global_clocks

SECTION GLOBAL_CLOCKS

Use the GLOBAL_CLOCKS section to describe the clock waveforms from the global clock distribution networks. Local clocks, such as gated clocks, are not directly supported. The clock waveforms are used to generate automatically the timing constraints of the paths between two sequential elements. To allow more user control when clocks interact, there are provisions to specify the clock period transitions, which should be considered. By default, the closest transitions are used when two clocks interact. The clock waveform specification has the following format:

WAVEFORM clkname RISE value FALL value PERIOD value [EXCEPT SOURCE {sequential list }| EXCEPT SINK{sequential list}].

  Arguments

clkname

Specifies the name of the macro driving the clock network.

RISE/FALL/PERIOD value

Specify the value as either an integer or a floating point number followed by an unit selected from {NS, US, PS}. The default time unit is 0.1ns.

EXCEPT {SOURCE|SINK} {sequential list}

Specify the list of sequential elements which should not be included as endpoints in the automatically generated paths involving sequential elements.

MULTICLOCK

Use the MULTICLOCK specification to specify which clock periods should be considered during the generation of the path constraints involving sequential elements. The default specification is to consider only the closest clock periods of the SOURCE and DESTINATION clocks. This specification has the following syntax:

MULTICYCLE SOURCE clkA CYCLE value EXCEPT {seqlist}; DESTINATION clkB CYCLE value EXCEPT {seqlist}.

MULTICYCLE SOURCE clkA CYCLE value EXCEPT {seqlist}.

Arguments

clkA/clkB

The name of the macro driving the clock network.

EXCEPT {seqlist}.

By default all sequential elements clocked by the clock driver are included. The EXCEPT seqlist is a list of all the sequential elements or specific pins to be excluded.

CYCLE value

By default, the closest transitions are considered. CYCLE provides the ability to use transitions from one or more clock periods past the closest transition. CYCLE zero indicates the closest transitions. CYCLE one skips the closest set of transitions and uses the next set of transitions. The term cycle is used to avoid confusion with the term period in the clock waveform specification. This allows you to specify a cycling-stealing clocking regime.