CMOS inputs on virtually all CMOS devices, from the humble 74HC00 to super complex microcontrollers, incorporate protection diodes internally to GND and to VCC.  Not all data sheets show them, but they are there to protect the device from being damaged by any over voltage on the input however brief.  Is it OK to actually use these diodes – yep.  If you want to drive an input from a higher voltage source you can connect it via a series resistor to limit the current.  The catch is you need to know what the safe amount of current is and for that you need to find the 'maximum diode current' specification in the devices datasheet.  If it omits to tell you then you really shouldn't as you don't know what a safe amount of current is, but often a device will have a 'Maximum diode current on any IO pin' type of specification (the diodes are also included on CMOS outputs, hence 'IO') and as long as you ensure you won't exceed this then you should have no problem.  Bear in mind:

Series resistors can be detremental on fast signals

If you are doing this on lots of inputs remember that the diode on each input is dissipating the excess voltage as heat, so ensure you won't cause a problem by too much heat being generated across multiple input diodes (maximum dissipation specifications in conjunction with the IC running current can be used to help with this).

 

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