First, there are no longer separate cell libraries for xc6s/xc7/xcu.
Manually instantiating a primitive for a "wrong" family will result
in yosys passing it straight through to the output, and it will be
either upgraded or rejected by the P&R tool.
Second, the blackbox library is expanded to cover many more families:
everything from Spartan 3 up is included. Primitives for Virtex and
Virtex 2 are listed in the Python file as well if we ever want to
include them, but that would require having two different ISE versions
(10.1 and 14.7) available when running cells_xtra.py, and so is probably
more trouble than it's worth.
Third, the blockram blackboxes are no longer in separate files — there
is no practical reason to do so (from synthesis PoV, they are no
different from any other cells_xtra blackbox), and they needlessly
complicated the flow (among other things, merging them allows the user
to use eg. Series 7 primitives and have them auto-upgraded to
Ultrascale).
Last, since xc5v logic synthesis appears to work reasonably well
(the only major problem is lack of blockram inference support), xc5v is
now an accepted setting for the -family option.
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.
