Nonlinear effects
You can account for nonlinear effects when assessing the value of scan compression.
Chris Allsup, Synopsys -- Test & Measurement World, 6/1/2006 2:00:00 AM
The linear relation for area A(x) described by equation 6 in the accompanying article ('Limits of test time reduction') is accurate if the number of gates added per scan chain remains constant with increasing compression. The silicon area overhead for some compression architectures, however, may increase disproportionately with higher compression levels. In addition, increasing the number of scan chains tends to increase the amount of wire routing congestion in designs, which may contribute to a further increase in area at higher compression levels.
This nonlinear increase in compression area is illustrated in Figure A, where A(x) is represented by:
(a)
where æ is a second-order area scaling coefficient.
 |
Figure A. Effect of nonlinear increase in compression area is illustrated here. |
The nonlinear increase in silicon area tends to offset the benefits of higher compression. Figure B compares ë for different die areas based on the linear relation for area (equation 6) and the nonlinear relation (equation a).
 |
Figure B. Shown here are the effects of a nonlinear increase in compression area on optimal compression level for different die areas A0. |
Initial compression area overhead
The linear relation for compression area does not describe some compression architectures that require an initial fixed amount of circuitry even for very small compression levels. All other factors being equal, the effect of this fixed area overhead is an increase in total cost across all compression levels. In this situation, it is useful to modify equation 6 to include this fixed area overhead AF:
(b)
In some architectures, the high initial area overhead for compression circuits can lead to a relatively lower unit increase in silicon area overhead as compression is increased, compared with implementations in which AF = 0. Figure C compares two hypothetical scan compression types, SC1 and SC2, where AF1 = 0.008 cm2, AF2 = 0, and ã2/ã1 = 1.25. The minimum total costs are measured at their optimal ë values. SC2 has a net cost advantage over SC1 that is greatest for A0 = 2.0 cm2.
|
Figure C. Cost difference per 1 million units for two compression types, SC1 and SC2, are illustrated here, where AF1 = 0.008cm2 and AF2 = 0. |
Formulas for ë and Äcost that account for the effect of significant initial compression area overhead are obtained by replacing A0 with A0 + AF in equations 16 and 17:
(c)
(d)
You can compare two different types of implementations as presented in Figure C—one with an initial fixed area overhead for compression, and one without—by comparing the total costs using equation 12 directly:
(e)
The difference in total cost Äcost is determined by first substituting AF1 and ã1 into equation e to reflect the cost of the fixed area overhead implementation, then substituting AF2 = 0 and ã2 in equation e to reflect the cost of the second implementation. Äcost is obtained by subtracting the two total costs at the ë values calculated from equation c.
Back to "Limits of test time reduction."
Also see: "Optimizing compression in scan-based ATPG DFT implementations"
Limits of test time reduction
06/01/2006Figure 2.2
01/28/2004Keeping the Navy calibrated: Figures
08/31/2007Figure 3.4
01/29/2004
-
FLIR offers IR camera for under $3000
-
Don't let the economy compromise quality (Guest commentary)
-
Danaher speeds up restructuring, acquires life-sciences businesses
-
Agilent’s Cover-Extend technology eliminates need for physical test points for in-circuit test
-
So many combinations: Testing a switch-matrix board
View All Webcasts
