Interfacing 5v with 3.8v serial device?

Discussion in 'The Projects Forum' started by wannaBinventor, Sep 19, 2010.

  1. wannaBinventor

    Thread Starter Member

    Apr 8, 2010
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    I'll be using the 16F818 (at 5 volts) to interface with a serial device with a max operating voltage of 3.8 volts. I'm planning on just using a voltage divider for outputs from the PIC going to inputs on the serial device. Does anyone see a problem with this?

    BTW, I've looked and the 16F818 minimum is 4 volts, so I cant just operate it below the 3.8 max on the serial device.
     
  2. windoze killa

    AAC Fanatic!

    Feb 23, 2006
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    Check the data sheet for your "serial device". Although its maximum operating voltage is 3.8V some devices are TTL tolerent. This just means they can happily accept 5V on their inputs. As you haven't mentioned what your serial device is I can't help any more.
     
  3. wannaBinventor

    Thread Starter Member

    Apr 8, 2010
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    MRF49XA/Si4421/RFM12B (all the same IC)

    I'll give it a look. Thanks for the info.
     
  4. windoze killa

    AAC Fanatic!

    Feb 23, 2006
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    Just looking at the datasheet you are out of luck. I would suggest using voltage dividers or drive transistors to provide the right voltage. There are level shifters you can use to but depending on how many I/O's you have transistors should be enough.
     
  5. Papabravo

    Expert

    Feb 24, 2006
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    Don't use voltage dividers. The voltage depends on the current delivered to the load. I don't understand why people keep thinking this is a reliable way to go. Get with the program
     
  6. wannaBinventor

    Thread Starter Member

    Apr 8, 2010
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    I wouldn't think the I/O pins would draw that much current. I guess I assumed since it uses CMOS technology that this would be the case.

    What would you suggest. A zener clamp?

    As far as getting with the program: I'm an accountant whose never had the first second of formal electrical engineering training. Six months ago I didn't know how to wire a light switch, so excuse me if I fail to consider these things that seem childishly obvious to you.
     
  7. Ghar

    Active Member

    Mar 8, 2010
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    The datasheet pegs the I/O input current at less than 1 uA
     
  8. windoze killa

    AAC Fanatic!

    Feb 23, 2006
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    Don't take it personally. He was referring to me. I also noted his suggestion was much better. (oopppsss he didn't give one)

    As far as using voltage dividers if the values of resistors is chosen correctly there shouldn't be a problem. We have been doing it in our systems here for years and we fly planes with them. It is not the best method but it can still work. Also as I said in my first response if you use the output of your micro to turn on/off transistors with the correct resistor biasing you can get the desired affect.
     
  9. retched

    AAC Fanatic!

    Dec 5, 2009
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  10. Papabravo

    Expert

    Feb 24, 2006
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    Specifically what you want for reliable operation is a device with TTL thresholds that runs on Vcc in the range 1.65V to 5.5V and has 5V tolerant inputs when Vcc is less than 5.0V. That way you can use the same part in either direction. They come in SOT-23 packages.

    You get in the habit of using voltage dividers and you connect it to an input that draws some current and ooops.

    BTW if you search the threads you will see that I've given the same advice over two dozen times. I guess I'm just getting blue in the face. Personally I wouldn't be caught dead using a voltage divider anyplace except to create a non-precision bias level for an opamp or comparator. They have no place in any kind of logic circuit.

    OK - here is the link to my favorite part
    http://www.onsemi.com/PowerSolutions/product.do?id=MC74VHC1GT50
     
    Last edited: Sep 21, 2010
  11. tom66

    Senior Member

    May 9, 2009
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    Most CMOS devices have ESD protection diodes. Basically these short out if your input exceeds Vdd+0.3V or Vss-0.3V. Putting a 10k resistor (or so) on the input can use the diodes to your advantage, effectively clamping the voltage. It has the same problems with the potential for input current, though.
     
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