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Revision: July 11, 2012
Note: This document applies to REV A of the board.
1300 NE Henley Court, Suite 3
Pullman, WA 99163
(509) 334 6306 Voice | (509) 334 6300 Fax
Doc: 502-237 page 1 of 4
Copyright Digilent, Inc. All rights reserved. Other product and company names mentioned may be trademarks of their respective owners.
Overview
The PmodGPS is an ideal solution for any
embedded system in need of satellite
positioning accuracy. It features the GlobalTop
Gms-u1LP GPS antenna module which utilizes
the MediaTek GPS MT3329.
Features include:
standard UART interface
input voltage: 3V – 3.6V
10Hz maximum update rate (1Hz
Default rate)
3m 2D accuracy without aid
super low power consumption (24mA
tracking and 30mA during acquisition)
ultra-high sensitivity: -165dBm
515m/s maximum velocity and 18,000m
maximum altitude
integrated ceramic GPS antenna
auto switchover to external antenna
12.5mm coin cell retainer for battery
backup of GPS RTCC and almanac
Functional Description
The Pmod GPS utilizes a standard 6-pin
connector and communicates via 2-wire UART.
Also available on the board is a 2-pin
connector for control of the NRST pin to the
module and also the RTCM pin for DGPS data
of RTCM protocol (this feature is disabled by
default, contact GlobalTop to enable).
Interface
The PmodGPS uses UART protocol for data
transmission and reception. By default, the
interface uses a baud rate of 9600, 8 data bits,
no parity, and a single stop bit. The module
provides the option for changing the baud rate
to predefined values which range from 4800 to
115,200.
The reset pin (NRST) on J2 is active low. If
the NRST pin is toggled, the device will
undergo a full reset. This reset performs
similarly to a power cycling of the device. The
1PPS pin on J1 provides a one pulse-per-
second output which is synchronized to GPS
time. See the timing diagram in fig.1 for a
visual representation.
1PPS Timing Diagram
Figure 1 – 1PPS Pin
The 3DF pin on J1 indicates a positional fix. .
When the module has a fix (2D or 3D) this pin
is held low, if the module is unable to get a fix
then the pin will toggle every second as seen
below in the diagram. LD1 also follows this
same behavior pattern in order to give the user
a visual representation.
Figure 2 – 3DF Pin output without a fix
PmodGPS Reference Manual
www.digilentinc.com page 2 of 4
Copyright Digilent, Inc. All rights reserved. Other product and company names mentioned may be trademarks of their respective owners.
The PmodGPS also comes with a coin cell
retainer designed for a 12.5 mm coin cell
battery. By installing a 3V coin cell battery the
user can significantly reduce the amount of
time that it takes to acquire the first fix. When a
battery is installed the module may be able to
perform a hot start or a warm start instead of a
cold start when power is first applied to VCC.
Note: The ground square solder pad of the coin cell
retainer may have an oxide build up that will keep the
battery from making a good connection. The user should
simply scuff up the square solder pad inside J3 to remove
any buildup. This can also occur if there has not been a
battery in the retainer for a while.
A cold start takes 1-2 minutes while outside in
good conditions, and can take several minutes
more if conditions are worse or the module is
indoors. A hot start takes 3-5 seconds, and a
warm start varies depending on how long the
module has sat unpowered. To cause a hot
start, the module must have acquired a fix
within approximately the last two hours and the
backup power (coin cell battery) must be
supplied. A warm start occurs when the
battery is still supplied but it has been more
than two hours since last power on.
If the user desires, an external antenna can be
used by installing a Linx Technologies Inc.
CONSMA 003.062 module on header J4. In
some conditions, the antenna can speed up
acquisition, especially if the antenna is
outdoors while the module is indoors.
For data output from the module, the
PmodGPS uses sentences based on the
NMEA protocol. Each NMEA message starts
with a ($) dollar sign. The next five characters
will be the talker ID and the arrival alarm. In the
case of this module, the talker ID will be “GP”
and the arrival alarm will be the specific
sentence output descriptor. Following this are
the individual data fields each separated by a
comma. After the data fields there is an
asterisk followed by the checksum. A
sentence is ended with <CR><LF>. For
example output sentences, see the end of this
manual.
