How to implement a 3D Lookup Table that can be converted with HDL Coder?

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Hello Community
I want to implement a non-linear function with 3 variables, i.e. f(x,y,z), in Simulink and convert it to VHDL code using HDL coder.
Since I avoid division blocks I implement non-linear functions with Lookup Tables. However, although there is a n-D Lookuptable in Simulink's HDL coder library, it does not support this block for code generation. I get an error saying that only 1D and 2D Lookup Tables are supportet.
Is there a workaround for this? My function is in the form of
where are constants and are unsigned integers from 0 to 31 (5 bit binary inputs), yielding a 32x32x32 Lookup Table.

Accepted Answer

Kiran Kintali
Kiran Kintali on 3 Sep 2020
Edited: Kiran Kintali on 3 Sep 2020
Can you consider the following modeling workaround until 3D LUT table support is available in HDL Coder?
  1 Comment
Dominik Hiltbrunner
Dominik Hiltbrunner on 4 Sep 2020
Today, I testet your approach in Simulink and implemented it on our target hardware. So far it seems to work very well.
Thank you!

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More Answers (2)

Kiran Kintali
Kiran Kintali on 2 Sep 2020
3D LUT support is on the HDL Coder near term product roadmap and will be available in an upcoming release.
In the interim we can guide you build such a model using supported blocks. Could your share the sample model (that generated the error) with the algorithm requirements shown in the question?
Are you planning to do fixed-point or floating-point model? What are the data types and interpolation options under consideration? What is your target FPGA and QoR (power, performance, area requirements)?

Dominik Hiltbrunner
Dominik Hiltbrunner on 2 Sep 2020
Hello Kiran Kintali
Thank you for your answer.
I do fixed-point arithmetic. The integer and fraction lenght are set by the workspace and later optimized by the fixed-point tool.
Some blocks have optimized fixed-point arithmetic that is pre-calculated. These blocks are locked against the optimization.
My target FPGA is a Altera Cyclone V SE (5CSEMA5F31C6N), running at 50MHz. My model runs on a much slower rate due to long logic paths.
There is no power / performance criterion set yet, i.e. many design choices are still open.
The most critical part is the resource requirement. Currently, with a highly optimized design the whole system occupies 80% of the available FPGA resources. Implementing a division or too large look-up tables exceeds this limit.
Next, some moder background:
I want to model a first-order z-transfer-function whose coefficients are adjustable at run-time. It is given by:
where is the sampling period, are constants, and are the inputs, i.e. this is what I called before.
This is solved by implementing the z-TF as IIR filter with adjustable gain blocks, i.e. a multiplication block in which one input is connected to a register that stores the adjustable value.
The register is load by an ADC as follows:
Any variable can be adjusted from [P/2^ADC_res ... P] where P is the maximum value that can take. The ADC yields a integer value from [0 ... 2^ADC_res-1].
Note that the value is not allowed to avoid division by 0.
To implement this, I need look-up tables that calculate the following expressions:
I attached a Simulink model and a corresponding m-file with all parameters that reproduces the problem.
The model has 3 paths: One for validation (top path), one with the current solution that uses division blocks (middle path), and one with a 3D lookup table (bottom path).


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