Global Resources in Actel Low-Power Flash Devices
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You can use the syn_global_buffers attribute in Synplify to specify a maximum number of global
macros to be inserted in the netlist. This can also be used to restrict the number of global buffers
inserted. In the Synplicity 8.1 version, a new attribute, syn_global_minfanout, has been added for
low-power flash devices. This enables you to promote only the high-fanout signal to global.
However, be aware that you can only have six signals assigned to chip global networks, and the rest
of the global signals should be assigned to quadrant global networks. So, if the netlist has 18
global macros, the remaining 12 global macros should have fanout that allows the instances driven
by these globals to be placed inside a quadrant.
Global Promotion and Demotion Using PDC
The HDL source file or schematic is the preferred place for defining which signals should be
assigned to a clock network using clock macro instantiation. This method is preferred because it is
guaranteed to be honored by the synthesis tools and Designer software and stop any replication
on this net by the synthesis tool. Note that a signal with fanout may have logic replication if it is
not promoted to global during synthesis. In that case, the user cannot promote that signal to
global using PDC. See Synplicity Help for details on using this attribute. To help you with global
management, Designer allows you to promote a signal to a global network or demote a global
macro to a regular macro from the user netlist using the compile options and/or PDC commands.
The following are the PDC constraints you can use to promote a signal to a global network:
1. PDC syntax to promote a regular net to a chip global clock:
assign_global_clock –net netname
The following will happen during promotion of a regular signal to a global network:
–
If the net is external, the net will be driven by a CLKINT inserted automatically by
Compile.
–
The I/O macro will not be changed to CLKBUF macros.
–
If the net is an internal net, the net will be driven by a CLKINT inserted automatically by
Compile.
2. PDC syntax to promote a net to a quadrant clock:
assign_local_clock –net netname –type quadrant UR|UL|LR|LL
This follows the same rule as the chip global clock network.
The following PDC command demotes the clock nets to regular nets.
unassign_global_clock -net netname
Note: OAVDIVRST exists only in the Fusion PLL.
Figure 3-8 PLLs in Low-Power Flash Devices
CLKA
GLA
EXTFB
POWERDOWN
OADIVRST
LOCK
GLB
YB
GLC
YC