3.3V -> 1.8V level shifter

Discussion in 'General Electronics Chat' started by bumclouds, Feb 16, 2012.

  1. bumclouds

    Thread Starter Active Member

    May 18, 2008
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    Hi everyone,

    I'm trying to connect a device which uses 3.3V logic to the input pins of a microprocessor that can only tolerate 1.8V.

    So I need to step down the signal from 3.3V to 1.8V.

    Can anyone on this forum recommend a common and easy to find IC which does this job? DIP package is the only option for me here.

    Thankyou!

    - Andrew
     
  2. kubeek

    AAC Fanatic!

    Sep 20, 2005
    4,670
    804
    What speed are we talking? A simple resistive divider should do the job for the 3.3->1.8v conversion. The other way, most devices nowadays accept anything greater than 0.8V as a logical 1, so you should be safe there, but check the datasheet.
     
  3. crutschow

    Expert

    Mar 14, 2008
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    Not quite. The maximum logic 0 it often 0.8V or less but the minimum logic 1 is usually at least 2-3V. The voltage in between those two levels is a no-man's-land where the circuit may interpret it as either a 1 or a 0 depending upon the particular circuit tolerances.

    But that still means that a 3.3 logic high will be correctly interpreted as a logic 1 by 5V logic.
     
  4. kubeek

    AAC Fanatic!

    Sep 20, 2005
    4,670
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    You're right, I misinterpreted the datasheet I was reading, the actual thresholds for 3.3Vcc were <0.6V for 0 and >2V for 1.
     
  5. steinar96

    Active Member

    Apr 18, 2009
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    A resistive divider may cause problems due to either too high input resistance (if its too stiff) or waste of power (too soft). There are many other ways to translate while maintaining proper conditions at the IO pins. These include diode clamping circuits.
     
  6. crutschow

    Expert

    Mar 14, 2008
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    If you can tell us the frequency and/or pulse width of the signals we can better recommend a solution.
     
  7. bumclouds

    Thread Starter Active Member

    May 18, 2008
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    The device I am using is a PSX game controller which (i think) has a baud rate of 9600kbps.

    The device I am using to read from the game controller is a Texas Instruments microprocessor whose input pins are not 3V tolerant (confirmed on the TI forums).

    Will using a simple resistor divider for each pin do the trick?
     
  8. Papabravo

    Expert

    Feb 24, 2006
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    ON Semiconductor and Fairchild have a line of single gate parts in SOT-23 packages that work on any Vcc from 1.65 V to 5.5V and have asymmetrical "TTL like" thresholds. This is in contrast to "CMOS like" thresholds at Vcc/2

    I can dig up links to actual datasheets if you are interested.
    http://www.onsemi.com/PowerSolutions/product.do?id=MC74VHC1GT50
     
  9. Brownout

    Well-Known Member

    Jan 10, 2012
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    I think you should be fine.
     
  10. crutschow

    Expert

    Mar 14, 2008
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    3,229
    A voltage divider of about 5K and 5k (to ground) should work well.
     
    Last edited: Feb 17, 2012
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