Designs With Latches

Consider a design with a scan chain and two clock trees as shown in Figure 8.6. Notice that the upstream flip-flops are driven by clock branch C1, and the downstream flip-flops by clock branch C2. When going from clock domain C1 to clock domain C2, a retiming latch is inserted between the scan-out pin of the last flop in the scan chain clocked by C1, and the scan-in pin of the first flop in the scan-chain clocked by C2. The enable of the latch is connected to C1 such that it is transparent when C1 is low. The latch holds the previous scanned value for the duration when the clock pulse is high. This approach works, provided the skew is not greater than the high pulse width of the clock. Alternatively, one must use DC to insert delays to frx the hold time violations. 

There was another moment of truth in the ballast project I described in Chapter 1, when we had ordered several new boards with its innovative 2N2222-2N2907 npn-pnp latch. But the protection latch on all these boards was not working at all. Design issue We looked hard at the transistors and thought they were OK. After a couple of days we took the transistors out, finally suspecting they were faulty (maybe inadequate hfe, etc.), and then made a discovery—we learned that so-called 2222 transistors do not even have the same pinouts Each manufacturer... 

The blast resistance of conventional doors is generally limited by the rebound capacity in the unseating direction. A conventional unreinforced hollow metal door with a cylindrical latch may be adequate to withstand a rebound force of 50 psf (2.4 kPa). Door with a mortised latch may be adequate for a rebound force of 100 psf (4.8 kPa). If the blast pressure exceeds this, other alternatives may be considered. These include placing interior or externa barrier walls, or installation of blast resistant doors and frames. Unlike conventional doors, blast doors are typically provided as a complete assembly including the door, frame, hardware and accessories. This is because all the components are dependent on each other to provide the overall blast resistance. Refer to Chapter 9 for performance requirements and design details for blast resistant doors. 

A synthesis tool on encountering such a directive on a case statement understands that all possible values (that can occur in the design) of the case expression have been listed and no other values are possible. Consequently, a variable assigned in all branches of the case statement will never infer a latch. Here is the case statement in the NextStateLogic module with the directive specified. 

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