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Strange Pin Problem — Parallax Forums

Strange Pin Problem

ReiserificKReiserificK Posts: 24
edited 2009-04-10 15:41 in Propeller 1
Hello everyone, I recently built my second protoboard setup, and have been having problems with the parallax GPS receiver. Instead of 1K current limiting resistors in series on the signal lines to the GPS and other 5v devices (first revision), there are 2.2K (current revision). Everything works correctly with the new 2.2k resistors, except for the GPS. After trying out a second identical GPS receiver, I tried running the same code on both of my protoboards. The 1k one works fine, the 2.2k does not. In this thread (http://forums.parallax.com/forums/default.aspx?f=25&m=124837) Chip Gracey said on a bi-directional signal line, a series resistor should work. Since the GPS module is supposedly TTL compliant (threshold voltage: 1.5v), I am very confused confused.gif . It seems like I could either a) have a faulty board b) have messed something up during soldering or c) the 2.2K resistor is causing a serial communication problem.

Any thoughts? Thanks in advance.

Edit: Forgot to mention that the GPS is able to get a fix on satellites, suggesting that the power connections are fine.

Post Edited (ReiserificK) : 4/9/2009 9:47:25 PM GMT

Comments

  • Cole LoganCole Logan Posts: 196
    edited 2009-04-09 22:09
    Looking at the diagram it looks like the GPS might be putting out a 3.3v signal. If this is the case then the higer resistance might not allow enought current to flow.

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  • ReiserificKReiserificK Posts: 24
    edited 2009-04-09 22:32
    Interesting. When I get a change (maybe in an hour) I'll try adding some new connectors.
  • ReiserificKReiserificK Posts: 24
    edited 2009-04-09 23:54
    I checked the output level of the GPS module. Turns out its around 4.2 v
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2009-04-10 02:05
    The problem is more likely to be the low voltage, not the high voltage. The GPS module uses a 4.7K pullup to +5V. When this is pulled down through a 2.2K resistor, the voltage on the SX pin will be about 1.6V. This may not be low enough to register as a logic "low" on the SX's TTL-programmed inputs.

    The obvious thing to try is to reprogram the GPS's SX for a CMOS input on the communication pin — either that, or replace the 4.7K pullup on the GPS board with a 10K resistor.

    -Phil
  • ReiserificKReiserificK Posts: 24
    edited 2009-04-10 15:41
    Thanks Phil, the logic-low voltage never crossed my mind!

    I changed the 2.2k resistor to a 1k and it works now. Thanks for the advice everyone.
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