After the computations in the CL are finished, the current consumption decreases down to the steady state value, and the OVD sends a signal of output validity.

**4.1 Information carrying signal**

Current consumption by CMOS CL contains useful information on CL state. CMOS CL is a network of CMOS gates, so the current consumed by CL is a superposition of currents consumed by CMOS gates included in the CL. Each CMOS gate contains PMOS transistor and NMOS transistor networks (Fig.5). While a gate is in a steady state either the PMOS or the NMOS network is in a conducting mode. When a gate switches the non-conducting transistor network becomes conducting. There is usually a short period in switching time when both networks are in a conducting mode.

Generally, current consumed by a CMOS gate includes three components [9,10]:

(a) leakage current *I**lk* passing between power supply and ground due to finite resistance of non-conducting transistor network;

(b) short-circuit current *I**sc* flowing while both networks are in a conducting mode;

(c) load capacitance *C**L* charge current *I**LC* flowing while a CMOS gate is switching from low to high output voltage via conducting PMOS network and *C**L* .

SPICE simulation has shown [5] that amplitude of current consumed by a typical CMOS inverter depends on *C**L* and is limited by the non-zero resistance of the conducting PMOS network (Fig.7). The integral of consumed current is proportional to *C**L* . When a gate switches from high to low output voltage, the component *I**LC* is negative by direction and negligible by value (Fig.7b). It is evident, the switchings from high to low output voltage occur at the expense of energy accumulated in *C**L* during the previous switching from low to high output voltage. The component *I**sc* does not depend on direction in which a gate switches.

The component *I**LC* equals to *I**LC* = *C**L**V**dd* *f *where *V**dd* is a power supply voltage, *f* is a gate switching frequency. Veendrick has investigated the component *I**sc* dependencies on *C**L* and rise-fall time of input potential signal [10]. He showed that if both input and output signal have the same rise-fall time, the component *I**sc* cannot be more than 20 percent of summary current consumption [10]. However, when the output signal rise-fall time is less than input one, the component *I**sc* can be of the same order of magnitude as *I**LC*. In that case it must be taken into account. As to the component *I**lk*, it entirely depends on CMOS process parameters and for state of the art CMOS devices *I**lk* is about 10-15 -10-12 A.

So, the analysis of CMOS gate current consumption allows us to conclude that in transient state a CMOS gate consumes a current *I*= *I**lk*+*I**sc*+*I**LC* and in steady state it consumes only *I**lk*<< *I* . The difference between two states from the viewpoint of current consumption is several orders of magnitude. So, CMOS gate output validity detection is possible, both in principle and in practice.

In Section 2 we presented series-parallel model of computations in CL. We showed that in every moment during switching current consumed by CL is a superposition of the currents consumed on the activated signal propagation paths (SPPs). Now, considering CL implemented by CMOS devices we should note that while logical signal propagates through SPP the neighbouring gates switch in opposite directions. That is why a curve of current consumed by a ten inverter chain (Fig.8) looks like a combination of crests and troughs. Nevertheless, in the very lowest point of the curve the current consumed by CL in a transient state remains several orders more than in a steady state.

**4.2 OVD implementation**

The proposed OVD circuit, shown in Fig.9, is a threshold circuit translating an analog current signal* I* into a logical signal *OV*.

The OVD circuit contains a current-to-voltage converter (CVC) consisting of the resistor *R*1 and the diode *D*1. The OVD also contains a comparator implemented by the MOS transistors *M*1-*M*7 and resistors *R*2,,,*R*3 . CMOS CL consumes the current *I* and introduces a capacitance *C*in . The capacitance *C*out represents the load caused by the interface circuitry. A low potential output signal of OVD corresponds to CL output validity. A high potential output signal corresponds to CL output invalidity. So, OVD generates *OV* signal in negative logic manner.

The transfer characteristics of CVC is determined by a system of three equations:

where *I* is an input current of CVC, *V* is a voltage drop on the CVC circuit, *I*r is a current flowing through the resistor *R*1, *I*d is a current passing through the diode *D*1, *I*0 is a leakage current of the diode, *r*b is a bulk resistance of the diode. Here stands for *kT*/*q* where *k* is Boltzmann's constant, *T* is absolute temperature, *q* is charge of an electron.

Equations (1)-(3) determine the functional connection *F* between input current *I* and voltage drop *V*: . Graphic solution of the system is shown in Fig.10.

CVC parameters to be calculated are *R*1 and *r*b. Initial data for calculating *R*1 are the threshold voltage drop *V*th and corresponding threshold input current *I*th . Value *I*th is determined by minimal current consumed by CMOS CL in transient state. Initial data for calculating *r*b are maximal voltage drop *V*max and corresponding maximal input current *I*max. Value *I*max is determined by the maximal number of gates in CL switching simultaneously and their load capacitances.

The comparator chosen is the CMOS ECL receiver proposed by Chappell et al.[11]. The circuit includes a single differential amplifier stage with built-in compensation for parameter variations, followed by a CMOS inverter. The comparator has 100-mV worst-case sensitivity in 1-m technology. Detailed static and dynamic analysis of the comparator circuit was given in [11].

The comparator compares input voltage signal *V*in with reference voltage *V*ref. If *V*in <*V*ref the comparator output signal equals to logical zero which means that CL outputs are valid. Otherwise, *V*in >*V*ref, the comparator output signal equals to logical "one" which means that the outputs are invalid.

As it follows from the OVD circuit configuration,

where* V*dd is a voltage of power supply.

Equations (4) and (5) allow us to calculate the threshold voltage drop *V* of the CVC circuit:

since , so

If 0<*V*<500mV then the diode *D*1 of CVC operates in the very small current region *I**d* 0 and *I**d* <<*I**r*. So the component *I**d* in the Equation (1) can be neglected and *II**r* =*V*/*R*1 .

For practical values of the threshold input current of the OVD circuit is reversely proportional to the resistance of *R*1 : . Substituting Equation (6) yields

.

As to choosing value of *r**b** * it must be done with regard to maximal voltage drop *V*max .

If V>750mV, the diode *D*1 is in active mode and while *r**b* <<*R*1 the condition *I**r* <<*I**d* is true. So, in the large current region *II**d* and Equation (2) determines an almost linear dependence between *I* and *V*. For instance, if the maximal voltage drop *V*max =900mV and maximal input current *I*max=2mA, then in accordance with the Equation (2) *r**b* 100. Typical element values for the OVD circuit with *V**th* =400mV are given in Table 1.

The turn-on *t*on and turn-off *t*off delays of the OVD circuit depend on the OVD itself and the CMOS CL as well. (Switching the OVD output from low to high voltage is called "turning-on" and reverse switching is called "turning-off".)

Consider a piece of CMOS CL and its interaction with OVD circuit (Fig.11). The piece is an SPP including *N* logic gates. Each gate is shown symbolically as a connection of PMOS and NMOS networks. All the capacitances affecting *t**on* and *t**off* can be brought down