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new member, RC Time accuracy — Parallax Forums

new member, RC Time accuracy

ArchiverArchiver Posts: 46,084
edited 2004-07-03 04:11 in General Discussion
Hello Everyone,

I am using the BS2p demo board and utilizing the RC time
capabilities. Is there any way to increase the accuracy of the RC
time measurement? My results vary +/- 200 units. My R is 20k and my
C is .1 uF, and I am reading anywhere from 22k to 18k, which isn't
accurate enough for my application. Any suggestions? Thanks

Comments

  • ArchiverArchiver Posts: 46,084
    edited 2004-07-03 00:40
    The problem is NOT the BASIC Stamp -- it's your RC components. Use
    really good components and you will get more consistent results.

    Jon Williams
    Parallax

    Original Message
    From: steven_hunting [noparse]/noparse]mailto:[url=http://forums.parallaxinc.com/group/basicstamps/post?postID=P7BYAyp55d9XrrF595GVMEo2l9FzBrkoC2gBErGAKHZdGI6JSTiame4qpHPOuZdxUZu5JrNE1EV-qWc]shunting@l...[/url
    Sent: Friday, July 02, 2004 3:07 PM
    To: basicstamps@yahoogroups.com
    Subject: [noparse][[/noparse]basicstamps] new member, RC Time accuracy


    Hello Everyone,

    I am using the BS2p demo board and utilizing the RC time
    capabilities. Is there any way to increase the accuracy of the RC
    time measurement? My results vary +/- 200 units. My R is 20k and my
    C is .1 uF, and I am reading anywhere from 22k to 18k, which isn't
    accurate enough for my application. Any suggestions? Thanks




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  • ArchiverArchiver Posts: 46,084
    edited 2004-07-03 01:07
    > The problem is NOT the BASIC Stamp -- it's your RC components. Use
    > really good components and you will get more consistent results.
    >
    Jon, is it likely that the value of the components are varying that much
    in real time? Seems to me that one should at least get a stable value with
    a fixed error.
    Tom Fisher
  • ArchiverArchiver Posts: 46,084
    edited 2004-07-03 02:17
    Again, not so with a sloppy capacitor -- the probable culprit.

    Jon Williams
    Parallax

    Original Message
    From: TBooneFisher [noparse]/noparse]mailto:[url=http://forums.parallaxinc.com/group/basicstamps/post?postID=FmRzKd0rODon26htdkXcb-PVUvdHyUQ62lo5V0XA1lhjzbOTdPiXP4MWYWEXKk5puAfFuQzZdQGlp0mcQfiQUg]tboonefisher@s...[/url
    Sent: Friday, July 02, 2004 7:07 PM
    To: basicstamps@yahoogroups.com
    Subject: Re: [noparse][[/noparse]basicstamps] new member, RC Time accuracy


    > The problem is NOT the BASIC Stamp -- it's your RC components. Use
    > really good components and you will get more consistent results.
    >
    Jon, is it likely that the value of the components are varying that
    much in real time? Seems to me that one should at least get a stable
    value with a fixed error. Tom Fisher




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  • ArchiverArchiver Posts: 46,084
    edited 2004-07-03 02:20
    > Again, not so with a sloppy capacitor -- the probable culprit.
    >

    Does even a "sloppy" capacitor's value vary from second to
    second?
    Tom Fisher
  • ArchiverArchiver Posts: 46,084
    edited 2004-07-03 03:03
    Hi Steven,

    Take a look at this URL about fine points and applications of the
    RCtime command...
    http://www.emesystems.com/BS2rct.htm
    especially the part about errors and noise.

    You may be able to improve the performance by changing your circuit
    layout. Look for a better, shorter ground path for the capacitor,
    with respect to Stamp ground. Ground loop voltages generated by
    digital operation of the Stamp and other circuits create a noisy
    switching threshold that makes your RCtime value jump up and down
    erratically. It is not possible to eliminate it completely, but it
    should be good for about 8--10 bits of accuracy with good layout.
    Use a film capacitor and resistor for best stability.

    You can even accentuate the noise effect by making a long ground loop
    on your capacitor, on purpose, and pick off RCtime values modulo 100
    as a random number generator.

    -- Tracy


    >Hello Everyone,
    >
    >I am using the BS2p demo board and utilizing the RC time
    >capabilities. Is there any way to increase the accuracy of the RC
    >time measurement? My results vary +/- 200 units. My R is 20k and my
    >C is .1 uF, and I am reading anywhere from 22k to 18k, which isn't
    >accurate enough for my application. Any suggestions? Thanks
  • ArchiverArchiver Posts: 46,084
    edited 2004-07-03 04:11
    > Take a look at this URL about fine points and applications of the
    > RCtime command...
    > http://www.emesystems.com/BS2rct.htm
    > especially the part about errors and noise.
    >
    Outstanding reference, Tracy. Thanks! Especially the section
    about using RCTIME to monitor a voltage source. I will use this
    in my heater controller instead of an ADC.
    Tom Fisher
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