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5V Garmin GPS 18 LVC and Propeller? — Parallax Forums

5V Garmin GPS 18 LVC and Propeller?

Jay KickliterJay Kickliter Posts: 446
edited 2008-08-19 23:45 in Propeller 1
What is needed to interface a Garmin GPS 18 LVC with the propeller? Does it require a MAX 232? Or just some limiting resistors? I dont need to send any data to the GPS, just receive from it.

This is from the spec. sheet:

CMOS Serial Output Levels: 0 V to Vin, between 4 and 5.5 V (Asynchronous Serial, TIA-232-F (RS-232) Compatible Polarity)

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Comments

  • Mike GreenMike Green Posts: 23,101
    edited 2008-08-19 14:17
    There's a whole long thread on interfacing the Propeller (3.3V logic) with other devices with 5V logic like this one. It's one of the "sticky" ones at the top of the topic list for this forum. You may be able to get by with a 1K resistor between the Propeller pin and the GPS output pin.
  • Jay KickliterJay Kickliter Posts: 446
    edited 2008-08-19 14:26
    I'm an idiot. Thanks. But it still leaves me with the question, how is it a 0 - 5 voltage swing but listed as RS 232? I though RS 232 had a negative component.
  • Mike GreenMike Green Posts: 23,101
    edited 2008-08-19 14:35
    It doesn't say that the signal conforms to RS-232, only that the polarity is compatible. Some (non-standard) RS-232 receivers allow 5V/0V signal levels in addition to the 5V/-5V signal levels that conform to the standard.
  • Sniper KingSniper King Posts: 221
    edited 2008-08-19 19:06
    A simple answer to your question is to put a 1K resistor between the GPS TX pin and the Propeller RX pin you choose. This is only applicable if the GPS is outputting at 5v or so. This has worked on more than one of my projects. I work with OEM GPSs everyday. I need a day off!

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    ·- Ouch, thats not suppose to be hot!··


    Michael King
    Application Engineer
    R&D
    Digital Technology Group
  • Jay KickliterJay Kickliter Posts: 446
    edited 2008-08-19 19:53
    I tried testing it with 1K resistor and Prop Plug, but all I'm getting is garbled text at all baud rates, even at 4800 which is factory default. Somewhere on the internet I read that the GPS 18 is inverted, does that mean anything?
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2008-08-19 20:17
    If it's 0-5V inverted, that means that the MARK (1) level is at 0V, and the SPACE (0) level is at 5V. If you're using one of the standard serial objects, there's a setting that allows you to invert the sense of the input so you can receive these kinds of signals.

    -Phil

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    'Still some PropSTICK Kit bare PCBs left!
  • ratronicratronic Posts: 1,451
    edited 2008-08-19 20:23
    I have a Garmin gps 18 pc and I open it w/ fullduplexserial at mode 1.

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    Fix it, if it ain't broke·
    D Rat


    Dave Ratcliff· N6YEE
  • Jay KickliterJay Kickliter Posts: 446
    edited 2008-08-19 20:52
    Thanks everyone for the help. I still haven't gotten it to work, but I think I get it now. For some reason the current drain of the GPS from the 5V regulator is causing the Propeller not to work.

    David, so you do fdserial.start( rx, tx, %1, baud)?
  • Sniper KingSniper King Posts: 221
    edited 2008-08-19 20:58
    Using the "FullDuplexSerial" object...



    GPS.start(Recv,Xmit,%000000,4800)



    This is what I use exactly...

    Also try using the "GPS_IO_mini_PNAV" which I think is on the Object exchange.· this is a brilliant litle control that does all the parsing for you.

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    ·- Ouch, thats not suppose to be hot!··


    Michael King
    Application Engineer
    R&D
    Digital Technology Group
  • ratronicratronic Posts: 1,451
    edited 2008-08-19 21:52
    I used 4,3,1,4800

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    Fix it, if it ain't broke·
    D Rat


    Dave Ratcliff· N6YEE
  • Paul RowntreePaul Rowntree Posts: 49
    edited 2008-08-19 22:27
    Jay, I am working on a balloon project (as I think you are!). I used the GSP18 and a MAX233 and it worked like a charm straight away; I wanted to have a serial port available for other functions so the MAX chip was great. I used the 233 to avoid the extra caps etc. I don't know if the extra voltages that the MAX generates can be used for any other purposes. My power for the GPS is coming from the TinyTrak3+ module though, just to avoid anything that might cause the Prop to reset.

    When is launch day?

    Keep us posted!
  • Jay KickliterJay Kickliter Posts: 446
    edited 2008-08-19 23:45
    Thanks again everyone. Finally got it to work. I did as suggested, 1k resistor between GPS and Propeller, and mode %0001 for FullDuplexSerial.

    Two things for any other beginners like myself: Pay attention to the significance of the digits in FullDuplexSerial mode, I was entering %1000, when I meant %0001. And don't forget to set _clkmode. That last one is what was tripping me up and causing the GPS serial strings to get garbled.

    Paul, launch day is a ways off. We (my classmate and I) are very new to microcontrollers and amateur radio. I'd like to launch before winter, but I'm not in any major hurry. We're also going to be using a TinyTrak, 3 or 4, plus a Motorola c168i cell phone for communications.
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