Controlling Output Slew Rate

Each output of a CPLD device is programmable to operate either at full speed or with limited slew rate. Limiting the slew rate reduces output switching surges in the device. Slew rate control becomes important when your design uses a large number of outputs or you have transmission lines on your board which are sensitive to fast edge rates.

By default, all outputs in a CPLD design have fast (unlimited) slew rate. You can change fitter options so that all outputs operate with slow slew-rate or so that the fitter automatically selects slew-rate based on timing constraints you enter for your design. To change output slew rate control for all outputs from the Design Manager, go to the Basic tab of the Implementation Options.

1. From the Design Manager, select Design Implement.
   
   
2. Click on the Options button in the Implement dialog box.
   
   
3. Select Edit Template
   
   
4. Select the tab labelled Basic.
   
   
5. You can instruct the fitter to automatically switch outputs to slow slew rate by changing the Default Output Slew Rate option to Slow.
   
   

If you are in the Flow Engine, this procedure is as follows:

1. From the Flow Engine, select Setup Options.
   
   
2. Select Edit Template.
   
   
3. Select the tab labelled Basic.
   
   
4. You can instruct the fitter to automatically switch outputs to slow slew rate by changing the Default Output Slew Rate option to Slow.
   
   

By setting Default Output Slew Rate to Timing Driven, outputs involved in timing-constrained paths will have their slew-rate automatically set to Slow only if the slew-rate-limited output propagation delay still allows the output to satisfy all applicable timing constraints. Outputs that do not participate in timing-constrained paths will operated in Fast slew-rate mode.

If you want to explicitly force an output to use slow slew rate, apply the SLOW attribute to an OPAD or IOPAD symbol or the pad net.

SLOW

If you want a device output to use fast slew, use the FAST attribute. Simply apply on an OPAD (or IOPAD) symbol or the pad net.

FAST

Applying the FAST or SLOW attribute to an output pad always overrides the Default Output Slew Rate options in the fitter template.