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Help With College project - Comms Level Shift

Discussion in 'Electronic Design' started by Kirky, Jan 2, 2005.

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  1. Kirky

    Kirky Guest

    I am working on my college project in which I need to perform a level shift
    but am unsure where to start.

    The levels are as follows


    Unit 1 Unit 2

    Logic 0 -0.6v 0v
    Logic 1 +5 to +6 2.5 to 4.1v

    Unit 2 Unit 1

    Logic 0 0 to 0.3v 0 to 0.8v
    Logic 1 +0.7v to 3.3v 0.7v(min 0.5mA)

    The Supply voltage I am working from is 3.6v.

    Any help would be much appreciated.
     
  2. Jim Thompson

    Jim Thompson Guest

    Which ones are SENDING levels, and which are RECEIVING? And provide
    some information about source impedances, loading impedances and speed
    required.

    ...Jim Thompson
     
  3. Kirky

    Kirky Guest

    Baud Rate is 1200, Unit 1 is a Data Logger which we use at work which has
    OPTO coupled comms lines and Unit 2 is a National Semiconducter LMX9820
    Blue-Tooth serial port module.

    Sorry for being vague but im a novice at the moment. Ive got the datasheet
    for the LMX9820 but am unsure as to what the impedances are.

    Unit 1(S) to Unit 2(R)

    Logic 0 -0.6v 0v
    Logic 1 +5 to +6 2.5 to 4.1v

    Unit 2(S) to Unit 1(R)

    Logic 0 0 to 0.3v 0 to 0.8v
    Logic 1 +0.7v to 3.3v 0.7v(min 0.5mA)

    In each case the Sending(S) voltage needs to be converted to the received(R)
    voltage.
     
  4. Fred Bloggs

    Fred Bloggs Guest

    Unit 1(R) Logic 1 makes no sense- you say 0.7V?
     
  5. Kirky

    Kirky Guest

    sorry my mistake, that should be 2v.
     
  6. Fred Bloggs

    Fred Bloggs Guest

    First off, the minimum baud rate on the LMX9820 is 2400 baud-
    you might want check for compatibility with your data logger. From your
    specs, the data logger appears to be running off Vcc=5V, and the
    Bluetooth interface is running off 3.6V. The Bluetooth receive looks to
    be CMOS leakage only- it will not load your data logger at all. You will
    need to reduce the data logger drive to the LMX9820 UART_RX/ _CTS pins.
    The LMX9820 Vih threshold is 0.7 x IOVCC, so that if IOVCC is 3.6V, this
    means a minimum input of 2.5V. The Vil is 0.2xIOVCC or 0.72V maximum.
    You can run the UART_TX and _RTS pins of the LMX9820 directly to the
    data logger. Something like this should work, the diodes should be BAS
    Schottky types- they come dual in little SOT packages:
    View in a fixed-width font such as Courier.




    IOVCC >------+
    |
    [10K]
    |
    UART_RX <-----+--|>|--[100]--<
    |
    |
    LMX9820 SIDE GND--|>|-+
    FROM DATA LOGGER


    IOVCC >------+
    |
    [10K]
    |
    UART_CTS <-----+--|>|---[100]--<
    |
    |
    GND--|>|-+




    IOVCC---|<|-+
    |
    |
    UART_TX >------+-[100]--------->
    | TO DATA LOGGER
    |
    |
    GND--|>|-+




    IOVCC---|<|-+
    |
    UART_RTS>------+-[100]--------->
    |
    |
    |
    GND--|>|-+
     
  7. Kirky

    Kirky Guest

    thanks for that, much appriciated.

     
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