Would an LED + resistor in parallel add latency

Discussion in 'General Electronics Chat' started by BrandonNC, Jun 15, 2012.

  1. BrandonNC

    Thread Starter New Member

    Jun 10, 2012
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    Hello again all,

    I've almost got my GPS clock circuit completed and am thinking about adding another LED (in addition to my solid power LED) to indicate the pulse signal.

    I am using a Garmin 18x LVC GPS which uses a PPS wire connected to the DCD pin of my serial port. This generates a very accurate pulse at 1Hz (every second) and I am just curious if wiring an LED in parallel to this would cause any additional latency or other problems? Basically I feed the GPS 5V of power and it transmits/receives to/from a serial port connector. The PPS wire is an additional wire coming from the GPS that generates a pulse to the DCD pin.

    Any advice would be appreciated!
     
  2. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    This guy says the pulse is only 20usec wide. I doubt that the line will provide enough current to make a visible flash from an LED. You could add a one shot (e.g., LM555) to make the pulse wider, and also provide higher current drive, if needed.
     
  3. BrandonNC

    Thread Starter New Member

    Jun 10, 2012
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    Thanks for the reply. I am not sure where he got that information though, as the PPS signal on a Garmin GPS is configurable and comes at a default length of 100ms. I have increased mine to 200ms but either way (100 or 200) I am clearly able to see the LED.

    I'm just not sure if adding it to the circuit would add any latency.
     
  4. Ron H

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    Can you post a link to the Garmin datasheet, so we can see the specs on the PPS drive capability?
     
  5. ErnieM

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    Apr 24, 2011
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    You can see incredibly short flashes of light on a LED. I would not be surprised if you can see 20uS.

    I did this experiment years back blocking out a low power indicator and the results amazed me, but I forget the exact details, meaning it may have been milliseconds and not microseconds I could see from across the production floor.
     
  6. BrandonNC

    Thread Starter New Member

    Jun 10, 2012
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  7. Ron H

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    I couldn't find anything in the specs about the drive capability of any of the outputs, but if you try it and find you can drive an LED and still get decent logic levels for whatever else the PPS is used for, consider this:
    It will almost certainly add no more than a few nanoseconds of delay. Considering that its timing is accurate to within 1 usec, then it might degrade the maximum error to a whopping 1.01 usec. :rolleyes: On the other hand, it might improve the error by a tiny amount.
    I don't think I would worry about latency. I would be more concerned with its ability to provide enough current for your LED.
     
  8. BrandonNC

    Thread Starter New Member

    Jun 10, 2012
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    Alrighty, thanks Ron!
     
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