There is information about original approach to DFT+ATPG linked together in the TwCAD. The TwCAD searchs automatically for testable modifications of digital circuit enouth the ATPG becomes posiible.
Suggested CAD is accepted in English and in Russian pages.
This approach is realized in general and especially applied for PLDs of Altera.
VHDL stands for very high-speed integrated circuit hardware description language. Which is one of the programming language used to model a digital system by dataflow, behavioral and structural style of modeling