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Pull-Up Resistor Question — Parallax Forums

Pull-Up Resistor Question

felixtcpipfelixtcpip Posts: 1
edited 2007-09-10 14:22 in BASIC Stamp
Hi,

I'm a bit confused on how to connect a Pull-Up Resistor, and it's purpose. I'm trying to connect the oled-128-gmd1 to my bs2 and the instructions say:
Instructions said...
The BASIC Stamp requires 10K Pull-Up Resistors on the TX and RX lines. These should be installed on the BASIC Stamp side of the connection.

Does that mean that I need to place a 10K Resistor between the TX-RX RX-TX lines of the Oled and BS2?

Also, what exactly are these 10K Resistors doing?

Thank you very much!
Felix

Comments

  • D FaustD Faust Posts: 608
    edited 2007-09-01 14:03
    You need to connect the 10K resistor between the line and Vdd.· Tx-resistor-vdd, Rx-resistor-Vdd. This pulls the pin to a logic level 1 when no information is being sent.· Otherwise the pin would float and you would get strange readings.

    TX and RX are not connnected with pull up resistors (well not directly, but·they are both·at Vdd)


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ON subnum GOTO Hope_this_helps, Thanks!, WOW!!
END
  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2007-09-04 15:59
    Hello, a connection diagram is being created right now and should be on our website later today on the uOLED product page. Take care.

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    Chris Savage
    Parallax Tech Support
  • quispqixquispqix Posts: 64
    edited 2007-09-05 20:26
    did you post that connect diagram yet?

    heres how I got mine working,...
    P10.........^^^10K^^^.......Vcc
    |
    |
    |
    Rx on uOLED


    P11.........^^^10K^^^.......Vcc
    |
    |
    |
    Tx on uOLED

    hey! this don't post correct! the connect should drop off the 10k resistor! check the attachment
    However I did not check the read back of the later!

    This is a beautiful display!

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    Keeping the price of milk competitive with gasoline!
    skype me: quispqix
    echolink: 184783 KC0LGV

    Post Edited (quispqix) : 9/5/2007 10:10:15 PM GMT
    bmp
    bmp
    10kmap.bmp
    838K
  • MarkSMarkS Posts: 342
    edited 2007-09-05 22:00
    I have a more specific question. If the data sheet does not give the resistor value for a pull up or pull down resistor, how do you determine it's value? I'm assuming that Ohm's law is still in effect, but what specific figures do you use to find out the value? R = E / I, so I'm assuming that you take the pin's source/sink current and the voltage difference you're aiming for and use these figures, correct?
  • NewzedNewzed Posts: 2,503
    edited 2007-09-05 22:06
    Mark, the rule of thumb is that about 4.7K is a strong pullup and 10K is a weak pullup.· If your triggering signal is a full 5VDC then use a 4.7K.· If the triggering singal is 3 or 4 volts, or if the current of the trigger is low level, less than .5ma, then use a 10K.

    Sid

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    Yesterday is history, tomorrow is a mystery, and today is a gift.

    That is why they call it the present.

    Don't have VGA?
    Newzed@aol.com
    ·
  • MarkSMarkS Posts: 342
    edited 2007-09-05 22:12
    Thanks for the tip, but I would still like to know how to arrive at those figures.
  • NewzedNewzed Posts: 2,503
    edited 2007-09-05 22:34
    Mark, just remember that if a pullup is brought to ground, you do not want the current through the resistor to exceed 2 or 3ma.· A 4.7K pullup will pass about 1 ma when grounded.· And remember that if your pullup is weaker than 10K, your circuit will be subject to triggering by noise or EMI.· That can even happen with a 10k if you are working in a noisy environment.· I have a lot of interference sources on my desk, so I usually use a 4.7K for 5VDC components unless the datasheet or other sources specifically recommend 10K.

    Sid

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    Yesterday is history, tomorrow is a mystery, and today is a gift.

    That is why they call it the present.

    Don't have VGA?
    Newzed@aol.com
    ·
  • JC68JC68 Posts: 19
    edited 2007-09-05 23:48
    i can't get the display to show anything but the built in demo screen.· is there any more info available.· the .pdf doesn't mention the 10k pull ups and the .bs2 doesn't mention the 1k on RX so...?


    Chris Savage (Parallax) said...
    Hello, a connection diagram is being created right now and should be on our website later today on the uOLED product page. Take care.

  • JC68JC68 Posts: 19
    edited 2007-09-05 23:53
    ok i got it. it's pretty much what quispqix said

    thanks!!
  • quispqixquispqix Posts: 64
    edited 2007-09-06 03:29
    reading voltage on the P's from Vss to the pulled up 10K P11 I am reading 3.3 Volts (as low as as 3.26V) reading on the cycle of uOLED Demo 3 on P10 It tends to a straight 3.0 Volts in cycle however it slightly fluxuates on after downloading a new BS2 program.
    I really do not want to try another resistor value to see any change.

    en.wikipedia.org/wiki/Pull-up_resistor

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    I can never find anything to eat at break-time!
    skype me: quispqix
    echolink: 184783 KC0LGV

    Post Edited (quispqix) : 9/6/2007 3:41:56 AM GMT
  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2007-09-06 15:54
    Sorry everyone, the diagram is being worked on today...it was unexpectedly delayed.

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    Chris Savage
    Parallax Tech Support
  • DufferDuffer Posts: 374
    edited 2007-09-07 05:38
    This might help.

    Note also, the current limiting 1K resistor in series between the Stamp Tx and the OLED Rx. This is REQUIRED for safe operation on a 5V logic system like the Stamp.

    steve
    DSCN0177.JPG
    2048 x 1536 - 506K
  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2007-09-10 14:22
    Hello everyone...The connection diagram/schematic has been posted to the product web page linked below. I hope this helps everyone. Take care.

    http://www.parallax.com/detail.asp?product_id=27925

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    Chris Savage
    Parallax Tech Support
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