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AM335x interface to MAX202 RS232 IC - Are the Voltages Compatible?

Brian Weir
Intellectual 810 points
Other Parts Discussed in Thread: MAX202, MAX3232

My  VDDSHV6 voltage on the AM335x is 3.3v, according to the spec the maximum voltage on the IO pins is  VDDSHV6 +.3v (3.6v).  The corresponding output voltages of the MAX202 RS232 IC are VCC +.3v (VCC is 5v) according to it's spec.  I did have my AM335x schematic reviewed by TI engineering and this was not mentioned as a problem.  Am I misunderstanding the spec's and this is not a concern?

Thanks,

Brian

  • 0
    TI__Guru*** 125430 points

    I agree with you.  Sounds like MAX3232 would be more appropriate.

  • 0
    TI__Guru**** 393215 points
    Brian,

    AM335x I/O pins are not fail-safe. Interfacing to 5V levels will probably damage the processor. Check section 5.1 from the AM335x Datasheet Rev. J, parameter "Steady state max voltage at all IO pins" and Note 8 below the table.
  • 0 in reply to Brad Griffis
    Intellectual 810 points

    Thank you Brad for the MAX3232 recommendation.  I see in the spec that besides being able to operate at 3.3v it requires less power supply current (1ma vs. 8ma, no load)  apparently a more efficient design than the 5v only MAX 2021.  But, however for the same -40/+85 temp range the MAX3232 is $0.95 more expensive (we have two in our design).  I am toying with a much less expensive approach of adding a series resistor on the drive line (input to the AM335x) with a 3.6v clamp diode to protect the AM335x input.  What do you think?

    Of interest, we have had a previous design in the field now for about 2 1/2 years with MAX2021's connected directly to the AM335x with no problems yet.  Also, we have our digital inputs clamped at 4.7v (also too high per the AM335x spec) again with no problems yet. The TI Engineer that reviewed our most recent design did make note of the 4.7v clamp being too hi.  So, the AM335x inputs must be more robust than the spec suggests.     

    Thanks, Brian 

  • 0 in reply to Biser Gatchev-XID
    Intellectual 810 points
    Thank you Biser, I already read that in the AM335x spec, but we have had this sort of 5v overdrive on our AM335x inputs for some time in our current design with no apparent problem. But, we are spinning our board now and wanted to get the experts opinion.
    Regards, Brian
  • 0 in reply to Brian Weir
    TI__Guru*** 125430 points

    I think that would work.  For communication going AM335x -> MAX202, the Vih, min is 2V for the MAX202, so the 3.3V signaling should still work fine in terms of the MAX202 being able to properly receive the data.  It was the MAX202 -> AM335x path that was problematic.  So yes, if you can simply clamp that output (for less money), then I don't see any issue there.  I would target 3.3V for your clamp voltage.  That way if you have some overshoot you'll still be within spec.