Floating pins on 74LS165

Hi I’m building a circuit to extend the number of input ports on a PIC device, by chaining 4 74LS165 (Shift registers). However there are a number of pins that are not used on the chips, and I remember from my college days that the unused pins should either be taken up to Vcc or down to Gnd via a resistor.

So do I take the unused pin high and low and what value resistor should I use.

Thanks in anticipation for your answers.

kernow boy said:

Hi I’m building a circuit to extend the number of input ports on a PIC device, by chaining 4 74LS165 (Shift registers). However there are a number of pins that are not used on the chips, and I remember from my college days that the unused pins should either be taken up to Vcc or down to Gnd via a resistor.

So do I take the unused pin high and low and what value resistor should I use.

Thanks in anticipation for your answers.

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Which way to pull them depends on what those pins do. In other words, if some of them are logic inputs and you dont need to use them, you will probably need to pull them a certain way as to not change the way your design operates - so that question is for you to answer based on what logic value they can be at and not screw up circuit operation.

How you pull them, the following will probably work:

For TTL inputs:

Pull down = 1k to ground
Pull up = 10k to 50k to Vcc

For CMOS inputs:

Pull down = 1k to ground
Pull up = 10k to 250k to Vcc