I want to interface a Teensy 3.5:


With a RockBlock 9603:


I will give 3.3V input to the Teensy, and 5V input to the Rockblock.

The Teensy is said to be 'all digital pins 5V tolerant'. Does that mean that I can just plug in the Teensy and RockBlock UART on each other and get good functioning? Or does this just mean that the Teensy will not be damaged, but the communication will fail due to the different logics levels?

Of course I could use a level shifter (MAX3323EEPE or similar), but this would be 'yet one thing to add to the design', so if I can do without I would like it.

  • \$\begingroup\$ Does this answer your question? how to choose level shifters for serial / UART \$\endgroup\$ Commented Feb 21, 2020 at 12:40
  • \$\begingroup\$ Part / sort of. This question is more specific, as the accepted answer points to (i.e., should be working even without any interface stuff in this specific case, though it may be not worth to take the chance) :) \$\endgroup\$
    – Zorglub29
    Commented Feb 21, 2020 at 13:34
  • \$\begingroup\$ (i.e. though I agree there is some level of overlap, the answer for the Rockblock + Teenesy seems to be that it is ok to just plug together, but I will need to use other breakouts / components that will absolutely require shifting anyways, which is why I asked both questions). \$\endgroup\$
    – Zorglub29
    Commented Feb 21, 2020 at 13:37

1 Answer 1


The Teensy will be fine with 5 V on the inputs; that's what tolerant means. The main question is whether the 3.3 V output from the Teensy will be recognised as a logic HIGH by the RockBLOCK. The answer is in the datasheet:

T/Rx High Minimum: 3.0 V

As long as the Teensy will drive the load to a voltage >3.0 V, it will be fine.

However, given the cost of the RockBLOCK module, the addition of a level shifting circuit seems like a trivial addition to ensure the correct operation. The chip you linked isn't a level shifter for logic level signals, it's for the RS-232 interface. Just use a buffer to boost up the microcontroller outputs to the 5 V you need on the RockBLOCK side.

  • \$\begingroup\$ Do you think any of the breakouts described there would do then? electronics.stackexchange.com/questions/482389/… \$\endgroup\$
    – Zorglub29
    Commented Feb 21, 2020 at 11:35
  • 1
    \$\begingroup\$ Either would be fine, but you only need to convert in one direction (3.3 V TX -> 5 V RX), so just a single transistor/FET will do it. \$\endgroup\$
    – awjlogan
    Commented Feb 21, 2020 at 11:39
  • \$\begingroup\$ Ok :) Will still use a breakout, since I have this kind of conversion problems with several chips, so as good to have just 1 breakout to solve all issues. \$\endgroup\$
    – Zorglub29
    Commented Feb 21, 2020 at 11:59

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