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I think understand the purpose of pull up resistors in general but in this particular case the DS1620 datasheet says that in the output mode it sources a current of 1ma (see footnote 5 on the last page). My understanding was that I would need a pull up resistor on the Atmega's input if the other side didn't source any current in the logical 1 state (high impedance), but in this case it says it sources current, why would I still need a pull up resistor enabled on the Atmega's input pin? (it doesn't work without the pull-up)

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The high output is specified as a minimum of 2.4V. This is enough if the ATmega is running off a supply of 4V or less, otherwise a pullup may be needed to pull it up to 0.6VCC.

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  • \$\begingroup\$ Both atmega and DS1620 are powered from 3.3 V. But I think you're right, Atmega's input high starts at 0.7 Vcc and DS1620 output high also starts at 0.7 Vcc so there's kind of a narrow margin and the Atmega may not detect the high without a pullup. Don't have an oscilloscope to to see what's really happening there. \$\endgroup\$ – axk May 23 '15 at 9:08

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