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Delay Balancing

Why Use Delay Balancing?

HDL Coder™ supports several optimizations, block implementations, and options that introduce discrete delays into the model, with the goal of more efficient hardware usage or achieving higher clock rates. Examples include:

  • Optimizations: Optimizations such as output pipelining, streaming, or resource sharing can introduce delays.

  • Cascading: Some blocks support cascade implementations, which introduce a cycle of delay in the generated code.

  • Block implementations: Some block implementations such as the Newton-Raphson architecture inherently introduce delays in the generated code.

When optimizations or block implementation options introduce delays along the critical path in a model, the numerics of the original model and generated model or HDL code can differ because equivalent delays are not introduced on other, parallel signal paths. Manual insertion of compensating delays along the other paths is possible, but is error prone and does not scale well to large models with many signal paths or multiple sample rates.

To help you solve this problem, HDL Coder supports delay balancing. By default, delay balancing is enabled on the model. The code generator detects introduction of new delays along one path, and then inserts matching delays on the other paths. When delay balancing is enabled, the generated model is functionally equivalent to the original model. It is not recommended that you disable delay balancing on the model. If you disable this setting, HDL Coder generates a warning that numerical differences can occur in the validation model. To fix this warning, enable Balance delays on the model or run the model check Check delay balancing setting.

Specify Delay Balancing

You can set delay balancing for an entire model. For finer control, you can also set delay balancing for subsystems within the top-level DUT subsystem.

Set Delay Balancing for a Model

Use the following makehdl properties to set delay balancing for a model:

  • BalanceDelays: By default, model-level delay balancing is enabled, and subsystems within the model inherit the model-level setting. To learn how to set delay balancing for a model, see Balance delays.

  • GenerateValidationModel: By default, validation model generation is disabled. When you enable delay balancing, generate a validation model to view delays and other differences between your original model and the generated model. To learn how to enable validation model generation, see the Generate validation model section in Model Generation Parameters for HDL Code.

For example, the following commands generate HDL code with delay balancing and generate a validation model.

dut = 'ex_rsqrt_delaybalancing/Subsystem';
makehdl(dut,'BalanceDelays','on','GenerateValidationModel','on');

Disable Delay Balancing for a Subsystem

You can disable delay balancing for an entire model or disable a subsystem within the top-level DUT subsystem. For example, if you do not want to balance delays for a control path, you can put the control path in a subsystem, and disable delay balancing for that subsystem.

To disable delay balancing for a subsystem within the top-level DUT subsystem, disable delay balancing at the model level. When you disable delay balancing for the model, the validation model does not compensate for latency inserted in the generated model due to optimizations or block implementations. The validation model can therefore show mismatches between the original model and generated model.

To disable delay balancing for a subsystem within the top-level DUT subsystem:

  1. Disable delay balancing for the model.

  2. Enable delay balancing for the top-level DUT subsystem.

  3. Disable delay balancing for a subsystem within the DUT subsystem.

When delay balancing is enabled on the model, the delay balancing setting on individual subsystems is ignored. To learn how to set delay balancing for a subsystem, see Set Delay Balancing For a Subsystem.

Delay Balancing Limitations

If delay balancing is unsuccessful, hdlcoder.optimizeDesign cannot optimize the generated HDL code.

Unsupported Blocks

The following blocks and subsystems do not support delay balancing:

  • Triggered Subsystem

  • Atomic Subsystem

  • HDLCosimulation

  • Data Type Duplicate

  • Decrement To Zero

  • Frame Conversion

  • Ground

  • FFT HDL Optimized

  • LMS Filter

  • Model Reference

  • To VCD File

  • Magnitude-Angle to Complex

Block Mode Restrictions

The following block implementations do not support delay balancing:

  • hdldefaults.ConstantSpecialHDLEmission

  • hdldefaults.NoHDL

Subsystem-Level Restrictions

HDL Coder does not support delay balancing, if:

  • There are multiple instances of an Atomic Subsystem in different conditional subsystems.

    In the Block Parameters dialog box of the Atomic Subsystem, you can set Function packaging to Nonreusable function.

  • The BalanceDelays block property for all instances of an Atomic Subsystem or Model Reference resolves to a different value.

    To fix this error, disable BalanceDelays for all instances of the Atomic Subsystem or Model Reference.

  • The block is inside a conditional subsystem and has pipeline delays.

  • A subsystem with BlackBox Architecture has the ImplementationLatency block property set to a negative value.

    To fix this error, for ImplementationLatency, enter a nonnegative integer.

Other Restrictions

HDL Coder does not support delay balancing, if:

  • Delays are introduced in a feedback loop and HDL Coder cannot balance the path delays. For example, if you apply clock-rate pipelining inside a feedback loop, HDL Coder introduces a delay at the clock-rate, and can cause delay balancing to fail.

    To reduce the number of clock-rate delays, increase the Oversampling factor.

  • The sample time is not discrete or the ratio of sample times of the fastest to the slowest rate is too large.

Delay Balancing with Constants

When you have Constant blocks as inputs inside the DUT Subsystem for which delay balancing is enabled, you see an initial simulation mismatch in the validation model. Consider this model inside a DUT Subsystem. The Constant block that outputs a value of 1 has the HDL block property OutputPipeline set to 1 .

This figure displays the generated validation model. You see that delay balancing added a matching delay to the input port to balance the pipeline register inserted for the Delay block. The code generator does not insert a matching delay on the parallel path containing the Constant block with the value 2 because the output value of the block is a constant. This delay not inserted results in an initial simulation mismatch

To resolve the simulation mismatch, in the validation model, manually add a matching delay at the output of the Constant block with the value 2.

Delay Balancing Report

To see the delay balancing information in the report, before you generate code for each subsystem or model reference, enable the optimization report. To enable this report, in the HDL Code tab, select Report Options, and then select Generate optimization report.

When you generate code for each subsystem, model reference, or MATLAB Function block, HDL Coder produces the optimization report. In the report, select the Delay Balancing section of the report.

The Delay Balancing Report shows latency changes, pipeline delay and phase delay at the output ports, and the number of pipelines added at the output ports to match the delays. If delay balancing fails, the report mentions the criteria that was violated and displays the link to any block or subsystem that caused delay balancing to fail.

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