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High value pullup resistor too noisy?

Discussion in 'General Electronics Discussion' started by eem2am, Aug 7, 2012.

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  1. eem2am


    Aug 3, 2009

    We have four different flashing LED lamps each mounted in different places on a lorry.

    The Lamps must FLASH ON together, in synchrony, and so therefore they need communication between them.

    This communication consists of a single wire, which connects into a microcontroller pin in each lamp.

    Here is the basic schematic of the situation, showing the microcontroller pin, and the interfacing resistors which are needed to allow the communication to work. (There is also an R/Zener protection in case the power positive cable is connected to the SYNC connection.


    The microcontrollers each transmit a 10ms low pulse on their SYNC port every 800ms. When not transmitting the low pulse, the microcontroller SYNC pin is made high impedance, as an input, and an 82K resistor pulls the SYNC port up to the rail.

    So basically, to stay in sync, the microcontrollers each send sync pulses, and if any microcontroller “hears” a logic low sync pulse, then it synchronises up to it, -by immediately flashing its lamp at that time.
    -this way, the lamps stay pretty much flashing in sync with each other.
    …the microcontrollers have oscillators which run at the same frequency but they still would get out of sync due to oscillator tolerance, so this sync’ing is necessary.

    ( each microcontroller has a “non-listening” time either side of sending their sync pulse, otherwise they would be constantly re-syncing.)

    Anyway, the question is, how high Ohmic value can the pullup resistor be before noise starts affecting the input?

    I have made it 82K here, and if just one microcontroller is outputing a low pulse, , whilst the other three are inputs (“listening” for a sync pulse), then the “listening” microcontrollers see a voltage level of 0.3V at their inputs, which is well below the microcontroller’s maximum logic low level of 0.8V.

    ……But could I make the pull-up resistor even higher and still not suffer from noise?

    How high value would the pull-up resistor be before noise started to affect the input reading?

    (We need to know because we’d like to have more then four lamps sync’ing with each other.)
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    you are probably better off having a line driver output (something capable of significant current), and a terminating resistor (say 100 ohms) at the end of the cable. The low impedance will tend to minimise noise.

    You don't say how long the cable is, but I'm assuming it's pretty long and in an (electrically) noisy environment.

    You can also help things by limiting the slew rate of your Sync signal. If you don't, you may have problems with the generation of your own noise!
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