It is possible to configure some of the
characteristics of the device by writing
command packets to the module. However,
this functionality is more advanced and not all
of the command packets are openly
distributed. In order to change the baud rate to
38,400 (minimum baud for 10Hz data
acquisition) the user must send the module
“$PMTK251,38400*27<CR><LF>” over the
UART. The “*27” corresponds to a checksum,
if a different baud is desired to be substituted,
a new checksum must be calculated using
GlobalTop’s Checksum Tool. The command
for changing the data acquisition to 10Hz from
1Hz is “$PMTK226,3,30*4<CR><LF>” (the
same checksum process applies here). For
more information on the individual command
packets contact GlobalTop for their complete
command list or see the FAQ for how to
change the baud rate.
Connector J1
Pin
Signal
Description
1
3DF
3D-Fix Indicator
2
RX
Receive
3
TX
Transmit
4
1PPS
1 Pulse Per Second
5
GND
Power Supply Ground
6
VCC
Power Supply (3.3v)
Connector J2
Pin
Signal
Description
1
~RST
Reset (active low)
2
RTCM
DGPS data pin (contact
GlobalTop for use)
For more information on the GPS module
interface, see the Gms-U1LP datasheet
available online or from GlobalTop at
www.gtop-tech.com.
PmodGPS Reference Manual
www.digilentinc.com page 3 of 4
Copyright Digilent, Inc. All rights reserved. Other product and company names mentioned may be trademarks of their respective owners.
Output Sentences
$GPGGA,064951.000,2307.1256,N,12016.4438,
E,1,8,0.95,39.9,M,17.8,M,,*65<CR><LF>
Example
Description
$GPGGA
Message ID
064951.000
UTC Time (hhmmss.sss)
2307.1256
Latitude (ddmm.mmmm)
N
N/S indicator
12016.4438
Longitude (dddmm.mmmm)
E
E/W indicator
1
Position Fix Indicator
8
Satellites used
0.95
HDOP
39.9
MSL Altitude
M
Units
17.8
Geoidal Separation
M
Units
Age of Diff. Corr.
*65
Checksum
<CR><LF>
End of message indicator
$GPGSA,A,3,29,21,26,15,18,09,06,10,,,,,
2.32,0.95,2.11*00<CR><LF>
Example
Description
$GPGSA
Message ID
A
Mode1 (see GlobalTop manual)
3
Mode2 (see GlobalTop manual)
29
Satellite used (CH1)
21
Satellite used (CH2)
….
Satellite Used (Ch12)
2.32
PDOP
0.95
HDOP
2.11
VDOP
*00
Checksum
<CR><LF>
End of message indicator
$GPGSV,3,1,09,29,36,029,42,21,46,314,43,
26,44,020,43,15,21,321,39*7D<CR><LF>
Example
Description
$GPGSV
Message ID
3
Number of Messages
1
Message Number
09
Satellites in View
29
Satellite ID (CH1)
36
Elevation (CH1)
029
Azimuth (CH1)
42
SNR (C/No)
….
15
Satellite ID CH(4)
21
Elevation (CH4)
321
Azimuth (CH4)
39
SNR (C/No)
*7D
Checksum
<CR><LF>
End of message indicator
$GPRMC,064951.000,A,2307.1256,N,12016.443
8,E,0.03,165.48,260406,3.05,W,A*55<CR><LF>
Example
Description
$GPRMC
Message ID
064951.000
UTC Time (hhmmss.sss)
A
Status (A = data valid)
2307.1256
Latitude (ddmm.mmmm)
N
N/S indicator
12016.4438
Longitude (dddmm.mmmm)
E
E/W indicator
0.03
Speed over ground (knots)
165.48
Course over ground (degrees)
260406
Date (ddmmyy)
3.05
Magnetic Variation (degrees)
W
E/W indicator
A
Mode (see GlobalTop manual)
*55
Checksum
<CR><LF>
End of message indicator
PmodGPS Reference Manual
www.digilentinc.com page 4 of 4
Copyright Digilent, Inc. All rights reserved. Other product and company names mentioned may be trademarks of their respective owners.
$GPVTG,165.48,T,,M,0.03,N,0.06,K,A*37<CR>
<LF>
Example
Description
$GPVTG
Message ID
165.48
Course (degrees)
T
Reference (true or false)
Course (degrees)
M
Reference (Magnetic)
0.03
Speed
N
Units (N = knots)
0.06
Speed
K
Units (K = km/hr)
A
Mode (see GlobalTop manual)
*37
Checksum
<CR><LF>
End of message indicator