3V ICs in a 5V circuit

Discussion in 'General Electronics Chat' started by aeroguy, Jul 28, 2011.

  1. aeroguy

    Thread Starter Member

    Sep 16, 2009
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    Howdy,

    I appologize for the extreme newbee question I'm about to pose.

    I am planning a controller circuit based around a PIC18, that will include an LCD display which operates at 5V.

    However, I want to make use of one of Microchip's RF transceiver modules which operates at 3V.

    The term "voltage divider" comes to mind.

    Before I start down a frustrating path, is it uncommon or ill-advised to attempt to mix components that require differing operating voltages into the same device?

    Digital communication between the PIC at 5V and the RF module at 3V will be required. It's not clear to me how that will work...

    I presume that this sort of topic has been discussed before, but I don't know what key phrases to search for.

    Thanks in advance,
    Eric
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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  3. aeroguy

    Thread Starter Member

    Sep 16, 2009
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  4. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    Heya Eric. Actually, today is is very common and well advised to mix components with different logic levels, and the world has not (will it ever?) make a solid transition on all logic going to 3.3V

    Generally using a voltage divider is not recommended though it can work going 5->3.3V. I just wouldn't use it for production.

    Similarly there are schemes using a diode and a resistor to go from 3.3->5V. Again, I'd toss in the chip to translate.

    Translation chips come in all makes and sizes, from single bit SOT-5's to 8 (or more probably) bits with mixes in direction. I've even seen one chip that is 2 bits wide, and has no direction control: it self-senses what side the input is on and adjusts itself (but it doesn't work at 5V).

    Do check the data sheets for the specific devices you are using. Some PICs running 3.3V have 5V tolerant pins, meaning you need not translate down on those pins (it's built in).

    The MRF24WB0MA/MB RF Transceiver Module (not sure which you use) allows up to 5.5V on the SDI, CS, and SCK pins, but will sense the over voltage and shut itself down after 0.5mS.
     
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