Introduction to the Combinational Logic Circuit

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The 74HC family has High-speed CMOS circuitry, combining the speed of TTL with the very low power consumption of the 4000 series. They are CMOS ICs with the same pin arrangements as the older 74LS family. Note that 74HC inputs cannot be reliably driven by 74LS outputs because the voltage ranges used for logic 0 are not quite compatible, use 74HCT instead.The 74HCT family is a special version of 74HC with 74LS TTL-compatible inputs so 74HCT can be safely mixed with 74LS in the same system. In fact, 74HCT can be used as low-power direct replacements for the older 74LS ICs in most circuits. The minor disadvantage of 74HCT is a lower immunity to noise, but this is unlikely to be a problem in most situations.General Characteristics:74HC and 74HCT family characteristics:

  • 74HC Supply: 2 to 6V, small fluctuations are tolerated.
  • 74HCT Supply: 5V ±0.5V, regulated supply is best.
  • Inputs have very high impedance (resistance),
  • Outputs can sink and source about 4mA if you wish to maintain the correct output voltage to drive logic inputs, but if there is no need to drive any inputs the maximum current is about 20mA. To switch larger currents you can connect a transistor.
  • Fan-out: one output can drive many inputs (50+), except 74LS inputs because these require a higher current and only 10 can be driven.
  • Gate propagation time: about 10ns for a signal to travel through a gate.
  • Frequency: up to 25MHz.
  • Power consumption (of the IC itself) is very low, a few µW. It is much greater at high frequencies, a few mW at 1MHz for example.

74LS family TTL characteristics:

  • Supply: 5V ±0.25V, it must be very smooth, regulated supply is best. In addition to the normal supply smoothing, a 0.1µF capacitor should be connected across the supply near the IC to remove the ‘spikes’ generated as it switches state, one capacitor is needed for every 4 ICs, etc.