level-sensitive scan design

(circuit design) (LSSD) A kind of scan design which uses separate system and scan clocks to distinguish between normal and test mode. Latches are used in pairs, each has a normal data input, data output and clock for system operation. For test operation, the two latches form a master/slave pair with one scan input, one scan output and non-overlapping scan clocks A and B which are held low during system operation but cause the scan data to be latched when pulsed high during scan. 

   ____
  |    |
 Sin ----|S   |
 A ------|>   |
  |   Q|---+--------------- Q1
 D1 -----|D   |   |
 CLK1 ---|>   |	  |
  |__|	  |    __
 	  |   |	   |
 	  +---|S   |
 B -------------------|>   |
 	      |	  Q|------ Q2 / SOut
 D2 ------------------|D   |
 CLK2 ----------------|>   |
 	      |____|
In a single latch LSSD configuration, the second latch is used only for scan operation. Allowing it to be use as a second system latch reduces the silicon overhead.

Last updated: 1995-02-15

Nearby terms:

level one cache ♦ level-sensitive scan design ♦ level two cache

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