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Multicycle Path Constraints Parameters

Synthesis tools require that data propagates from a source register to a destination register within one clock cycle. Multicycle paths cannot complete their execution within one clock cycle and cannot meet the timing requirements. To meet the timing requirement of multicycle paths, use the multicycle path constraints parameters to generate a register-to-register path information file or to generate enable-based constraints that use the timing controller enable signals.

Enable-based constraints

To meet the timing requirement of the multicycle path in your model, use enable-based constraints. The constraints are applied to a model that has Clock inputs set to Single. This option is useful for a multirate model to create a constraint file for relaxing timing of the slow-rate regions.

Enable-based constraints relax the timing requirement by enabling multiple clock cycles for data to propagate between the registers. The constraints use the timing controller enable signals to create enable-based register groups that have registers in each group driven by the same clock enable.

Settings

Default: Off

On

When you enable this setting and generate HDL code, HDL Coder™ generates a constraints file with the naming convention dutname_constraints. The format of the file name depends on the synthesis tool that you specify. The constraints file defines the timing requirements of multicycle paths. The file contains information about the clock multiples for calculating the setup and hold time information.

Off

Do not generate a multicycle path constraints file.

Dependency

  • When you select the Enable-based constraints check box, make sure that Clock-rate pipelining is Off. Using enable-based multicycle path constraints is an alternative to the clock-rate pipelining optimization. You can clear the clock-rate pipelining check box in the HDL Code Generation > Optimization > Pipelining tab.

  • Set the Clock inputs to Single. You can set the clock input in the HDL Code Generation > Global settings > Clock settings > Clock inputs.

  • To generate multicycle path constraints file, oversampling factor must be greater than 1. You can specify the Oversampling value in the HDL Code Generation > Global settings > Clock settings > Oversampling.

Command-Line Information

Parameter: MulticyclePathConstraints
Type: character vector
Value: 'on' | 'off'
Default: 'off'

To set this property, use the functions hdlset_param or makehdl. To view the property value, use the function hdlget_param.

For example, you can enable the MulticyclePathConstraints setting when you generate HDL code for the symmetric_fir subsystem inside the sfir_fixed model by using either of these methods:

  • Pass the property as an argument to the makehdl function.

    makehdl('sfir_fixed/symmetric_fir', ... 
            'MulticyclePathConstraints','on')
  • When you use hdlset_param, set the parameter on the model, and then generate HDL code by using makehdl.

    hdlset_param('sfir_fixed','MulticyclePathConstraints','on')
    makehdl('sfir_fixed/symmetric_fir')

See Also

Register-to-register path info

Generate a text file that reports multicycle path constraint information. The text file describes one or more multicycle path constraints that are agnostic to the synthesis tool. You must convert this information to the format required by the synthesis tool. It is recommended that you use the enable-based constraints setting instead to meet the timing requirements of multicycle paths. When you use that setting, the generated constraints are more robust for name changes in synthesis tools, and are supported on Xilinx Vivado, Xilinx ISE, and Altera Quartus II.

Settings

Default: Off

On

Generate a text file that reports multicycle path constraint information for use with synthesis tools.

The file name for the multicycle path information file is derived from the name of the DUT and the postfix '_constraints',

DUTname_constraints.txt

For example, if the DUT name is symmetric_fir, the name of the multicycle path information file is symmetric_fir_constraints.txt.

Off

Do not generate a multicycle path information file.

Command-Line Information

Parameter: MulticyclePathInfo
Type: character vector
Value: 'on' | 'off'
Default: 'off'

To set this property, use the functions hdlset_param or makehdl. To view the property value, use the function hdlget_param.

For example, you can enable the MulticyclePathInfo setting when you generate HDL code for the symmetric_fir subsystem inside the sfir_fixed model by using either of these methods:

  • Pass the property as an argument to the makehdl function.

    makehdl('sfir_fixed/symmetric_fir', ... 
            'MulticyclePathInfo','on')
  • When you use hdlset_param, set the parameter on the model, and then generate HDL code by using makehdl.

    hdlset_param('sfir_fixed','MulticyclePathInfo','on')
    makehdl('sfir_fixed/symmetric_fir')

Compatibility Considerations

Starting in R2021b, it is not recommended to use Register-to-register path info for generating multicycle path information. This option is removed from the HDL Code Generation > Optimization tab in HDL Coder Properties. It is recommended to use Enable-based constraints for generating multicycle path information. You can still enable this option in the MATLAB command-line interface by using hdlset_param or makehdl commands.

See Also