What is Scan Compression, EDT and/or CoDec in DFT?

Scan Compression is a DFT technique that reduces the testing time and thus cost by compressing(or reducing) the scan chain lengths. The need for such a technique arises due to the fact that we have limited pins at the SoC, but millions of Flip Flops & Gates; now due to the huge number of flops in a chain the test time is very large due to the large number of shift operations.

In scan compression two hardware blocks are primarily introduced : the Decompressor and the Compactor. The decompressor is the block that converts scan in pins to multiple internal chain pins ; the compactor does the opposite operation. The compressed scan chains are called channels.

CoDec is the name given to Compactor-Decompressor. EDT stands for Embedded Deterministic Test. The embedded logic includes a decompressor located between the external scan channel inputs and the internal scan chain inputs, and a compactor located between the internal scan chain outputs and the external scan channel outputs.

The cons of compression are Lower Test Coverage ,pattern Inflation and Routing Congestion ; thus there is a limit till what a design can be compressed.

Lets take an example of a design that has 1000 scan elements (flip-flops) and 10 scan-in pins and scan-out pins each.

So for a standard non-compressed design we will has 10 scan chains with 100 flops (1000/10) in each chain as shown below.

But if we consider the same design to be compressed by a factor of 10x then there are 100 internal channels ; and each channel has 10 flops (1000/(10x10)) each in a chain. The extra cost paid is of course the EDT hardware that are introduced.

Another important terminology used here is “Compression Ratio” which is defined as the?ratio of number of? scan chains to the number of scan channels.

Source:

- CMOS VLSI Design: A Circuits and Systems Perspective by Weste & Harris

- Essentials of Electronic Testing for Digital, Memory and Mixed-Signal VLSI Circuits by Bushnell & Agrawal

- Semicon Shorts

etc