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Meaning of LEVEL-SENSITIVE SCAN DESIGN

 
Computing Dictionary
 
 Definition: 

(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.

 

 

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