Do the inputs of the 74HC374-SMD D-type flip flop have high impedance inputs? In datasheet is stated that it has buffered inputs, I assume that means high impedance, right?

In other words, do I have to put resistors in the input pins?

  • \$\begingroup\$ No buffering and high impedance are not the same thing although a buffer may have an high impedance input. Its good practice to pull up (or down) unused inputs with a resistor. \$\endgroup\$ – JIm Dearden Jun 24 '13 at 9:07
  • \$\begingroup\$ All input pins are connected to a micro-controller port. Do I have to put some small resistor between the port and flip flop, or do I risk to fry something? \$\endgroup\$ – Chris Jun 24 '13 at 9:14
  • \$\begingroup\$ No you won't fry anything. As long as the working (supply) voltages are the same the outputs of the micro and inputs of the chip are compatible. The only problem is if the micro port pin goes high impedance. This allows the input of the chip to float. If there is a chance that this might happen you need a pull up resistor. \$\endgroup\$ – JIm Dearden Jun 24 '13 at 9:19
  • \$\begingroup\$ No, the micro has already pull up resistors internally. My only fear is that the flip flop input has a small impedance. \$\endgroup\$ – Chris Jun 24 '13 at 9:25
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    \$\begingroup\$ @user1410908 - microcontroller internal pullup resistors tend to be software configurable, so if they get misconfigured (or inactive in a shutdown mode) you won't get the floating input protection from them. \$\endgroup\$ – Chris Stratton Jun 24 '13 at 16:53

With the 74HC series and other logic families that include a large number of devices rather than the individual datasheets repeating the basic characteristics of the logic family normally the manufacturers publish some form of family guide. For example from NXP Semiconductor:

74HC/T HCMOS Family Specifications

Page 28 shows some more detailed information on the inputs of the 74HC series and mentions a current flow of only a few nA. There's also the host of other information you might be used to seeing in a complete datasheet.


The D Inputs of a 74HC374 are extremely high impedance.


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