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A couple basic questions regarding a ADC

Discussion in 'Electronic Basics' started by Michael Noone, Apr 24, 2005.

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  1. Hi - I'm working on interfacing a TI ADS1252 ADC (http://www-
    s.ti.com/sc/ds/ads1252.pdf) to an Atmel AVR (an 8b microcontroller, an
    ATMEGA48). This one was marked as having SPI, which I thought would make it
    easy to interface with. Now that I'm sitting down and wiring it up, I have
    noticed something very strange. There are only *two* digital io lines,
    while SPI typically has three. Can anybody tell me if the built in SPI
    device in an AVR will be able to handle this?

    Also - it asks for a clock input, but it only has one pin for the clock.
    I'm used to just attaching a crystal and two capacitors for a clock signal,
    but that doesn't exactly seem possible with only one clock pin. So - how
    would I go about getting a 16Mhz clock signal?

    Thanks for your help,

    -Michael J. Noone
     
  2. If you are not hooking any other devices to the AVR's
    SPI port, it can be made to work. If the SPI bus is
    shared among devices, you have some work to do
    getting that thing to be quiet until told to speak up.
    You can buy crystal oscillators at that frequency.
    You're welcome.
     
  3. Right now it'll be the only device on the SPI bus, so hopefully that
    won't be an issue.
    Yes - heck I think I even have some 16Mhz crystals lying around. But my
    problem is that I'm used to connecting two leads from a crystal to two
    pins on an IC. Now there is only one pin - thus my confusion. Perhaps do
    you just have one pin from the crystal connected to the clock pin and
    also connected to a grounded capacitor, and then have the other crystal
    pin only connected to a grounded capacitor? Thanks,

    -Michael J. Noone
     
  4. If the pin only has one input clock pin then it is expecting a
    single-ended signal input and cannot be connected directly to a crystal.
    Some chips have two pins and allow you to connect a crystal directly
    (with appropriate external passives). The difference is that the
    feedback circuitry is inside the chip in the latter case.

    A device that provides that feedback circuitry is a called an oscillator
    (or crystal oscillator). Its output is at some logic level (see
    datasheet of specific oscillator for this info). This sounds like what
    you need. Make sure you pick one that matches the input requirements
    for the chip you want to connect e.g. input voltage level, jitter, ...

    James.
     
  5. John Fields

    John Fields Guest

    ---
    You need to supply a 16MHz clock signal to the CLK input. if you don't
    already have that in your system, then you need to buy a 16MHz crystal
    oscillator. Check Digi-Key, they've got bunches. Here's one:

    http://www.digikey.com/scripts/DkSearch/dksus.dll?Detail?Ref=182864&Row=272631&Site=US
    ____
    As for the operation of SCLK and DOUT/DRDY it's all spelled out in the
    data sheet at:

    http://focus.ti.com/lit/ds/symlink/ads1252.pdf
    ____
    Basically, you wait until DOUT/DRDY goes low, (which will signal the
    end of a conversion, then you exercise SCLK and data will come out of
    ____
    dout/DRDY.
     
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