Shop OBEX P1 Docs P2 Docs Learn Events
5v to 3v — Parallax Forums

5v to 3v

DorlingDorling Posts: 32
edited 2007-05-23 20:24 in General Discussion
I am connecting the javelin up to the Linkmatik (by flexipanel). Linkmatik operates at a Vdd of 3.3v. javelin operates at a Vdd of 5v. It needs 2 UART, 2 output and one input to connect to it where Output highs are Vdd and Input highs must not exceed Vdd.

How can I do this?

Many Thanks

Jonathan

Post Edited (Dorling) : 5/16/2007 11:42:27 PM GMT

Comments

  • FranklinFranklin Posts: 4,747
    edited 2007-05-17 01:28
    Take a look in the propeller forum. The propeller works on 3.3v and there is some discussion on connecting to 5v. If your chip is 5v tolerant you can use a current limiting resistor or connect the two with a mosfet, transistor or optoisolator.

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    - Stephen
  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2007-05-17 01:30
    Franklin has a point but the Propeller is 5V tolerant when you limit the current to the pins. Without knowing the electrical specifications of the Linkmatic you would have to go with a standard solution of you risk damaging the unit. There are converter chips, but we don’t have any manufacturer part numbers.

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    Chris Savage
    Parallax Tech Support
  • PJAllenPJAllen Banned Posts: 5,065
    edited 2007-05-17 01:31
    Attached is a re-drawing of Beau Schwabe's 3V-to-5V interface.
    ¿Eso es OK?

    Post Edited (PJ Allen) : 5/17/2007 1:38:03 AM GMT
    3V_5V.JPG
    363 x 284 - 9K
  • DorlingDorling Posts: 32
    edited 2007-05-21 20:12
    thanks for all your great reply

    PJ Allen. I need to go from 5v to 3.3v to do you have a drawing of that pleas

    Many Thanks

    Jonathan
  • Mike GreenMike Green Posts: 23,101
    edited 2007-05-21 20:41
    Beau's drawing is actually for 3.3V, but should work for voltages over a fairly wide range, say from 3V at least to 4V. The output voltage to the 3V terminal is roughly 2.7V for a logic high and roughly 0.6V for a logic low depending on the currents involved.
  • DorlingDorling Posts: 32
    edited 2007-05-21 22:13
    I am a bit lost here. This Beau Schwabe's does it work both way 3v to 5v and 5v to 3v?

    Is it negative logic?

    Is there any more information on the web about this about how it works?

    Many Thanks

    Jonathan
  • Aaqil KhanAaqil Khan Posts: 60
    edited 2007-05-22 14:48
    Hi Dorling,

    I've done a similar thing in one of my robotics projects, where I had to interface a BS2 microcontroller with a 3.3V XBee from Maxstream. I used level translator ICs to communicate between 5V and 3.3V circuits. What you need is:

    5V -> 3.3V : 74LVC245 IC

    3.3V -> 5V : 74HC245 IC

    I was able to google a schematic for your reference: http://ciam.dyndns.org/~vitamin/tof/AXIC/arduinoxbeebuffered.jpg

    Hope this helps.

    Cheers,

    Aaqil

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    E=mc^2
  • Mike GreenMike Green Posts: 23,101
    edited 2007-05-22 14:53
    Dorling,
    1) It's positive logic: 0V for logic low, anything over about 2.7V for logic high

    2) It works both ways.

    3) Using the 74HC245 and 74LVC245 is faster and provides for several signal lines (they're octal parts), but these parts are unidirectional (which may not matter). Beau's circuit switches more slowly, only provides a single signal line (unless you have multiples of the circuit), but is bidirectional.

    4) Some companies (like TI) now have high speed bidirectional level shifters. See the "sticky thread" in the Propeller forum for examples and links.

    Post Edited (Mike Green) : 5/22/2007 3:00:52 PM GMT
  • Aaqil KhanAaqil Khan Posts: 60
    edited 2007-05-23 14:58
    Hi,
    I looked up TI's website for bidirectional level shifters. For those interested, heres a datasheet for the SN74LVC1T45

    http://focus.ti.com/lit/ds/symlink/sn74lvc1t45.pdf

    Aaqil

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    E=mc^2

    Post Edited (Aaqil Khan) : 5/23/2007 3:07:02 PM GMT
  • DorlingDorling Posts: 32
    edited 2007-05-23 17:19
    Aaqil,

    Many Thanks for the for the link and the IC what could be used.

    It state in the schematic that the need for capacitor (http://ciam.dyndns.org/~vitamin/tof/AXIC/#proto1) but doesn't state which type are thy electrolytic or ceramic?

    Many Thanks

    Jonathan
  • Mike GreenMike Green Posts: 23,101
    edited 2007-05-23 17:37
    Generally, capacitors from 1uF and higher are electrolytic while capacitors lower than 1uF are ceramic (or mica or mylar insulated). There are exceptions, but this is a fairly good rule.
  • Aaqil KhanAaqil Khan Posts: 60
    edited 2007-05-23 20:24
    The 100nF are basically bypass capacitors used to filter noise in the power lines. Generally use monolithic ceramic capacitors with 50Volt +-20% rating. Electrolytic capacitors, like Mike mentioned, are typically larger values and not suitable for high frequency signals.

    Heres an interesting article if you're looking to learn more about bypass caps:
    www.seattlerobotics.org/encoder/jun97/basics.html

    Hope that helps.

    Aaqil

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    E=mc^2
Sign In or Register to comment.