What is main differences between if else and case statement in VHDL. Although both look similar and sometime replace each other.but What logic circuit appear after synthesis . When should we go for if else or case statement ?
Assuming an if-statement and a case-statement describes the same behavior, then the resulting circuit is likely to be identical after the synthesis tools done the translation and optimization.
As Paebbels writes in the comment, the details are described for each tool in the relevant synthesis guide, and there are probably tool-dependent cases where the result may differ, but as a general working assumption, then the synthesis tool will get to the same circuit for equivalent if-statements and case-statements.
The critical point is usually to make correct and maintainable VHDL code, and here readability counts, so choose an if-statement or a case-statement depending on what makes the code most straight forward, and don't try to control the resulting circuit through VHDL constructions, unless there is a specific reason that this is required.
Note that in the if-statement early conditions takes priority over later, but in the case-statement all when have equal priority.
Remember that VHDL is parallel programming language and a form of declarative programming see here as opposed to procedural programming like c/c++ and another other sequential language.
This means in essence, you are telling or attempting to describe to the compiler with your code what the behavior should be, and not specifically telling it what to do or what the behavior is like with procedural programming. This might be what prompted you to ask the question.
Now remember however, that the sequencing of the if or case will affect synthesis. With FPGA's nowadays, all combinatorial part of the logic are in the form of Loop up tables which are internally designed as cascaded arrays multiplexers grouped together to form LUTs with input number N commonly 4 See here for more details, and the compiler decides how to configure these arrays of LUTs.
The ordering can affect the number of cascaded multiplexer that the compiler calculates before the output is resolved.
Note that although in theory, it is possible to get the same behaviour for both if and switch. Case is looking at a single variable and deciding cases for each possible outcome while an If statement can be applied to multiple variables at the same time.
So flexibility? I would say goes to If. However with great power comes great responsibility, if is easy it use several signals from everywhere and if not done properly can lead to bad design, ie coupling of too many variable and any change is subject to failure due to too many dependency issues. Case is suitable for state machines but that is also true for procedural languages I suppose.
In addition, if you use too many different signals to act as conditions to your If, it can affect timing. which may mean limitation in your clock frequency, if you are working with high speed and the list goes on. clock skew, need to constrain signals etc.
Related
I'm trying to write the control logic module for a toy processor. It cycles through the fetch/decode/execute states, reads and writes from various bits of memory, and sets a bunch of control signals. It's somewhat large, and as far as I can tell it can't really be subdivided into smaller modules.
I don't want to put the logic for all of the states into one process -- it's hard to read, and the mass of intermediate aliases & signals are a pain when using the simulator.
I tried splitting each state's logic into its own process, but then I had problems with multiple drivers.
I also tried declaring separate procedures for each state's logic in the head of one main process, and had the process just call the correct procedure based on the current state. This worked quite nicely, with modular "functions" and a more readable structure... but each procedure's intermediate signals aren't visible in the simulator (and maybe not accessible to a testbench? I gave up before trying that.). I was using ISim in case that's relevant.
Was I doing something wrong? Is there some trick I can use to avoid having one massive monolithic process?
EDIT: code for the module is here.
It could just be that you need to use an editor better suited to reading large VHDL files. I regularly work with 3000+ line VHDL files where most of the space is the logic of a single process, and have no difficulties reading them due to an editor that supports code folding.
I use Notepad++, but I'm sure there are other editors that can support folding on VHDL syntax. When I open a file, I press alt+0 to fold every possible syntax folding point then expand as needed to the part I'm working on. You can also use line hiding to fold arbitrary sections of your file, although that's a little more awkward to work with.
If you have large groups of related concurrent statements you can easily group them into a folding point with a name : if true generate which also allows you to declare intermediate signals outside of the scope of your main architecture (block statements work, but aren't supported by all tools). To force a folding point within a process I use if true then.
If you are designing a processor that implements the different operations in a giant case statement, then what you are really describing is a series of parallel functional units, feeding an output multiplexer. You might have an output that is driven, depending on the op mode, by the output of either a multiplication, addition, subtraction, some logic operation, a shift, etc.
