What exactly does "AVR I/O clock to be present" mean?

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I want to use the PE6 int6 on my atmega32u4 but read that the AVR I/O clock needs to be present. I always thought that meant the internal clock, thus implying its presence is a must (can not exactly remove it). Does this refer to the fuse setting? I'm using an external clock in my case. 

Last Edited: Wed. Dec 16, 2020 - 10:32 PM
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Last Edited: Wed. Dec 16, 2020 - 10:49 PM
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The I/O clock is derived from whatever your clock source is. In your case an external clock. The I/O clock is not active in any sleep modes except idle mode.  "11. External Interrupts" in the datasheet says:

 

Note that recognition of falling or rising edge interrupts on INT6 requires the presence of an I/O clock, described in “System Clock and Clock Options” on page 27.

So, you can't trigger INT6 on a falling or rising edge in any sleep mode except idle.

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Ah, very good guys thank you.