Synchronous preset

What if we want to model a flip-flop with synchronous preset and clear In such a case, simply describe the synchronous preset and clear logic within a clocked always statement (an always statement with a clock event). Here is an example. 

Figure 2-54 Synchronous preset clear synthesized as combinational logic.

There are two approaches to synthesize this model. One approach is to direct the PresetData input into the synchronous preset input of the synthesized flip-flops alternatively, the PresetData could be directed directly into the data input of the flip-flops. The synthesized netlist shown here shows the latter option a synthesis system may optionally synthesize to the alternate approach. 

Figure 2-55 Not a synchronous preset and clear flip-flop.

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. 

Here is a model for a parameterized N-bit binary up-down counter with synchronous preset and preclear controls. The counting is synchronized to the rising edge of a clock. 

Negative edge-triggered, positive synchronous // preset, static D-type FF. endmodule... 

Figure S3 Inferring a flip flop with a synchronous preset and dear...

Summing up, a robust and easy to handle SMB-design uses 4 zones, a recycling pump fixed in respect to the columns and two pumps for the control of the outlet flow rates. Extremely high precision of all technical components of the SMB is needed. All pumps and valves have to be exactly synchronized. The flow rates should not vary by more thanl % from the preset value. All connections between the different parts of the system must be carefully optimized in order to minimize the dead volume. All columns should be stable and nearly identical in performance. If the SMB-technology is to be used in Biotechnology, GMP issues (cleaning, process and software validation) also have to be considered. In addition and as with any continuous process in that particular area, the definition of a batch could be a problem. 

Time resolution of changes in the mass spectra can be accomplished in several ways. A TOFMS accessory is available from CVC Products, Inc. (Rochester, NY), which rasters mass spectra across the oscilloscope screen both vertically and horizontally. This device counts a preset number of spectra, n, and then oflFsets the trace both vertically and horizontally. After n more spectra, the trace is again offset. This procedure continues up to the desired number of traces. The device can be used to display 1 16 traces with 1-64 spectra per trace, thus permitting time resolution as low as 0.1 msec. However, the total display time, 16 traces times 64 spectra/trace, is only 0.1 sec. Thus, synchronization of sample loading and/or failure with this data acquisition device is critical. This data acquisition method provides both quantitative and qualitative data about the evolved compounds. However, the mass range which can be displayed effectively is limited to about 100 amu. 

The type of load-transfer switch used in the UPS system is a critical design parameter. The continuity of AC power service to the load is determined by the type of switching circuit used. An electromechanical transfer switch, shown in Fig. 10.195, is limited to switch times of 20-50 ms. This time delay may cause sensitive load equipment to malfunction, and perhaps shut down. A control circuit actuates the relay when the sensed output voltage falls below a preset value, such as 94% of nominal. A static transfer switch, shown in Fig. 10.196, can sense a failure and switch the load in about 4 ms. Most loads will ride through this short delay without any malfunction. To accomplish a smooth transition, the inverter output must be synchronized with the power line. 

The slit function can be observed well in the imaging of line sources when the inlet and outlet slits (and the middle slit if present) are moved synchronously. If the slit is too wide the line of a mercury lamp does not appear as the expected Gaussian-shaped curve intensity distribution, but as a triangle. If the inlet and exit slits are different, a trapezoid is obtained. Triangular shapes in the spectrum indicate defective adjustment of slit widths in the equipment. These effects are observed mainly in spectrometers in which only a small number of preset slit widths can be selected. Distortion of a spectral band is negligible only if... 

The most common and definitely the best way of achieving this is to attach an external asynchronous or synchronous reset signal to each and every storage element in the circuit. When an external initialization signal is specified, the compiler will select an appropriate component from the library (if the elements are inferred). For example, if a flip flop is required to be initialized to V, the selected element will possess an input for a preset signal. A variety of designs based on this form of initialization are given in the examples in this chapter. 

The RESET signal is synchronous and is part of the combinational logic input to each flip flop. The circuit behaviour could have been specified as asynchronous by performing the reset sequence outside the clocked part of the process. This would have inferred flip flops with Preset and Clear inputs. The actual construction of a behavioural description to produce this circuit is left for the reader to attempt. Beware, oAer changes will also be required to the process ... 

Restrictions on the content of clock statements. It is far too easy to create storage elements that require components which may it may not be possible to synthesize (e.g. a device with bodi a synchronous dear and asynchronous preset) using a particular library. (Qiapter 5 showed how a number of different D-types could be inferred.)... 

The checking procedures just described are similar to the ones that may be applied to check the logic when the crossover mode is selected. In this mode, the output of All (pin 5) is at logic 1 when no input pulse is present. Note, in particular, that the SCA output signal is always synchronized with the timing discriminator output, and that it is delayed by A7 for a preset time interval relative to the All triggering time. 

The counter generally provides several counting decades whose output is available in BCD format. Normally, only the first (faster) decade, or decades directly associated with some types of preset control, are of the synchronous type. Frequently, ICs with two decades are used, but LSI circuits with four or six decades are already present in various instruments. 

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