Guidelines to avoid Negative Slack with State Machines

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When you get a negative slack in the route report, how do you systematically debug it?. The route report shows long combinational path in terms of the net names generated in the synthesis, and how would you relate it to your code?

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I'm not sure if it is related to your problem, but at least the following expressions are perhaps inferring too much combinational and sequential logic due to the non covered status on if expressions, particularly because you have a lot of states:

STATE1:
    begin
        if(condition2)
            nextstate = STATE2;
        else 
            if(condition3)
                    nextstate = STATEX;
            else
                    nextstate = IDLE;
    end

I would recommend you start thinking about use the when-case statement instead.

Jeebu -

Your state machine coding looks fine to me, although I don't care for the split combinational nextstate with registered presentstate style any more. There was a day when synthesis tools needed the help of that coding style, but modern tools don't need it and I find it a bit archaic. I know a lot of books still push that style, but don't believe everything you read.

That aside, do you have the design fully constrained for timing? And is that host machine that your state machine is interacting with in the same clock domain? If not, did you structure the design to handle clock domain crossing on the interface?

Bob

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Jeebu -

Your state machine coding looks fine to me, although I don't care for the split combinational nextstate with registered presentstate style any more. There was a day when synthesis tools needed the help of that coding style, but modern tools don't need it and I find it a bit archaic. I know a lot of books still push that style, but don't believe everything you read.

That aside, do you have the design fully constrained for timing? And is that host machine that your state machine is interacting with in the same clock domain? If not, did you structure the design to handle clock domain crossing on the interface?

Bob

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Dear Bob,

Thanks for the reply,

Yes I m working with the same clock domain. So the framework I m working with, takes care of clock domain crossing.and gives me data in the same clock domain. Can you point me in some direction on how to systematically debug it?.

Sometimes the same code when build, will give positive slack, and sometimes negative slack. How do we reliably implement this state machine without any slack?.

Regards

Jeebu

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Your state machine coding looks fine to me, although I don't care for the split combinational nextstate with registered presentstate style any more. There was a day when synthesis tools needed the help of that coding style, but modern tools don't need it and I find it a bit archaic. I know a lot of books still push that style, but don't believe everything you read.

Bob

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A two- or a three-process state machine is the better approach, unless you have something simple that fits perfectly in the synchronous process.

Back to the main issue: if a state machine is failing timing there are 2 possibilities:

1. the required clock frequency is simply too high

2. there is too much decision logic in the state transitions

Usually it is the second case. This happens e.g. when you test a large counter for a certain value.

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A two- or a three-process state machine is the better approach, unless you have something simple that fits perfectly in the synchronous process.

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This could definitely be debated, josyb!

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This could definitely be debated, josyb!

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Try me!

Almost every state machine I write, inside an Avalon_ST module, has asynchronous outputs. Can you show me how to do that with a synchronous (only) process?

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Try me!

Almost every state machine I write, inside an Avalon_ST module, has asynchronous outputs. Can you show me how to do that with a synchronous (only) process?

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I believe by asynchronous you mean not assigned in a clocked process (but in a comb. section following a clocked process and so as part of comb cloud between registers).

I have done designs for last 16 years including Avalon interfacing and never used but a single clocked process.

In fact in the last two years I gave up state machine methodology altogether and just use a counter (which by itself is a state machine) and you can run as many states as your counter size allows with only drawback being using numbers instead of names. If a need a change to occur before next state then I can start the assignment at end of current state.

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Try me!

Almost every state machine I write, inside an Avalon_ST module, has asynchronous outputs. Can you show me how to do that with a synchronous (only) process?

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Sure, just encode the combinational outputs separately, either with explicit assign statements or using a separate case statement. That's a separate issue than encoding the state machine itself. As a general rule I think it's bad practice to have combinational signals flow through a state machine unclocked to create combinational outputs. I understand that there are times where that's the only option, but those times are pretty rare in my experience. If you allow yourself to code that way as general practice then you probably end up creating combinational outputs even when NOT necessary. Bottom line is that I think the multi-process approach makes reading and debugging the code much more difficult. Especially if you don't give special names to clocked vs. non-clocked state machine outputs. If you don't do that then it can become very confusing. If combinational outputs are used as an exception then you can assign them separately with a comment block to explain the what and why.

I've been a consultant for a very long time so have worked in many different organizations. I try to adopt coding practices that make it as easy as possible for my customers to maintain my code after I leave. Good business practice. State machine coding is a big part of that, and I use state machines as often as I can because they can be self-documenting if you take the time to add appropriate comments.

