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A few new attributes are defined for use in cell libraries:
- iopad_external_pin: marks PAD cell's external-facing pin. Pad
insertion will be skipped for ports that are already connected
to such a pin.
- clkbuf_sink: marks an input pin as a clock pin, requesting clock
buffer insertion.
- clkbuf_driver: marks an output pin as a clock buffer output pin.
Clock buffer insertion will be skipped for nets that are already
driven by such a pin.
All three are module attributes that should be set to a comma-separeted
list of pin names.
Clock buffer insertion itself works as follows:
1. All cell ports, starting from bottom up, can be marked as clock sinks
(requesting clock buffer insertion) or as clock buffer outputs.
2. If a wire in a given module is driven by a cell port that is a clock
buffer output, it is in turn also considered a clock buffer output.
3. If an input port in a non-top module is connected to a clock sink in a
contained cell, it is also in turn considered a clock sink.
4. If a wire in a module is driven by a non-clock-buffer cell, and is
also connected to a clock sink port in a contained cell, a clock
buffer is inserted in this module.
5. For the top module, a clock buffer is also inserted on input ports
connected to clock sinks, optionally with a special kind of input
PAD (such as IBUFG for Xilinx).
6. Clock buffer insertion on a given wire is skipped if the clkbuf_inhibit
attribute is set on it.
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