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74HCXX voltage

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gogoman:
Hello the 74HCXX family is rated to operate with a VCC between 2 and 5V.
    because its, considered a 5V device, Is there any reason not to use the device at 3.3v?
    What are the limitations?
    what are the  voltage input high and low at 3.3V ?
    what are the  voltage output high and low at 3.3V ?
thanks

MarkS:
All of that information will be in the datasheet under "DC Characteristics".

David Hess:
At 3.3 volts, 74HC logic is slower because the channel resistance of the CMOS transistors is higher.  The input threshold voltage is half way between the ground and supply voltages, so 1.65 volts.

RoGeorge:
There are the 74HC and the 74HCT, both are CMOS devices with a pinout compatible with the initial TTL family (Transistor Transistor Logic).  The last T in the 74HCT series means the logic levels are compatible with the voltage logic levels used in the old TTL series.

CMOS digital circuits, by default have the logic level variable, it's at the half of their supply voltage, so it depends of the voltage you are using to power them.

As a thumb rule, any CMOS will work faster at a higher voltage.  By how much faster it depends with each chip, you'll need to consult the datasheet.

Kleinstein:
Lower supply voltage also mean higher output resistance and thus less capable to drive loads (e.g. LEDs, relays).

The lower speed with a reduced supply voltage also has its good sides: usually less EMI produced.

The CMOS chips have the threahold usually at 30 and 70% of the supply. The range inbetween is undefined and can lead to significant higher supply current. Even the 74HCT chips may take more current with a 3 V input signal, even though in the range of defined high for this series.

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