You can easily design this in a modular way, by implementing each functional unit in its own entity, some of which might be quite simple. In the first instance, these blocks would operate unconditionally, and their outputs would feed an output multiplexer. You might later add enable signals, driven by your instruction decoding logic, that enable only the blocks that will be used in a particular operation, in order to save power. It might sound like you will end up with a lot of control signals using this approach, but if you put them all in a record, it makes the code quite compact, while at the same time allowing verbosity and readability at the point where a control signal is used, for example:
AddSub : entity work.AdderSubtractor
port map (
clk => clk,
enable => decoded_instruction.addsub_enable,
a => a,
b => b,
mode => decoded_instruction.addsub_mode, -- This might be an enumerated type
output => addsub_output
);
There would be other _output signals, and at the end you would have something like
OutputMux : process (all)
begin
case decoded_instruction.output_mux_select is
when ADD_SUB => output <= addsub_output;
when MULT => output <= mult_output;
when LOGIC => output <= logic_output;
end case;
end process;
One bonus of doing it this way is that you might find it efficient for several of the functions to be implemented in a DSP block in the FPGA; you can easily design a functional block for add, subtract, multiply, written to target the DSP block in your device. The output of this would be just another input to your 'output' multiplexer. In my experience you should be able to efficiently implement many of your processing functions using a single DSP block (or a single entity that describes a few cascaded DSP blocks, depending on your data path width).
Personally I much prefer this approach of making the design very modular. In a recent multicore DSP project, I have only a couple of files that have ~500 lines, with the majority having 200 or less. This means that when I come back to a part of the design, it usually fits on one page, and can easily be picked up and understood in a very short amount of time. I also find that when implementing heavy pipelining to improve the performance of the design, having too much going on in one process or entity can make this job an order of magnitude more difficult.
Lastly, if functional elements are contained in small entities, you can more easily simulate, test, and verify just that bit of code in isolation, which in my experience allows the block to be signed off more quickly, while at the same time giving more confidence in the code. If everything is in one process, it is harder to have confidence that making a change that fixes or improves one thing, isn't going to break something else. Again in a heavily pipelined design, I find that it can be quite easy to change something that inadvertently causes the design to fail an aggressive timing constraint, so the simpler the entities, the smaller the chances of this happening.
As said, your question is hard to answer. How many lines are we talking about?
You could look up good VHDL code practises though:
- aliases should be avoided (not all tools even support then AFAIK)
- give signals/variables a clear name
- try to group functionality
- try not to change a signal/variable on 2 places separated by 500lines, usually there is a way
- if really needed you could consider using shared variables, introduced in VHDL93. (this will, however, not solve your multiple driver issue)
- do not forget the availability of records to group signals
About making your "intermediate signals visible", you could write
junk_proc: process(clk, rst) is
variable a,b,c: of_some_types;
begin
if rst then
//do reset stuff
elsif rising_edge(clk)
b:=func1(a);
c:=func2(b);
end if;
end process;
variables a,b and c (plain wires in this case) could obviously be visualized in any simulation tool.
If, however, you write b=func1(func2(func3(func4(a)))), do not forget that you describe all this to happen in a single clock cycle. Considering your description I bet you'll run into problems, but perhaps that's a good way of learning.
This is mostly out of curiosity.
One fragment from some VHDL code that I've been working on recently resembles the following:
led_q <= (pwm_d and ch_ena) when pwm_ena = '1' else ch_ena;
This is a mux-style expression, of course. But it's also equivalent to the following basic logic expression (at least when ignoring non-binary states):
led_q <= ch_ena and (pwm_d or not pwm_ena);
Is one "better" than the other in terms of logic utilisation or efficiency when actually implemented in an FPGA? Is it preferable to use one over the other, or is the compiler smart enough to pick the "best" on its own?
(For the curious, the purpose of the expression is to define the state of an LED -- if ch_ena is false it should always be off as the channel is disabled, otherwise it should either be on solidly or flashing according to pwm_d, according to pwm_ena (PWM enable). I think the first form describes this more obviously than the second, although it's not too hard to realise how the second behaves.)
