For XC9000 devices each output 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, the CPLD fitter will apply a fast slew rate to all outputs. If you want to limit the slew rate of a device output to decrease its switching speed, use the “set_pad_type” command in dc_shell. Enter the following in dc_shell:
set_pad_type -slewrate HIGH port_list
where port_list is a list of output ports that are to operate with slow output slew rate.
If you need to explicitly set an output to use fast slew rate, enter the following in dc_shell:
set_pad_type -slewrate NONE port_list
The set_pad_type command must be executed before the insert_pads command in dc_shell.
By default, the fitter uses Fast for slew rate for all output drivers. To change the default to slow slew rate in the Design Manager:
ImplementTo specify default slow slew-rate from the UNIX command line, use the parameter -slowslew in the cpld command:
cpld -slowslew design_name
The fitter also has an option whereby it will automatically apply Slow slew rate to each output unless that would cause any propagation delay to that pin to fail to meet a timing specification.
To enable slew-rate optimization in the Design Manager, go to the Implementation Options template and check the Fast box on the Default Output Slew Rate line.
To enable automatic slew rate control from the Unix command line, use the parameter -autoslewpwr when executing the cpld command.
cpld -autoslewpwr design_name
