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I'm running an Atmega32u4 on a 5V circuit. However, I'm also using a Bluetooth IC [MDBT40] on a 3.3V circuit that I'm setting down from 5V with a linear regulator. I'm turning both SCK and MOSI to 3.3V using 74LVC2G34.

However, I'm a little bit stuck on the MISO line to the Atmega. From Adafruit's schematic here, it seems like MISO [despite being 3.3V] is passed off directly. Can I connect this 3.3V signal wire to my 5V Atmega and have it function?

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In datasheet ATMEGA328, Page 258,

enter image description here

Vhigh(min) = 0.6Vcc, considering Vcc=5V, then, Vigh = 3V.

Although MISO acting as input will work on 3.3V but you would be working on the edge. If the Vcc goes low, you would not be able to read a signal.

Recommendation: Try using a level-shifter(easy to build,3.3V to 5V) or optocoupler(different voltage references) for the same.

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  • \$\begingroup\$ Thanks a lot for the help! This cleared things up! \$\endgroup\$
    – Felix Jen
    Jul 9, 2020 at 5:19
  • \$\begingroup\$ I'm glad I was able to help. :) \$\endgroup\$
    – DivB
    Jul 9, 2020 at 5:21

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