No Longer Recommended: 20 microseond-accuracy UTC clock via the USGlobalSat MR350P GPS

No Longer Recommended: 20 microseond-accuracy UTC clock via the USGlobalSat MR350P GPSEdit

Schematic for MR350P GPS to EVAL232R / Beaglebone connections

Note: this schematic had previously incorrectly reversed the TX and RX lines.  This was corrected 2014 April 2.

The layout shows the view of the end of the plug on the MR350P GPS, which is the same view as on the inside of the mini-din jack; i.e. the side on which you need to solder the wires (or solder the rectangular header pins, with wires already attached to them. The PPS line is a straight-through  connection to slot 15 of header P9 on the beaglebone itself.  The end of that wire should be soldered to a rectangular connector pin, which will fit into the hole at slot 15 on P9.  The connection may be loose, but this can be fixed by very slightly bending the connector pin before insertion.  The top panel of the beaglebone case will help ensure the pin remains inserted.  The attached PDF version of the schematic can be zoomed.

The +5VDC power and ground are obtained from pins 1 and 8 on jumper J2 of the Eval232R board.  RX and TX connect directly from the PS/2 cable to the appropriate (and opposite sense) pins on the DB9 serial connector, as shown.

Note: connecting to the DB-9 within the constraints of the black project box may require removal of some or all of the metal shielding around that connector.

Photos of Example Project Box

Here is a simple project box that achieves this - note the cut-away portion of the metal housing on the DB-9 connector. 
This could be avoided by use of a larger project box:

The EVAL232R board is simply heat-glued to the bottom of the project box, into which appropriate
holes have been cut/drilled for the USB port, the miniDIN connector, and the PPS wire.

The wire emerging from the top side of the box gets plugged into pin 15 of header P9 on the beaglebone computer:

Reprogramming the EVAL232 Board so it is recognized as a PPS-GPS by the SensorGnome

Note: this step requires a computer running Windows with Microsoft .NET 4.0 framework, because FTDI doesn't provide a Mac version of their FTPROG utility.

Download, install, and run the FTPROG utility from ftdichip.comto reprogram the EVAL232R USB/Serial adapter to give it the unofficial VID/PID we're using to indicate presence of a PPS GPS (vid=0xbd09, pid=0x0001).  There is a template file called SensorGnome_PPS_GPS.xml attached to this page which should be used to do this.  This rebranding allows us to distinguish the MR350P + PPS combination from other devices which might use the FTDI 232R USB/Serial interface. 

Internal software details (for SensorGnome developers)

• we force recognition of the reprogrammed EVAL232R device using a udev rule to modprobe ftdi_sio (see attached udev rules file)
• for some versions of the MR350P, you might have to force the device into using NMEA mode.  This is only required if the device does not appear to work and does not give sensible data via the gpsmon utility.  To do this, use gpsctl from the gpsutils package (version 3.7 or later) like so:

gpsctl -n -t SiRF /dev/ttyUSB0

• add kernel PPS support.  You need these settings in the kernel configuration, as well as one of the kernel patch files attached below:

# PPS support
#
CONFIG_PPS=y
# CONFIG_PPS_DEBUG is not set

#
# PPS clients support
#
CONFIG_PPS_CLIENT_KTIMER=y
# CONFIG_PPS_CLIENT_LDISC is not set
# CONFIG_PPS_CLIENT_PARPORT is not set
CONFIG_PPS_CLIENT_GPIO=y