AR# 4330: Exemplar: How to force a clock signal to use an IBUF instead of the tools insert of BUFG
AR# 4330
Exemplar: How to force a clock signal to use an IBUF instead of the tools insert of BUFG
Description
Keywords: IBUF, BUFG, clock, buffer, normal, I/O
Urgency: Standard
General Description:
My design has multiple clocks and I want to force one clock to use a non-clock buffer, an IBUF, instead of the BUFG. Using the Leonardo GUI and the default options, I have instantiated an IBUF on the clock signal that goes to the clock of a flip flop. The tools automatically replaces the IBUF with a BUFG. How can I force the IBUF to be used?
Solution
1
By default the Leoanardo GUI will force clock signals to global buffer when the resources are available. If the design has multiple clocks then the best way to get around this is to turn off global buffer insertion, then use the buffer_sig attribute to push BUFG's onto the desired signals, if needed. By doing this the user will not have to instantiate any BUFG components, and as long as "chip" options is used in optimize the IBUF's will be auto-inserted..
Do the following:
For LeonardoSpectrum: -------------------------------------
1) In the Technology --> Advanced Settings Tab DESELECT the option 'Add Clock Buffer'
2) Load the library
3) Read the HDL files.
4) After 'Read' use the buffer_sig to get BUFG's on the desired signals, if needed.
3) In Optimize->Advanced DESELECT the option to 'Map to Clock/Global Buffers' This will prevent Exemplar from changing an IBUFG to a BUFG, and will also prevent an auto-insertion of a BUFG.
4) Verify the proper buffers were used on the proper signal by using the 'View Schematic' feature.