Scan Styles

Scan technique is the most widely used Design For Testability(DFT) technique, and more importantly, is supported by most test synthesis tools. Iliis technique involves replacing the sequential non-scan cells by scan cells of the desired scan style. This transformation enables the sequential scan cells to be connected as a shift register in the scan mode. Further, for ATPG each of these scan cells behave as a pseudo primaiy input as well as a pseudo primaiy output. In this section, we discuss four basic issues related to Test Synthesis using TC, namely. Scan methodology. Scan Style, Scan Insertion and ASIC Vendor issues. 

In TC terminology, the scan style refers to the kind of scan cell used. The four commonly used scan styles are as follows  

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The most popular DFT technique in ASIC design is the Scan Design Technique. Scan techniques involve replacing sequential elements in the design with equivalent scan cells. There exist different styles of scan cells. Based on the scan style selected the design is required to meet certain design rules. The most commonly used scan style is the multiplexed flip flop. 

Figure 8.1 shows a multiplexed flip-flop scan cell. In this chapter, we discuss only the multiplexed flip-flop scan style. However, most of the test design rules discussed are also applicable to other scan styles. This scan style is supported by most ASIC vendors. For a multiplexed flip-flop scan style the scan ports required are the scan-iny scan-enabley and scan-out ports. The normal clock is used in the test mode in this scan style. 

What scan style does the ASIC vendor support ... 

ASIC vendors usually support only some scan styles and not all the available scan styles. Most ASIC vendors support the multiplexed scan flip-flop style. 

TC uses the signaLtype attribute to identify scan ports. Functional ports can be identified as scan ports, by assigning this attribute using the set.signaLtype command. TC creates scan ports automatically if no functional ports are identified with the signaLtype attribute. In the muxed flip-flop scan style, where normal clock is used as test clock, one must not associate a signaLtype attribute, testjclock with the clock port. 

Set your currenLdesign to the top level and specify the test methodology and scan-style. 

During the synthesis phase, ensure that DC does not use any sequential cells which do not have scan-equivalents of the desired scan style in the technology libraiy. There are two ways to ensure this. One approach is to place a dont use attribute on all the scan cells in the library as well as all non-scan sequential cells which do not have a scan equivalent. 

Another simpler alternative is to use the Test Smart Compile approach. In this approach the user must specify the scan style before compile. The Test Smart Compile is turned on by specifying the scan style (using set scan style command) before compile. Then, DC automatically ensures that only those sequential cells which have scannable equivalents in the specified scan style, are inferred during optimization. However, for partial scan designs, users may choose to use non-scan cells functionally in critical paths, and choose not to replace diem during scan insertion. 

Specify the test methodology, scan style, number of scan chains, mixing of clock domains in the scan chain etc. using the set scan configuration command. 

To find all muxed flip-flops cells in the library libA, use the following dc shell script. For other scan styles this script can be easily modified. The file scan, cells contains a list of all the scan cells in the library and their pins. 

You have a design with scan inserted. The scan style used is multiplexed flip-flop and methodology, full scan. Running check test on the post scan design gives a clean report... 

What should the current design be set to in order to generate these vectors The two possible choices are A and D. If you set current design to D, TC thinks it has access to inputs that it my not have. If you set current design to A, TC has to create vectors for a design buried 4 or 5 levels deep into the hierarchy. The scan style used is the multiplexed flip-flop scan style. 

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Styles, P., Gratwohl, C Brown, F.F. (1979). Simultaneous multinuclear NMR by alternate scan recording of 31P and 13C spectra. J. Magn. Reson. 35, 329-336. 

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