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Another 3.3V I2C to 5V I2C Topic — Parallax Forums

Another 3.3V I2C to 5V I2C Topic

DavidMDavidM Posts: 626
edited 2008-04-21 06:59 in Propeller 1
HI,

I have made ( and I am using ) the following circuit and I am using this with the PROP and a 5V I2C LCD ( the MODTRONICS 2X16 LCD one I have mentioned in a previous post)

It seams to work but I would like an expert opinion on it as I have seen other solution involving level shifter IC's and mosfets.

But note, I did not have any 9K resistors so I am using 10K instead.


Thanks


Dave M
pdf
pdf
5VI2C TO 3.3V I2C CIRUIT REV-1.pdf
21K

Comments

  • JavalinJavalin Posts: 892
    edited 2008-04-16 13:05
    Well I don't know if I qualify as expert but thats fine, bar I usually pull them to 3.3v with a 4.7k. Either will work. The resistor level in the pull up really is determined by whats on the bus.

    J
  • stevenmess2004stevenmess2004 Posts: 1,102
    edited 2008-04-17 12:39
    If it is working and you don't have any problems than just use it. Depending on the chip the voltage requirements for a logic 1 will be different. In some chips you need more than 3.5V for a logic 1.

    Its probably a good idea to keep the pullups connected to the 5V as this will help ensure that the voltage is greater than whats required for a level 1. See this thread http://forums.parallax.com/showthread.php?p=721553 for some calcs on what voltages you will get. The spec sheet for you device should have the levels you need if you are worried about it.
  • DavidMDavidM Posts: 626
    edited 2008-04-18 22:38
    HI,

    I built a small vero-board adaptor with this circuit to test and it seems to work,

    But my LCD which is 5V also seems to work running the I2C buss at 3.3v ? go figure?


    Regards


    Dave M
  • JavalinJavalin Posts: 892
    edited 2008-04-19 19:47
    no not really - digital IO just has a threshold between a "1" and a "0". I.e. vdd/2 for the propeller (3.3v/2=1.65v), so below is a "0" and above is a "1".

    So in the case of a 5v - (5v/2=2.5v) a 3.3v level will read as a "1" and obviously the 0v will read as "0".

    James
  • stevenmess2004stevenmess2004 Posts: 1,102
    edited 2008-04-19 22:45
    Javalin, not quite. There is a range in which the input could be read as either a 1 or a 0. On a prop this is specified as 0.3Vdd to 0.7Vdd. The actual range is often smaller than this though. In this thread http://forums.parallax.com/showthread.php?p=719164 someone actually measured these voltages.
  • JavalinJavalin Posts: 892
    edited 2008-04-21 06:59
    I stand corrected. Sounds like I was correct - in that they were aiming for a vdd/2 - but missed a bit. So the above post should read:

    ..... i.e 1v for the Propeller, so below is ....

    J
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