Trouble getting GNSS working on SIM868

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    Eivind

    I have a board with an ATSAMD21 Cortex M0 microcontroller connected to a SimCom SIM868. The MCU is at 3.0V instead of the usual 3.3V, so that it can communicate with the SIM868 without level shifters. Thus, the MCU digital outputs are 3V, and the SIM868′s are 2.8V.

    I’ve been able to use the GSM features without problem, but I am struggling with getting the GNSS features to work. I am able to communicate with the module over GPS_RXD/GPS_TXD. I also have GPS_EN connected to a digital pin on the MCU, and the result depends on how I drive this pin.

    When I drive GPS_EN to either 0V or 3.0V I receive the following from GPS_TXD:
    $PMTK010,001*2E
    $PMTK011,MTKGPS*08
    $PMTK010,001*2E
    $PMTK011,MTKGPS*08
    $PMTK010,001*2E
    $PMTK011,MTKGPS*08
    $PMTK010,001*2E
    $PMTK011,MTKGPS*08
    (etc…)
    And the unit does not respond to input.

    I get the best results I get when I disable the GPS_EN output (set it to high-impedance). In this case, I can communicate with the module. However, it does not seen to ever get a fix:

    $GNGGA,235952.800,,,,,0,0,,,M,,M,,*54
    $GPGSA,A,1,,,,,,,,,,,,,,GA,000004.800,,,,,0,0,,,M,,M,,*5A
    $GPGSA,A,1,,,,,,,,,,,,,,,*1E
    $GLGSA,A,1,,,,,,,,,,,,,,,*02
    $GPGSV,1,1,00*79
    $GLGSV,1,1,00*65
    $GNRMC,000004$GNGGA,000005.800,,,,,0,0,,,M,,M,,*5B
    $GPGSA,A,1,,,,,,,,,,,,,,SV,1,1,00*65
    $GNRMC,000005.800,V,,,,,0.00,0.00,060180,,,N*51
    $GNVTG,0.00,T,,M,0.00,N,0.00,K,N*2C
    $GPACCURACY,9999000.0*08
    $GNGGA,000006.800,,,,,0,0,,,M,,M,,*58
    $GPGSA,A,1,,,,,,,,,,,,,,9000.0*08
    $GNGGA,000007.800,,,,,0,0,,,M,,M,,*59
    $GPGSA,A,1,,,,,,,,,,,,,,$GNGGA,000008.800,,,,,0,0,,,M,,M,,*56
    $GPGSA,A,1,,,,,,,,,,,,,,$GNGGA,000009.800,,,,,0,0,,,M,,M,,*57
    $GPGSA,A,1,,,,,,,,,,,,,,$GNGGA,000010.800,,,,,0,0,,,M,,M,,*5F
    $GPGSA,A,1,,,,,,,,,,,,,,$GNGGA,000011.800,,,,,0,0,,,M,,M,,*5E
    $GPGSA,A,1,,,,,,,,,,,,,,$GNGGA,000012.800,,,,,0,0,,,M,,M,,*5D
    $GPGSA,A,1,,,,,,,,,,,,,,$GNGGA,000013.800,,,,,0,0,,,M,,M,,*5C
    $GPGSA,A,1,,,,,,,,,,,,,,$GNGGA,000014.800,,,,,0,0,,,M,,M,,*5B
    $GPGSA,A,1,,,,,,,,,,,,,,$GNGGA,000015.799,,,,,0,0,,,M,,M,,*55
    (..)
    $GPGSA,A,1,,,,,,,,,,,,,,$GNGGA,000058.799,,,,,0,0,,,M,,M,,*5C
    $GPGSA,A,1,,,,,,,,,,,,,,$GNGGA,000059.799,,,,,0,0,,,M,,M,,*5D
    $GPGSA,A,1,,,,,,,,,,,,,,,*1E
    $GLGSA,A,1,,,,,,,,,,,,,,,*02
    $GPGSV,1,1,00*79
    $GLGSV,1,1,00*65
    $GNRMC,000059.799,V,,,,,0.00,0.00,060180,,,N*57
    $GNVTG,0.00,$GNGGA,000100.799,,,,,0,0,,,M,,M,,*50
    $GPGSA,A,1,,,,,,,,,,,,,,,,,N*5A
    $GNVTG,0.00,T,,M,0.00,N,0.00,K,N*2C
    $GPACCURACY,9999000.0*08
    $GNGGA,000101.799,,,,,0,0,,,M,,M,,*51
    $GPGSA,A,1,,,,,,,,,,,,,,$GNGGA,000102.799,,,,,0,0,,,M,,M,,*52
    $GPGSA,A,1,,,,,,,,,,,,,,$GNGGA,000103.799,,,,,0,0,,,M,,M,,*53
    $GPGSA,A,1,,,,,,,,,,,,,,$GNGGA,000104.799,,,,,0,0,,,M,,M,,*54
    $GPGSA,A,1,,,,,,,,,,,,,,$GNGGA,000105.799,,,,,0,0,,,M,,M,,*55
    $GPGSA,A,1,,,,,,,,,,,,,,$GNGGA,000106.799,,,,,0,0,,,M,,M,,*56
    $GPGSA,A,1,,,,,,,,,,,,,,$GNGGA,000107.799,,,,,0,0,,,M,,M,,*57
    $GPGSA,A,1,,,,,,,,,,,,,,$GNGGA,000108.799,,,,,0,0,,,M,,M,,*58
    $GPGSA,A,1,,,,,,,,,,,,,,$GNGGA,000109.799,,,,,0,0,,,M,,M,,*59
    (…)

    The module is responsive to commands, but it never seems to get a fix on the satellites. I have tried setting the correct current UMT time with command PMTK740, and activating GPS and GLONASS with $PMTK353,1,1,0,0,0*2B, but it did not help. I am using a patch antenna connected with a u.fl connector, which is connected with an as short as possible trace to the antenna pin on the SIM868. I have not done impedance matching, but I would have thought that I should at least be able to get an approximate location.

    Question one: Why is it that the module does not seem to work when I set GPS_EN to 3V? Is it OK to set it to High-Z, as I am doing?

    Question two: What could be the reason I am not getting a location?

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