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WHAT DOES ENERGY GET DISSIPATED FOR IN CMOS CIRCUITS in Software Creator Code 39 in Software WHAT DOES ENERGY GET DISSIPATED FOR IN CMOS CIRCUITS




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9.1 WHAT DOES ENERGY GET DISSIPATED FOR IN CMOS CIRCUITS use software bar code 39 generating tocreate 3 of 9 barcode in software Mobile Barcode Usage two s complement sign & magnitude 0 0 MSB 6 8 bit position 14 LSB Fig. 9.4 Ave Code 3/9 for None rage activities in noisy speech signals (reprinted from [239]).

. This is beca use most VLSI circuits contain numerous registers the contents of which change very infrequently when compared with the busy datapath and pipeline registers. This includes registers that hold con guration data, calibration parameters, mode or status information, or slowly-varying coe cients. Most nodes that are part of register les or RAMs also exhibit low activities and the same applies to exception-handling circuitry and human interfaces.

. 9.1.2 Crossover currents As illustrat ed in g.8.4b, both the n- and the p-channel transistor in a CMOS inverter are partially conducting when the voltage at the input satis es Uth n < Uin p < Udd + Uth p .

This means that charge is then allowed to ow from vdd to vss via the two MOSFETs without ever reaching the load, see g.9.1b.

We will refer to this phenomenon as crossover current, but the reader is cautioned that many synonyms exist.7 The energy dissipated by crossover currents per charge discharge cycle depends on numerous factors. The approximation Ec r c y c k = (Ud d 2Ut h )3 tr a k 12 (9.

11). was derived Software barcode 39 by the author of [219] under a number of simplifying assumptions: Analysis refers to CMOS inverters exclusively. Electrical symmetry is assumed, which is to say that gain factors and threshold voltages are the same, i.e.

= n = p and Uth = Uth n = Uth p . The output load is assumed to be zero.8 The input voltage rises and falls linearly with ramp time tr a k = tr i k = tf a k .

. Such as over lap current, short-circuit current, sho otthrough current, rushthrough current, contention current, Class A current, and even dynam ic leakage current. Any capacitance attached to the output slows down the build up of drain source voltage across the o -going M OSFET and so curbs the current that ows from vdd to vss directly. This e ect e ectively m akes (9.

11) an upp er b ound for E c r k , a nding con rm ed in [240], where a much m ore detailed p ower m o del is presented.. Design of VLSI Circuits A Sah mode l is a good enough approximation for the transistors. As one naturally expects, losses augment with input ramp time and with transistor size because of = W . This gives rise to a dilemma.

The slower the input ramps to a gate, the more energy gets L wasted by crossover currents. Making transitions faster helps to cut dissipation in the gate being driven, but the generous sizing of the MOSFETs in the driving gate necessary to do so in ates the energy losses there. A compromise must be sought.

As a general guideline, circuits should be designed so as to (a) make signal rise and fall times approximately the same, and (b) make them comparable to the propagation delay of a typical gate from the cell library being used [241]. Only drivers that handle very heavy loads such as o -chip loads, vast clock nets, or long busses require a more sophisticated analysis to better balance area, delay, and dissipated energy.9 Otherwise, any step taken in pursuit of a lower charge/discharge dissipation Ech such as cutting down Udd , k , Ck , transistor sizes, and total node count K at the same time also helps to abate the energy losses Ecr that are due to crossover currents.

The argument is in support of a rough but popular approximation: k k Ec r k = (9.12) Ec r c y c k k Ec h c y c k = k Ec h k 2 2 k 2 Ed y n k = Ec h k + Ec r k (1 + k )Ec h k = (1 + k ) Ck Ud d (9.13) 2 As a rule of thumb, k is generally between 0.

05 and 1.5 [241], with the larger numbers applicable to those situations where (almost) no load is attached to the driving gate. The average value for digital VLSI circuits with adequately sized bu ers has been found to be k 0.

2 or less [182]. With supply voltages and overdrive factors being lowered from one process generation to the next, crossover losses and hence also k continue to diminish. Observation 9.

3. Crossover currents are not normally addressed in any speci c way during digital CMOS circuit design, apart from a) keeping ramp times within reasonable bounds and b) pad drivers that handle very heavy loads and more substantial voltages. From a practical perspective, note that library vendors refrain from modelling capacitive and crossover currents separately.

Instead, they characterize each library cell with a single energy gure g ate in their datasheets. This quantity is obtained by dividing the cell s power dissipation Pg ate by the frequency fou p of the signal at the output, which explains why it is expressed in W/MHz rather than in pJ. g a t e = Pg a t e 2 2 Ed y n c y c g a t e = (1 + )Cg a t e Ud d 1.

2 Cg a t e Ud d fo u p (9.14). Before compa Software bar code 39 ring such numbers or calculating with them, it is extremely important to understand a vendor s tacit assumptions about output loads, ramp times, node activities, contributions from input and clock activities (included or excluded), de nition of toggle rate (equal to or two times frequency), and static currents (neglected or included in Pg ate ).. It is needle ss to say that drive con icts cause signi cant crossover currents, they should b e avoided anyway. Exam ples of soft-switching pad drivers are given in section 10.4.

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