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.
Clifford has commented on PR #387 stating that he does not like the
driver script and would prefer to only have the core script with
appropriate notes in the documentation.
Also rename to .cc (rather than .cpp) for consistency.
