Digilent 410-237P User Manual

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Revision: September 06, 2012
Note: This document applies to REV A of the board.

1300 NE Henley Court, Suite 3

Pullman, WA 99163

[phone] Voice | [phone] Fax

Overview

The PmodGPS can add satellite positioning
accuracy to any embedded system. The
PmodGPS features a GlobalTop Gms-u1LP
GPS antenna module that utilizes the
MediaTek GPS MT3329.

Functional Description

The PmodGPS uses a standard 6-pin
connector and communicates via a 2-wire
Universal Asynchronous Receiver/Transmitter
(UART.) The PmodGPS also has a 2-pin
connector for control of the NRST pin to the
module and the Radio Technical Commission
for Maritime services, or RTCM pin for
Differential Global Positioning System (DGPS)
data using RTCM protocols.

Note: The PmodGPS arrives with the RTCM
feature inactive, to enable RTCM capabilities
users should contact GlobalTop at: www.gtop-

tech.com.

Interface

The PmodGPS uses UART protocol for data
transmission and reception. The interface
operates at a default baud rate of 9.6 kBd, 8
data bits, no parity, and with single stop bits.
However, users can change the baud rate to
predefined values that range from 4.8 kBd to

115.2 kBd.
The reset pin (NRST) on J2 allows normal

operation in active low. If users toggle the
NRST pin it will completely reset the module.
This reset performs similar to a power cycling
of the device. The 1 PPS pin on J1 provides a
one pulse-per-second output synchronized
with GPS time. (See the timing diagram in
figure one)

Features include:

 Integrated ceramic GPS antenna
 Standard UART interface
 Input voltage: 3V – 3.6V
 A 10Hz maximum update rate (1Hz Default

rate)

 3m 2D accuracy without aid
 Low power consumption (24mA tracking

and 30mA during acquisition)

 Ultra-high sensitivity: -165dBm
 A 515m/s maximum velocity and 18,000m

maximum altitude

 Auto switchover to external antenna
 12.5mm coin cell retainer for battery

backup of GPS, RTCC, and almanac

The 3DF pin on J1 indicates the status of the

user’s positional fix. When the module has a

constant fix (2D or 3D) this pin stays low, if the
module is unable to get a fix then the pin will
toggle every second. (See figure two) LD1
also follows this same behavior pattern in order
to give the user a visual representation.

Doc: 502-237 page 1 of 4

PmodGPS Reference Manual

Figure 1:

1PPS Pin Timing Diagram

Figure 2:

3DF Pin output without a fix

The PmodGPS also comes equipped with a
coin cell retainer for a 12.5 mm coin cell
battery. Users can significantly reduce the
amount of time that it takes to acquire the first
positional fix by installing a 3V coin cell battery.
With the battery installed the module can also
perform a hot start or a warm start instead of
cold starting where users have to supply power
to the VCC.

Note: The ground square solder pad of the
coin cell retainer may develop an oxide build
up that will keep the battery from making a
good connection. Users should simply scuff up
the square solder pad inside J3 to remove any
buildup. Oxide buildup may also occur if there
has not been a battery in the retainer for a
while.

A cold start takes one or two 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
three to five seconds and a warm start varies
depending on how long the module has sat
unpowered. Users can hot start the PmodGPS
only if it has acquired a fix within approximately
the last two hours and with accessible backup

power (coin cell battery) connected to the coin
cell retainer. A warm start occurs when the
battery is connected but the module has been
without power for more than two hours.

Users may utilize an external antenna at their
discretion by installing a Linx Technologies Inc.
CONSMA 003.062 module on header J4. The
antenna can speed up acquisition of GPS
signal in some conditions, especially if the
antenna is outdoors and the module is indoors.

The PmodGPS uses sentences based on
National Marine Electronics Association
(NMEA) protocols for data output. Each NMEA
message begins with a ($) dollar sign. The
next five characters are the talker ID and the
arrival alarm. The PmodGPS talker ID is “GP”
and the arrival alarm is the specific sentence
output descriptor. Individual comma separated
data fields follow these five characters. After
the data fields there is an asterisk followed by
a checksum. Each sentence should end with
<CR><LF>. For example output sentences
refer to tables two through six at the end of this
manual.

User may configure some of the PmodGPS
characteristics by writing command packets to
the module. However, these functions are
more advanced and not all of the command
packets are openly distributed. Users may
change the baud rate to 38.4kBd (minimum
baud for 10Hz data acquisition) by issuing the
following command over the UART:

“$PMTK251,38400*27<CR><LF>”
The “*27” corresponds to a checksum, if users

want to substitute a different baud rate, then
they must calculate a new checksum using
GlobalTop’s Checksum Tool. The command
for changing the data acquisition from 1Hz to
10Hz is:

“$PMTK226,3,30*4<CR><LF>”
The same checksum process for changing the

baud rate applies to this change too. Contact
GlobalTop for more information on the
individual command packets, their complete

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