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I have a microcontroller (ATmega328) that runs on 5V and another chip (MAX31855), connected via SPI as a slave (that only using the MISO line), that runs on 3.3V.

  1. Do I need to convert the output of the slave from 3.3V to 5V?
  2. The chip select pin (active low) is held high via a pullup resistor connected to 3.3V. Is this a problem since it is also connected to the 5V uC? I assume so, yes? Would I need to use a logic level converter for that as well?
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    \$\begingroup\$ Try it, it should work. If it is not reliable, then you will want to use a level converter. \$\endgroup\$ – Matt M May 19 '14 at 21:24
  • \$\begingroup\$ @Matt: Bad advice. It may appear to work, but you have no way to know how much margin there is. Some other day at a different temperature it might not work. Worst case is works 99.99% of the time. \$\endgroup\$ – Olin Lathrop May 19 '14 at 22:21
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Do I need to convert the output of the slave from 3.3V to 5V?

Unlikely because the input logic high acceptable level for the ATmega will probably cover 3.3V devices.

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  • \$\begingroup\$ I haven't looked it up, but I wouldn't be so quick to assume this. Many PICs, for example, have a minimum guaranteed logic high input level of .8 Vdd, which would be 4 V in this case. \$\endgroup\$ – Olin Lathrop May 19 '14 at 22:20
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Follow this simple 7-step process:

  1. Open datasheet.

  2. Find electrical characteristics section.

  3. Find logic levels section within that.

  4. Find the minimum guaranteed voltage to be interpreted as a high level by the input pin you are using for MISO.

  5. Compare to the 3.3 V the MISO signal will be when high.

  6. If #4 < 3.3V, then it will work, else it won't.

  7. Realize all the above steps should have been obvious.

If the answer is it won't work, then you have to find a solution. There are many possibilities, one of which is to use a logic level converter.

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