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The original CMOS logic families were the 4000-seriesand the 74C-series circuits. The 74C-series circuits arefunctional equivalents to the corresponding numbered74-series TTL circuits, but have CMOS logic levels andretain the other well known characteristics of CMOSlogic.These characteristics are: low power consumption, highnoise immunity, and slow speed. The low power consumptionis inherent to the nature of the CMOS circuit.The noise immunity is due partly to the CMOS logiclevels, and partly to the slowness of the circuits. Theslow speed is due to the technology used to constructthe transistors in the circuit.CONTENTS PAGECMOS EVOLVES 1WHAT IS CHMOS? 1THE MCS-51 FAMILY IN CHMOS 1LATCHUP 2LOGIC LEVELS AND INTERFACINGPROBLEMS 2NOISE CONSIDERATIONS 2UNUSED PINS 3PULLUP RESISTORS 4PULLDOWN RESISTORS 4DRIVE CAPABILITY OF THEINTERNAL PULLUPS 5POWER CONSUMPTION 6Idle 7Power Down 7USING THE POWER DOWN MODE 8USING POWER MOSFETs TOCONTROL VCC 9BATTERY BACKUP SYSTEMS 9POWER SWITCHOVER CIRCUITS 1180C31BH a CHMOS EPROM 12SCANNING A KEYBOARD 13DRIVING AN LCD 16Using an LCD Driver 17RESONANT TRANSDUCERS 18Frequency Measurements 18Period Measurements 20Pulse Width Measurements 21HMOS/CHMOSINTERCHANGEABILITY 21External Clock Drive 21Unused Pins 22Logic Levels 22Idle and Power Down 22REFERENCES 23