For a simple logical expression, like the one shown, where the synthesis tool can easily create a complete truth table, the expression is likely to be converted to an internal truth table, which is then directly mapped to the available FPGA LUT resources. Since the truth table is identical for the two equivalent expressions, the hardware will also be the same.
However, for complex expressions where a complete truth table can't be generated, e.g. when using arithmetic operations, and/or where dedicated resources are available, the synthesis tool may choose to hold an internal representation that is more closely related to the original VHDL code, and in this case the VHDL coding style can have a great impact on the resulting logic, even for equivalent expressions.
In the end, the implementation is tool specific, so the best way to find out what logic is generated is to try it with the specific tool, in special for large or timing critical parts of the design, where the implementation is critical.
In general it depends on the target architecture. For Xilinx FPGAs the logic is mostly mapped into LUTs with sporadic use of the hard logic resources where the mapper can make use of them. Every possible LUT configuration has essentially equal performance so there's little benefit to scrutinizing the mapper's work unless you're really pushing the speed limits of the device where you'd be forced into manually instantiating hand-mapped LUTs.
Non-LUT based architectures like the Actel/Microsemi device families use 2-input muxes as the main logic primitive and everything is mapped down to them. You can't generalize what is best across all types of FPGAs and CPLDs but nowadays you can mostly trust that the mapper will do a decent enough job using timing constraints to push it toward the results you need.
With regards to the question I think it is best to avoid obscure Boolean expressions where possible. They tend to be hard to decipher months later when you forgot what you meant them to do. I would lean toward the when-else simply from a code maintenance point of view. Even for this trivial example you have to think closely about what behavior it describes whereas the when-else describes the intended behavior directly in human level syntax.
HDLs work best when you use the highest abstraction possible and avoid wallowing around with low-level bit twiddling. This is a place where VHDL truly shines if you leverage the more advanced features of the language and move away from describing raw logic everywhere. Let the synthesizer do the work. Introductory learning materials focus on the low level structural gate descriptions and logic expressions because that is easiest for beginners to get a start on but it is not the best way to use VHDL for complex designs in the long run.
Of course there are situations where Booleans are better, particularly when doing bitwise operations across vectors in parallel which requires messy loops to do the same imperatively. It all depends on the context.
Is it a good design practice to use combinatorial logic to drive the output of a module in VHDL/Verilog?
Is it okay to use the module input directly inside a combinatorial block,and use the output of that combinatorial block to drive another sequential block in the same module?
An answer to the two questions really depends on the overall design methodology
and conditions, and will be opinion based, as Morgan points out in his comment.
The questions are in special relevant for a large design with timing pushed to
the limit, and where multiple designers contribute with different modules. In
this case it is important to determine a design methodology up front which
answers the two questions, in order to ensure that modules provided by
different designers can be integrated smoothly without timing issues.
Designing with flip-flops on all outputs of each module, gives the advantage
that when an output is used as input to other module, then the input timing is
reasonable well defined, and only depends on the routing delay. This makes it
a Yes to question 1.
Having a reasonable well-defined input timing makes it possible to make complex
combinatorial logic directly on the inputs, since most of the clock cycle will
be available for this. So this also makes it a Yes to question 2.
With the above Yes/Yes design methodology, the available cycle time is only
used once, and that is at the input side of the module, before the flip-flops
that goes on the output. The result is that multiple modules will click nicely
together like LEGO bricks, as shown in the figure below.
If a strict design methodology is not adhered to in different modules, then
some modules may place flip-flops on the input, and some on the output. A
longer cycle time, thus slower frequency, is then required, since the worst
case path goes through twice the depth of combinatorial logic. Such a design
is shown in the figure below, and should be avoided.
A third option exists, where flip-flops are placed on all inputs, and the
design will look like the figure below if two different modules use the same
output.
One disadvantage with this approach is that the number of flip-flops may be
higher, since the same output is used as input to multiple flip-flops, and the
synthesis tool may not combine these equivalent flip-flops. And even more
flip-flops than this may be required, if the module that generates the output
will also have to make a flip-flopped version for internal use, which is often
the case.
