A programmable logic device (PLD) that provides the capability to observe and control the logic state of buried internal nodes is disclosed. The PLD provides shadow storage units for internal nodes such as logic element registers, memory cells, and I/O registers. A sample/load data path includes bidirectional...http://www.google.com/patents/US6014334?utm_source=gb-gplus-sharePatent US6014334 - Sample and load scheme for observability of internal nodes in a PLD
