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Generate Valid In, Valid Out with HDL Coder - Best practice?

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TS on 20 Nov 2017
Commented: Ludo Visser on 7 Aug 2018
I am using Matlab HDL Coder to generate VHDL Code and want to add a "valid in/out" Port to my subsystems to ensure the timing in sequential operations on a macro level.
Valid in: Triggers the calculation of the subsystem with the current values of the subsytem input ports
Valid out: Signals (e.g. with an rising edge) that the calculation of the subsystem is finished
I have tinkered with several idears:
- Enabled subsystem with a state machine outside of the subsystem to control if the output values have changed
- A parallel path inside the subsystem which just routes the valid in directly to valid out - a rising edge is sourced when the calculation is started -> with "balenced delays" the valid out signal (should) rise when the calculation is ready
- some minor variations of the above / unsucessful approches
So , what is best practice when implementing a valid in / out control for a HDL Subsytem?

Answers (1)

Ludo Visser
Ludo Visser on 7 Aug 2018
I typically follow your second suggestion. I used enabled subsystems to split up the computations into blocks that can either be done in a single clock cycle (like addition, logic, etc.) or that require a predictable number of pipeline registers (e.g. multiplication). In this way, I can manage the results of the delay balancing feature (which I really dislike, but that's another discussion).
A typical subsystem looks like this:
You can see the "input valid" to "output valid" line in the top and the data line directly below it.
As you can see, I'm generous with unit delays, so I can predict better where the delay balancing will insert delays. In later stages, if I run into performance issues, I can optimize a bunch of them away, but typically it's not necessary in my applications.
Ludo Visser
Ludo Visser on 7 Aug 2018
I also turn it off at the top level. It's annoying that HDLWA pushes this setting so hard - it generates an error if you turn it off, which you can only bypass by setting "ignore warnings", and who knows what other warnings go unseen because of that.

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