This is all just my personal opinion based on many years of doing this stuff. Please don't take offense. I'm just trying to share my experiences and what works for me. All the Mealy/Moore multi-process state machine crap that gets taught in schools and perpetuated in books is just bad information IMO.

Edit: While I'm at it I may as well make half the world angry and do a little VHDL trashing. For the life of me I can't understand why anyone writing rtl code would want to use VHDL vs. verilog/systemverilog. It used to be that VHDL was much more capable for modeling, but systemverilog has changed that. For synthesizable rtl code VHDL is just way too much work to write and to read. If I never had to read a line of VHDL code again I would be a happy man!!

Bob

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Sure, just encode the combinational outputs separately, either with explicit assign statements or using a separate case statement.That's a separate issue than encoding the state machine itself.

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To do this you have to copy-paste transition conditions from the state machine? Distributing them all over the code?

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As a general rule I think it's bad practice to have combinational signals flow through a state machine unclocked to create combinational outputs. I understand that there are times where that's the only option, but those times are pretty rare in my experience. If you allow yourself to code that way as general practice then you probably end up creating combinational outputs even when NOT necessary.

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That's pure conjecture ... see also below.

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Bottom line is that I think the multi-process approach makes reading and debugging the code much more difficult. Especially if you don't give special names to clocked vs. non-clocked state machine outputs. If you don't do that then it can become very confusing. If combinational outputs are used as an exception then you can assign them separately with a comment block to explain the what and why.

I've been a consultant for a very long time so have worked in many different organizations. I try to adopt coding practices that make it as easy as possible for my customers to maintain my code after I leave. Good business practice. State machine coding is a big part of that, and I use state machines as often as I can because they can be self-documenting if you take the time to add appropriate comments.

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You must assess that these combinational outputs eventually are used in some other synchronous process further down or up the hierarchy. E.g. I always instantiate a counter component if I need to count something. To load a downcounter with some value by the state machine I either need a combinatorial output, or in the case of a synchronous output load one value less. If I want to test the counter in the next clock, as the state machine has stepped to another state too, I have a problem with the external counter being loaded with the synchronous output, as the counter has not been loaded with that new value yet.

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This is all just my personal opinion based on many years of doing this stuff. Please don't take offense. I'm just trying to share my experiences and what works for me. All the Mealy/Moore multi-process state machine crap that gets taught in schools and perpetuated in books is just bad information IMO.

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None taken.

I too couldn't care less either whether Moore or Mealy or the two together are doing the job. But your style is exactly one of those two, or am I mistaken? I had to look it up (searching at least 5 VHDL books) and it is moore. So I most often do mealy things :)

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Edit: While I'm at it I may as well make half the world angry and do a little VHDL trashing. For the life of me I can't understand why anyone writing rtl code would want to use VHDL vs. verilog/systemverilog. It used to be that VHDL was much more capable for modeling, but systemverilog has changed that.

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I nowadays mostly code in myhdl (http://www.myhdl.org) ... and for VHDL I use a good editor: sigasi (http://www.sigasi.com) (Which also does some Verilog)

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For synthesizable rtl code VHDL is just way too much work to write and to read. If I never had to read a line of VHDL code again I would be a happy man!!

Bob

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If we compare VHDL as being Pascal/Ada like and Verilog as C-like (as you Verilog-guys classify them), I can quote an old boss of mine: if you get a pascal program compiled without errors, you have a good chance it will run. with c on the other hand you are nowhere yet.

I once wrote some VHDL code to compare all these styles and posted it on all programmable planet, but the sponsor of that web-site pulled it off the air without a warning, and I seem to have lost the text ... I put the VHDL source code here: https://gist.github.com/josyb/a84d067f0a468d0931599f1891bc83ff

I haven't got much free time lately, but let's define a non-trivial exercise and compare the coding styles and the QoR.

One more defence: the latest book on VHDL: effective coding with vhdl, principles and best practice by ricardo jasinsky Page 510:"The two-process styles offers several advantages [over the one-process style]".

Josy

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I believe by asynchronous you mean not assigned in a clocked process (but in a comb. section following a clocked process and so as part of comb cloud between registers).

I have done designs for last 16 years including Avalon interfacing and never used but a single clocked process.

In fact in the last two years I gave up state machine methodology altogether and just use a counter (which by itself is a state machine) and you can run as many states as your counter size allows with only drawback being using numbers instead of names. If a need a change to occur before next state then I can start the assignment at end of current state.

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I'd love to see that code. I might learn something new.