Multiplexed flip-flop

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. 

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. 

From this example, it appears that all the inputs to NextState, the value 12, the value 5, and the variable CurrentState, should be multiplexed using appropriate select lines into the D-input of the inferred flip-flops for Next-State. This is exactly what occurs as shown in the synthesized netlist in Figure 2-55. So then, how can we infer flip-flops with synchronous preset and clear A synthesis system may provide a solution for this by providing a special option for directing the synthesis system to generate a synchronous preset clear flip-flop. 

HDL code can be behavioral or RTL. In the synthesis domain, the latter is usually considered to be the synthesizable form of HDL code. Since the focus of this book is on logic synthesis, all examples discussed are in synthesizable RTL code. As a preliminary introduction to HDL coding and synthesis let us consider the following simple examples. Four examples of HDL code which infer on synthesis a D-flip flop, a latch, an AND gate, and a multiplexer (referred to as mux throughout this book) respectively are discussed. 

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. 

Determines the type of flip flop that will be used in a sequential circuit. Options are generally for multiplexed data input type (M) or single input D-type (D),... 

The 4-bit shift register is a simple circuit that suitably demonstrates the aspects of sequential logic design discussed above. Like the multiplexer example in Chapter 4, a number of different design methods are illustrated for the same circuit. Also, other new constructs are introduced as more complex flip flop functions are developed. Again, all the VHDL descrip-... 

The optimized circuits for the asynchronous architectures are, however, quite different. ASYNCJFI, shown in Figure 5.19, has imdergone a dramatic transformation that has doubled its cell count but actually reduced the number of transistors. This has been achieved by replacing each all-in-one multiplexer and flip flop element with simpler D-type flip flops and front-end combinational logic. The flip flops still retain their asynchronous initialization inputs and feedbadc connection but the multiplexed data inputs have gone. Note that the feedback connection now uses the inverted output of each flip flop. The result of area optimization is a slightly smaller... 

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