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When using ICs that use enable pins, what is the best practice for driving them in a microcontroller system (particularly when there's no especially pressing need to disable the device at any time)?

For example, I'm using a chain of parallel-to-serial shift registers to aggregate a large number of physical device inputs into a single serial bitstream to my uC, and these registers only operate when a clk_en clock enable pin is driven low.

The inputs will be polled for the lifetime of the uC, so at no point do I NEED to explicitly disable the clk_en signal.

Is it considered bad practice to have this pin permanently pulled low (i.e. with a pull-down resistor)?

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I don't think that would be bad practice, just choose an adequate pull-down resistor.

In order to make the design flexible though, you could also add a zero ohm jumper to that enable pin (a do-not-stuff component) along with the pull down resistor, just in case you need to do some debugging in the future, never know. That zero ohm jumper could connect to one of the GPIOs, Vcc, or some header pin you may want to add for debugging or whatnot.

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  • \$\begingroup\$ I like the idea of a jumper, especially for debugging. Thank you for the input. \$\endgroup\$ – Connor Spangler May 24 '17 at 23:47
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If the device is to be permanently enabled, I would tie the Enable pin to Vcc or Ground, whichever will enable the device - should be no need for pull-up or pull-down resistors.

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  • \$\begingroup\$ I'm confused exactly when I would or wouldn't need a pull up or down resistor. Wouldn't omitting one create a short? \$\endgroup\$ – Connor Spangler May 24 '17 at 23:22
  • \$\begingroup\$ Unless clearly stated in the datasheet, digital IC inputs can be connected directly to Vcc or Ground without damage. I do recall some early TTL logic where a pull-up resistor rather than direct Vcc connection was recommended. You would require a pull-up or -down resistor if you thought you might want to pull the input to the opposite state occasionally. \$\endgroup\$ – Peter Bennett May 24 '17 at 23:45
  • \$\begingroup\$ Interesting, thank you. I've dealt in TTL circuits only up until now, so growing pains and all that. \$\endgroup\$ – Connor Spangler May 24 '17 at 23:46
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The purpose of an enable pin is to allow disabling the part (or some function of it) when you do not need it.

Typically, you want the output(s) of a chip to be disabled while it is powering up, so you tie the enable pin through a resistor to "disabled", and later force it on with a GPIO output from your microcontroller, after the initialization is done.

If you do not care anout the state while powering up, and if you do not want to disable it later, you can connect the enable pin directly to VCC or ground. (But for debugging, it might be useful to add a jumper or pull-down resistor to allow to change it.)

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