Level-sensitive scan design

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Level-sensitive scan design (LSSD) is part of an integrated circuit manufacturing test process. It is a DFT scan design method 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 used as a second system latch reduces the silicon overhead.

This article is based on material taken from the Free On-line Dictionary of Computing prior to 1 November 2008 and incorporated under the "relicensing" terms of the GFDL, version 1.3 or later.