[SOLVED] XOR gates used in compression logic for DFT

[neofeoe]

hi all,
what is the function of xor gates employed in the compression logic used in DFT?

 

[swethapr565]

The logic of the xor function:

A B OUT
--- ----
0 0 0
0 1 1
1 0 1
1 1 0
---------
Here, if the paths A,B is compressed to one path as OUT. The xor gate waits for input A and also for input B and gives an equivalent output thus helping in compressing the path to a single equivalent path. thus this helps in observing the inputs provided to any untestable or testable block - a requirement of observability and controllability thus compacting the output. unlike the and gate or "OR" gate, that ignores the other input, xor gate takes care of both the paths to compact to a single path. xor gate reduces the no. of paths as well as improves the testability measure of a ldesign.

 

[dftrtl]

For every node in design we cannot place one flop as observability point. So we xor the inputs and reduce the no of observability flops.