And then go over to more complex designs as serial port, vga diaplay.
Thank you for your reply. So I have to go to practical training. Nothing really to be read. Thank you again :-)
Imagine that you have a really huge design with lots of interfaces and FSMs that depend on each other and you need to control some machine with it. Without simulation models every change will require you to dive deep into the design and check that all circuits still work as intended. If you have simulation models that do some abstraction you only need to run the simulator, see whether the interfaces behave correctly (e.g. a ramp input to the component that feeds that DAC will lead to a ramp output) and that's it. Without simulation you'll just die from the complexity. On the other hand, thing that work in my simulation typically also work in hardware.
I also bought that book wishing to learn VHDL, wasted money. I bought another book but about Verilog instead and I have been making quite a bit of progress. I decided again to give "The designer's..." a try... there is something wrong with that book. It is not that simulation is not important is the approach to the whole that I find not very useful. I haven't used any delays in my simulations and I'd like to include them because well hardware has them... the problem is how to do that (And I do not mean the syntax!) but how to realistically estimate how much time every component needs. Any tips on how to approach this problem ? Thanks!
> I haven't used any delays in my simulations and I'd like to include them > because well hardware has them... What hardware? Your FPGA? The external components? Its not your task as developer to calcutlate the FPGAs internal timing! > the problem is how to do that (And I do not mean the syntax!) but how > to realistically estimate how much time every component needs. For timings you read the data sheet of your (external) components. For the internal delays inside the FPGA the usual way ist not to calculate whatsoever delays, but to put constraints on your design. So you tell the toolchain, what timing you want. Thats the completely other way! Rather than saying "let me see what the resulting delays are", you say "i want the tools to reach my specific timing". And this you say with timing constraints. Its like pin assignment constraints: you tell the toolchain where to put the IO signals. Timing constraints are similar for Verilog and VHDL.
Just quickly writing about a book: My absolute favorite is "RTL Hardware Design USING VHDL" by Pong P.Chu. I simply love it and it is never trying to teach you what you don't know but always what you should know.
He (Pong P. Chu) some other books like FPGA prototyping by Verilog/VHDL examples. I have been considering one of those, too.
> My absolute favorite is "RTL Hardware Design USING VHDL" by Pong P.Chu. A book having one complete chapter about clocking and synchronizing and at least 10 pages about metastabiliy, but using the asynchronous reset scheme throughout, having only 4 lines about the second most critical asynchronous external signal and stating a simple and uncritical behaviour to it, is only halfway finished... :-o Just to say it simple: Every asynchronous external signal must be synchronized. The reset is a asynchronous external signal. If a part of a design doesn't need a reset, then don't implement it. (For Xilinx users: read the whitepaper WP272) Ale wrote: > He (Pong P. Chu) some other books like FPGA prototyping by Verilog/VHDL > examples. Theres also one book specially for the Spartan 3 and even in this the Xilinx specific considerations with sync and async reset is completely ignored...
I'd still say Chu wrote a great introductory book. I tried to approach the topic with some others and i got nothing but frustrated. With this book and also the other one which is in particular written for the Spartan 3, I was able to understand and work right away. Might be a question of taste and also super FPGa professionals might have a completely different view than less experienced guys.
> Might be a question of taste To this: Yes, of course. I don't like the 3-process-fsm style... > and also super FPGa professionals might > have a completely different view than less experienced guys. To this: No. In a book specially for Spartan 3 there MUST be at least a hint on the specific behaviour of Xilinx flipflops! When you use the async reset, the FFs will be set in the async mode, and therefore they cannot be set/reset synchronous afterwards. Heres a German topic on that: Beitrag "Re: Hardware mit VHDL "richtig" beschreiben." Maybe Babelfish or Google translator can get it right... ;-)
(Not a criticism but a note): Where you use the unsynthesizable "wait" :(
> Where you use the unsynthesizable "wait" In processes without sensitivity list. See attachment from the XST User Guide. Duke
> Where you use the unsynthesizable "wait" Of course wait is synthesizeable. Keep in mind: a process with a sensitivity list is a special case of a generic process. These two simple samples are similar:
process (a,b,c) begin
wait on a,b,c;
Duke Scarring wrote: > See attachment from the XST User Guide. And even this isn't correct: XST allows several wait statements and the wait also don't need to bee the first statement in the process... :-o http://www.lothar-miller.de/s9y/archives/47-wait-im-Prozess.html
Above you say that "wait for" can't be synthesized and then you say it can... (I only know Verilog!), are there then two "wait xx" statements ? I see that a wait on would be equivalent to a sensitivity list, what is the other one for ?
> are there then two "wait xx" statements ? No. There are three conditional waits (on, until, for) and one "wait forever". The only waits that are synthesizable is a wait on and a wait until.
>I'm a PhD student physicist who needs to use >FPGA's for an experiment. Then take a look at 'HDL Chip Design' from Douglas J. Smith. It´s main focus is more on the things you are looking for, I guess.
Dave wrote: >>I'm a PhD student physicist who needs to use >>FPGA's for an experiment. > > Then take a look at 'HDL Chip Design' from Douglas J. Smith. > It´s main focus is more on the things you are looking for, I guess. Thank you. I'm checking it. Hopefully it's available for download :-).
I'd check at the University's library... you may be surprised by the large amount of books available, well at least here in Germany :).
Ale wrote: > I'd check at the University's library... you may be surprised by the > large amount of books available, well at least here in Germany :). Thank you for the suggestion. But I found it already online and downloaded it, and it's on my tablet queued for reading xD. The situation with our library here is fairly complicated. The library is at city center, and I work in the Clinic of the university, which is far from there. I don't like hard books. I like to have my books on my tablet and read them wherever I'm ;-).
I also got a tablet and have the publications on it. I'm glad I do not have to move stacks of paper around!
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