So the short answer to the questions is: Yes and Yes.
The answer to both questions as expressed is basically yes, provided the final design meets speed targets, and the input signals are clean.
The problem with blocks designed this way are that the signal timings through them are not accurately defined, so that combining several such blocks may result in an absurdly slow design, or one in which fast input signals don't propagate cleanly through the design.
If you design such a circuit, and it meets ALL your input and output timing constraints as well as any clock speed constraints you set, it will work.
However if it fails to meet the clock constraints you will have to insert registers to "pipeline" the design, breaking up long slow chains of combinational logic. And you will have to observe the input and output timings reported by synthesis and PAR, and they can get complicated.
In practice (in an FPGA : ASICs can be different) registers are free with each logic block (Xilinx/Altera, not true for Actel/Microsemi) and placing registers on each block's inputs and/or outputs makes the timings much simpler to understand and analyse.
And because such a design is pipelined, it is normally also much faster.
Although I'm somewhat proficient in writing VHDL there's a relatively basic question I need answering: When to break down VHDL?
A basic example: Say I was designing an 8bit ALU in VHDL, I have several options for its VHDL implementation.
Simply design the whole ALU as one entity. With all the I/O required in the entity (can be done because of the IEEE_STD_ARITHMETIC library).
--OR--
Break that ALU down into its subsequent blocks, say a carry-lookahead adder and some multiplexors.
--OR--
Break that down further into the blocks which make a carry-lookahead; a bunch of partial-full adders, a carry path and multiplexors and then connect them all together using structural elements.
We could then (if we wanted) break all of that right down to gate level, creating entities, behaviours and structures for each.
Of course the further down we break up the ALU the more VHDL files we need.
Does this affect the physical implementation after synthesis and when should we stop breaking things up?
You should keep your VHDL at the highest level of abstraction, so don't ever "break it down" as you described. What you are proposing is that you do the synthesis yourself (like creating a carry-lookahead adder) which is a bad idea. You don't know the target device (FPGA or ASIC library) as well as the synthesizer does and you shouldn't try to tell it what to do. If you want to do an addition, use the + operator and the tools will figure out the best structure that fits your design constraints.
Dividing the design into many modules will often make it more difficult to optimize your design, since optimizations between modules are generally harder to do than optimizations within modules.
Of course, major functional blocks that have well defined interfaces between them should be in separate modules for the sake of maintaining the design and readability. The ALU can be one module, the instruction ROM another, and so forth. These modules have distinct, well-defined functions and there is not much opportunity for intramodule optimization. If you want to get the last possible bit of optimization available, just flatten the design before optimization and let the tools do the work.
Is it synthesizable to use:
case statement within a case statement
case statement within an if statement
if statement within a case statement
I can compile it without any errors, but I'm still not sure if it would mess up the hardware structure and make it to complex.
Reason why I'm doing this:
I have a couple of states (state machine), and to make them go through all states I use case statements. But I also need to make some conditions (cases and ifs) within some of these states, some of them are quite big.
There's no reason the synthesiser shouldn't handle nested ifs and cases. And indeed I have done so many times in the past.
I imagine the algorithms of the synthesiser treats an if as just a 2-branch version of a case statement when it comes to logic implementation, so the type of decision function is not an issue. Nesting them will just cause it to create a set of logic for each decision, which is cascaded in the case of the nested decision.
If you find it doesn't work, file a bug report!
Of course, if you have very aggressive timing constraints, and many nested conditions, you may find that the logic the synthesiser produces, while correct, is not quick enough to meet your clock period target. In that case, there's nothing much for it but to refactor your logic to reduce the depth of the decisions.
Annex J of IEEE Std 1076-2008 (the LRM) references IEEE Std 1076.6-2004, IEEE Standard for VHDL Register-Transfer Level (RTL) Synthesis, wherein case statements are supported and a case statement alternative (the actual choice and associated sequence of statements) may specify sequential statements including case statements.
So the answer is yes, you should in general expect to have cases statements in case statement alternatives be capable of being synthesized. Whether or not a particular vendor fully supports 1076.6 or not is a separate question.