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CMOS, the Ideal Logic FamilyCMOS

cmos lí thưởng và một số đạc tình như thời gian trễ, đặc tuyến | I Fairchild Semiconductor CMOS the Ideal Logic Application NOte77 FAIRCHILD Family January 1983 tm INTRODUCTION Let s talk about the characteristics of an ideal logic family. It should dissipate no power have zero propagation delay controlled rise and fall times and have noise immunity equal to 50 of the logic swing. The properties of CMOS complementary MOS begin to approach these ideal characteristics. First CMOS dissipates low power. Typically the static power dissipation is 10 nW per gate which is due to the flow of leakage currents. The active power depends on power supply voltage frequency output load and input rise time but typically gate dissipation at 1 MHz with a 50 pF load is less than 10 mW. Second the propagation delays through CMOS are short though not quite zero. Depending on power supply voltage the delay through a typical gate is on the order of 25 ns to 50 ns. Third rise and fall times are controlled tending to be ramps rather than step functions. Typically rise and fall times tend to be 20 to 40 longer than the propagation delays. Last but not least the noise immunity approaches 50 being typically 45 of the full logic swing. Besides the fact that it approaches the characteristics of an ideal logic family and besides the obvious low power battery applications why should designers choose CMOS for new systems The answer is cost. On a component basis CMOS is still more expensive than TTL. However system level cost may be lower. The power supplies in a CMOS system will be cheaper since they can be made smaller and with less regulation. Because of lower currents the power supply distribution system can be simpler and therefore cheaper. Fans and other cooling equipment are not needed due to the lower dissipation. Because of longer rise and fall times the transmission of digital signals becomes simpler making transmission techniques less expensive. Finally there is no technical reason why CMOS prices cannot approach present day TTL prices as sales volume