5vdc mcu to 3.3vdc mcu serial comm

Discussion in 'Embedded Systems and Microcontrollers' started by bewter, Dec 29, 2008.

  1. bewter

    Thread Starter New Member

    Dec 29, 2008
    Greetings All, Simply, I have an atmega128 5vdc chip I need to serial comm to a 3.3 wifi module. I have been told I need something like a max3232 chip to do the conversion. I've read the data sheet on this chip and am not certain it meets my needs nor what would be required wiring it in. Can anyone point me in the right direction, maybe a schematic of a proper hook up?
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    You will need to see what the WiFi module requires in the way of an interface, and satisfy that.
  3. leftyretro

    Active Member

    Nov 25, 2008
    Well first you need to look over the specs on the wifi module if you can. Some 3.3v devices can accept 5vdc signals and are called "5 volt input tolerant". Those that can't, can usually be handled by either a series current limiting resistor or a two resistor voltage divider between a 5volt logic output signal pin to a 3.3volt logic input signal pin. A 3.3 volt digital output signal pin can usually wire directly to a 5 volt digital input signal pin as 3.3 volts is usually above what a 5vdc system will consider a logical one.

    The Max3232 sounds like a RS-232 to TTL level converter and is not applicable to hooking 5 volt systems to 3.3 volt systems.

  4. John Luciani

    AAC Fanatic!

    Apr 3, 2007
    You probably do not want a MAX3232. If your WIFI module does not have 5V tolerant
    inputs I would use a buffer like the 74AHC14 which does have 5V tolerant inputs.
    Power the 74AHC14 from the 3.3V supply.

    I have a schematic that shows a 3.3V level translator for an FTDI USB to RS232
    cable that uses the 74AHC14. See the "FTDI Level Translator" application hint at

    (* jcl *)
  5. Alberto

    Active Member

    Nov 7, 2008
    Connect 3 10K resistors in series and connect one end to ground and the other end to the micro (5V device) then connect the 3 V device to the 2nd 10K resistor so that you should have 20K from ground for the 3 V device and 30K for the 5 V device. this devider works pritty well and is what I use normally to interface Xbee module to pics.

    You will use max3232 only if you need to connect your 3.3 volts device to your RS232 PC port.

    Last edited: Dec 31, 2008
  6. Papabravo


    Feb 24, 2006
    What you want is a buffer that has 5V tolerant inputs when run from a 3.3V supply, AND TTL input thresholds when run from a 5V supply. A CMOS 3.3V device can meet the TTL thresholds on a device powered from 5V.

    You need something like the following:
  7. bewter

    Thread Starter New Member

    Dec 29, 2008