Keywords: dual port ram di unsupported pack merge dpram ram16x2
General Description: CLB configurations involving the F and G combinational outputs from the same CLB driving a 2- or 3-input function that could fit into the H LUT are not supported by the M1.3 Mapper.
There is no one error message that is symptomatic of the Mapper to handle such configurations, but most likely it would be one that starts with,
"Unable to obey design constraints which require the combination of the following symbols into a single CLB"
This configuration is illustrated in the following figure:
F and G combinational outputs from the same CLB drive 2- or 3-input LUT that could be packed into the H LUT.
The same problem presents itself when trying to map a 16x2 RAM with additional logic that could fit into an H-LUT.
The only workaround available is to create the desired CLB configuration in EPIC, generate a physical macro from it, and instantiate it into your design.
For more information on creating physical macros, refer to the (Xilinx Manual EPIC Design Editor Reference/User Guide).