GHDL implementation of VITAL¶
This chapter describes how VITAL is implemented in GHDL. Support of VITAL is really in a preliminary stage. Do not expect too much of it as now.
The VITAL standard or IEEE 1076.4 was first published in 1995, and revised in 2000.
The version of the VITAL packages depends on the VHDL standard. VITAL 1995 packages are used with the VHDL 1987 standard, while VITAL 2000 packages are used with other standards. This choice is based on the requirements of VITAL: VITAL 1995 requires the models follow the VHDL 1987 standard, while VITAL 2000 requires the models follow VHDL 1993.
The VITAL 2000 packages were slightly modified so that they conform to the VHDL 1993 standard (a few functions are made pure and a few one impure).
VHDL restrictions for VITAL¶
The VITAL standard (partially) implemented is the IEEE 1076.4 standard published in 1995.
This standard defines restriction of the VHDL language usage on VITAL model. A VITAL model is a design unit (entity or architecture) decorated by the VITAL_Level0 or VITAL_Level1 attribute. These attributes are defined in the ieee.VITAL_Timing package.
Currently, only VITAL level 0 checks are implemented. VITAL level 1 models can be analyzed, but GHDL doesn’t check they comply with the VITAL standard.
Moreover, GHDL doesn’t check (yet) that timing generics are not read inside a VITAL level 0 model prior the VITAL annotation.
The analysis of a non-conformant VITAL model fails. You can disable the checks of VITAL restrictions with the –no-vital-checks. Even when restrictions are not checked, SDF annotation can be performed.
Backannotation is the process of setting VITAL generics with timing information provided by an external files.
The external files must be SDF (Standard Delay Format) files. GHDL supports a tiny subset of SDF version 2.1, other version number can be used, provided no features added by the next version are used.
Hierarchical instance names are not supported. However you can use a list of instances. If there is no instance, the top entity will be annotated and the celltype must be the name of the top entity. If there is at least one instance, the last instance name must be a component instantiation label, and the celltype must be the name of the component declaration instantiated.
Instances being annotated are not required to be VITAL compliant. However generics being annotated must follow rules of VITAL (e.g., type must be a suitable vital delay type).
Currently, only timing constraints applying on a timing generic of type VitalDelayType01 has been implemented. This SDF annotator is just a proof of concept. Features will be added with the following GHDL release.
Negative constraint calculation¶
Negative constraint delay adjustment are necessary to handle negative constraint such as a negative setup time. This step is defined in the VITAL standard and should occur after backannotation.
GHDL does not do negative constraint calculation. It fails to handle models with negative constraint. I hope to be able to add this phase soon.