INIT signal

The better approach to asynchronous initialization is demonstrated in the ASYNCJF2 architecture and the synthesized process shown in Figure 5.17. The If statement is completed in this case by the clocking condition and therefore a much simpler circuit is produced. The only function of the INIT signal in this case is to feed the appropriate asynchronous input of each flip flop. The synthesis statistics for these two circuits in Table 5.2 indicate that the latter is roughly one-third smaller than the former. 

Figure 1.4. Experimental and theoretical femtosecond spectroscopy of IBr dissociation. Experimental ionisation signals as a function of pump-probe time delay for different pump wavelengths given in (a) and (b) show how the time required for decay of the initally excited molecule varies dramatically according to the initial vibrational energy that is deposited in the molecule by the pump laser. The calculated ionisation trace shown in (c) mimics the experimental result shown in (b).

Fig. 1.32. Phosphorylation of the C-terminal domain of RNA polymerase II and the beginning of transcription. The transition from the initiation complex to actual begin of transcription is regulated via phosphorylation of the C-terminal domain (CTD) of RNA polymerase II. In the above model it is assumed that initially a complex is formed between TFIID and a holoenzyme of RNA polymerase consisting of RNA polymerase II and associated factors (mediators, SRB proteins) and the basal transcription factors. Phosphorylation of the C-terminal domain effects the dissociation of the RNA polymerase from the initation complex and the transition to the elongation phase. A protein kinase, which is part of TFIIH, is responsible for the phosphorylation. The nature of the signal that induces phosphorylation of RNA polymerase II remains unknown. SRB suppressor of RNA polymerase B. After Koleske and Young (1995).

Signal transduction involves a transfer of information from cell surface receptors to the cytoplasm and ultimately to the nucleus where cellular activation and response are initated (1). Protein-tyrosine kinases (PTKs) serve central roles in many of these pathways by phosphorylating tyrosyl residues, which result in the introduction of phosphotyrosyl (pTyr, 1, Fig. 1) pharmacophores... 

It is well known that even modern Fourier-transform instruments require concentrations of at least 10 M to give a reasonably resolved NMR spectrum. Also, the minimum time necessary for a spectrum to be taken or for one given signal to be scanned is of the order of a minute. These constraints rule out the possibility of identifying short-lived transients or any compound present in low concentrations formed during a cationic polymerisation. Thus, while carbenium ions have been characterised by this technique under suitable conditions, their concentration in a polymerising solution is always too low to permit detection by NMR spectroscopy. On the other hand, one can easily follow the disappearance of the monomer and/or the formation of the polymer and of any other product (such as an ester) formed in appreciable quantities, an operation which is extremely convenient, not only from the point of view of the kinetics of pol)onerisation, Init also because it can lead to mechanistic information concerning the nature and the structure of various products. 

This is because the x-rays can generally escape from any point in the excitation volume shown in Figure 2. However, Auger electrons initated beneath the top layers of atoms either cannot escape from the solid or they lose energy such that they cannot contribute to the Auger electron signal. 

The spectroscopic tool to be considered here is femtosecond pump/probe spectroscopy. This experimental technique uses two ultrashort laser pulses which are time-delayed with respect to each other. They are sent into a molecular sample and a signal is recorded as a function of the delay-time between the pulses. To be more specific, we assume the molecule to be in an inital state 0o) O). Here o) denotes the wave function for the nuclear motion and 0) the wave function of the electrons (the adiabatic separation of nuclear and electronic motion is assumed throughout). The pump pulse induces a transition and the resulting wave function which describes the molecule after the interaction with the electric field may be assigned as 0i l). We treat electronic excitation so that the molecule is prepared in another electronic state 1). After the pump pulse passed the sample, the molecule evolves unperturbed until the probe pulse starts interacting. This interaction results in a second excitation to (in our case) a final electronic state 2) with the respective nuclear wave function 1 2) The scheme just described is depicted in Figure 1 and illustrates the idea of many pump/probe experiments. 

If the initialization signal, INTT, is high the normal shift operation occurs. The register will be initialized if INIT goes low. These operations inside a process are synchronous when a Wait statement is used with the dock expression. The trigger in this case is a rising clock edge. See Box 5.7 for more details on this construct. 

Each of the behavioural architectures consists of a process that describes a particular variant of the 4-bit shift register and an output inverter. These are two concurrent statements that will appear at the top level of the synthesized circuit. All behavioural architectures except SIMPLE employ an external initialization signal called INIT and therefore only SIMPLE has a different top-level circuit. Figure 5.13a shows the top level of SIMPLE and Figure 5.13b shows the same for all the other behavioural architectures. In each case the block represents the Process statement, the output of which is fed into an inverter. 

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