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Does anyone see a problem with Interfacing a Propeller to 5V devices through PC — Parallax Forums

Does anyone see a problem with Interfacing a Propeller to 5V devices through PC

Marc GebauerMarc Gebauer Posts: 60
edited 2007-02-28 23:58 in Propeller 1
I have been studying the datasheet for the PCF8574 and see no reason why I can't feed it's VDD·with +5V,·have it's I/O·pins connect·to other +5V devices,·yet directly connect the SDA and SCA to my Propeller with pullups to +3.3V. The PCF8574's·SDA line, when it is an output, only syncs current to 0V (nothing drives to +5V). I have attached a drawing for you to see what I am talking about.

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pdf
pdf
Interface Propeller to 5V devices with PCF8574.pdf
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Comments

  • crgwbrcrgwbr Posts: 614
    edited 2007-02-28 23:40
    The prop is 5v tolerent. Therefore it should be fine (If the logic level of the PCF8574 is below 3.3v).

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  • Paul BakerPaul Baker Posts: 6,351
    edited 2007-02-28 23:45
    crgwbr said...
    The prop is 5v tolerent. Therefore it should be fine (If the logic level of the PCF8574 is below 3.3v).

    The Propeller is not 5V tolerant. The Propeller can drive the input of a 5V device, however you must protect the input of the Propeller when receiving a 5V input, there is a sticky thread that explains this.

    5V tolerant means that no special steps need to be taken to interface.

    When you are interfacing via an open collector output situation (such as I2C where you are using pullup resistors on SDA and SCL) connect the pull-ups to 3.3V and you should be able to interface the two.

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    Paul Baker
    Propeller Applications Engineer

    Parallax, Inc.

    Post Edited (Paul Baker (Parallax)) : 2/28/2007 11:50:07 PM GMT
  • Mike GreenMike Green Posts: 23,101
    edited 2007-02-28 23:54
    As you noted, SCL is an input to the PCF8574 and SDA is an input and open-drain output which ought to work when the PCF8574 is powered from 5V. Unfortunately, the PCF8574's I2C inputs are not spec'd for the switch threshold for logic inputs. VIH is defined as 0.7Vcc or 3.5V when powered by 5V. That doesn't say whether it would be happy with say 2.5V for a logic high. The Propeller's outputs are likely to get to maybe 2.6V for a logic high. Whether that's high enough for reliable operation is anyone's guess. The "sticky" thread on using 5V devices with the Propeller has several suggestions for interfacing to 5V I2C devices. You can do it with as little as an MOSFET and a couple of resistors (per line).
  • crgwbrcrgwbr Posts: 614
    edited 2007-02-28 23:55
    huh...guess I'l have to be more carefull with my prop from now on. Seems like I remeber someone saying that it was 5v tolerent.

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    NerdMaster
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  • Paul BakerPaul Baker Posts: 6,351
    edited 2007-02-28 23:58
    Jon Williams stated that when the Propeller first came out, but you'll notice he mentioned using a 1k resistor. This is not the technical definition of 5V tolerant, 5V tolerant I/O means you can feed it directly into the device without ill effect.

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    Paul Baker
    Propeller Applications Engineer

    Parallax, Inc.
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