Known Issues


Synthesis of tri-state signals

Tri-state signals should be only used when they are connected (through the hierarchy) to top-level bidirectional or output pins.

Descriptions which infer a tri-state driver like:

pin <= data when tri = '0' else 'Z';

should not be included in any IP core description because these hinder or even inhibit block-based design flows. If a netlist is generated from such an IP core, the netlist may contain only a simple internal (on-chip) tri-state buffer instead of the correct tri-state I/O block primitive because I/O buffers are not automatically added for netlist generation. If the netlist is then used in another design, the mapper, e.g. Xilinx ISE Map, may fail to merge the internal tri-state buffer of the IP core netlist with the I/O buffer automatically created for the top-level netlist. This failing behavior is not considered as a tool bug.

Thus, if tri-state drivers should be included in an IP core, then the IP core description must instantiate the appropiate I/O block primitive of the target architecture like it is done by the Xilinx MIG.

Synthesis of bidirectional records

Records are useful to group several signals of an IP core interface. But the corresponding port of this record type should not be of mode inout to pass data in both direction. This restriction holds even if a record member will be driven only by one source in the real hardware and even if all the drivers (one for each record member) are visible to the current synthesis run. The following observations have been made:

  • An IP core (entity or procedure) must drive all record members with value ‘Z’ which are only used as an input in the IP core. If this is missed, then the respective record member will be driven by ‘U’ and the effective value after resolution will be ‘U’ as well, see IEEE Std. 1076-2008 para. 12.6.1. Thus simulation will fail.

    But these ‘Z’ drivers will flood the RTL / Netlist view of Altera Quartus-II, Intel Quartus Prime and Lattice Diamond with always tri-stated drivers and make this view unusable.

    Note: Simulation with ModelSim shows correct output even when the ‘Z’ driver is missing, but a warning is reported that the behavior is not VHDL Standard compliant.

  • Altera Quartus-II and Intel Quartus Prime report warnings about this meaningless ‘Z’ drivers. Synthesis result is as expected if each record member is only driven by one source in real hardware.

  • The synthesis result of the Lattice Synthesis Engine (3.7.0 / 3.8.0) is not optimal. It seems that the synthesizer tries to implement the internal (on-chip) tristate bus using AND-OR logic but failed to optimize it away because there was only one real source. Test case was a simple SRAM controller which used the record type T_IO_TRISTATE to bring-out the data-bus so that the tri-state driver could be instantiated on the top-level.

Use separate records for the input and output data flow instead.

Aldec Active-HDL

  • Aliases to functions and protected type methods

Altera Quartus-II / Intel Quartus Prime

  • Generic types of type strings filled with NUL


  • Aliases to protected type methods

Xilinx ISE

  • Shared Variables in Simulation (VHDL-93)

Xilinx Vivado

  • Physical types in synthesis
  • VHDL-2008 mode in simulation
  • Shared variables in simulation (VHDL-93 and VHDL-2008))