Sparkfun NEO-M8P-2 Document technique

GPS-RTK Hookup Guide

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Introduction

The NEO-M8P-2 module is the top-of-the-line module for high accuracy GNSS and GPS location solutions

including RTK. The NEO-M8P-2 is unique in that it is capable of both rover and base station operations. The ‘-2’

designation means this module has survey-in mode allowing the module to become a base station and produce

RTCM 3.x correction data. From here on we will refer to the module as NEO-M8P but it should not be confused

with the NEO-M8P-0 module (which is not able to produce RTCM data).

SparkFun GPS-RTK Board - NEO-M8P-2 (Qwiic)

 GPS-15005

Product Showcase: SparkFun GPS-RTK Board

Suggested Reading

Before getting started, be sure to checkout our What is GPS RTK? tutorial and if you want to pre-read a bit have a

look at our Getting Started with U-Center.

Hardware Overview

Communication Ports

The NEO-M8P-2 in unique in that it has four communication ports which are all active simultaneously. You can

read NMEA data over I2C while you send configuration commands over the UART and vice/versa. The only limit is

that the SPI pins are mapped onto the I2C and UART pins so it’s either SPI or I2C+UART. The USB port is

available at all times.

I2C

An introduction to I2C, one of the main embedded

communications protocols in use today.

Serial Basic Hookup Guide

Get connected quickly with this Serial to USB adapter.

What is GPS RTK?

Learn about the latest generation of GPS and GNSS

receivers to get 2.5cm positional accuracy!

Getting Started with U-Center

Learn the tips and tricks to use the u-blox software tool

to configure your GPS receiver.

USB

The micro-B connector makes it easy to connect the NEO-M8P to u-center for configuration and quick viewing of

NMEA sentences. It is also possible to connect a Raspberry Pi or other SBC over USB. The NEO-M8P

enumerates as a serial COM port and it is a seperate serial port from the UART interface. See Getting Started with

U-Center for more information about getting the USB port to be a serial COM port.

A 3.3V regulator is provided to regulate the 5V USB down to 3.3V the module requires. External 5V can be applied

or a direct feed of 3.3V can be provided. Note that if you’re provide the board with 3.3V directly it should be a clean

supply with minimal noise (less than 50mV VPP ripple is ideal for precision locating).

I C (a.k.a DDC)

The u-blox NEO-M8P has a “DDC” port which is really just an I2C port (without all the fuss of trademark issues).

All features are accessible over the I2C ports including reading NMEA sentences, sending UBX configuration

strings, piping RTCM data into the module, etc. We’ve written a handful of sketches and an Arduino library to aid in

using the NEO-M8P over I2C in a snap. You can get the library through the Arduino library manager by searching

‘SparkFun Ublox’. Checkout the GPS-RTK Arduino Library section for more information.

UART/Serial

The classic serial pins are available on the NEO-M8P but are shared with the SPI pins. Because USB covers most

serial needs we didn’t label the UART pins explicitly. By default the UART pins are enabled. Be sure the DSEL

jumper on the rear of the board is open.

MISO = TX out from NEO-M8P

MOSI = RX into NEO-M8P

SPI

The NEO-M8P can also be configured for SPI communication. By default the SPI port is disable. To enable SPI

close the DSEL jumper on the rear of the board. Closing this jumper will disable the UART and I2C interfaces.

Control Pins

The control pins are highlighted below.

These pins are used for various extra control of the NEO-M8P:

FENCE: Geofence output pin. Configured with U-Center. Will go high or low when a geofence is setup.

Useful for triggering alarms and actions when the module exits a programmed perimeter.

RTK: RTK output pin. Remains high when module is normal GPS mode. Begins blinking when RTCM

corrections are received and module enters RTK float mode. Goes low when module enters RTK fixed

mode and begins outputting cm-level accurate locations.

PPS: Pulse-per-second output pin. Begins blinking at 1Hz when module gets basic GPS/GNSS position

lock.

RST: Reset input pin. Pull this line low to reset the module.

SAFE: Safeboot input pin. This is required for firmware updates to the module and generally should not be

used or connected.

INT: Interrupt input/output pin. Can be configured using U-Center to bring the module out of deep sleep or to

output an interrupt for various module states.

Antenna

The NEO-M8P requires a good quality GPS or GNSS (preferred) antenna. A U.FL connector is provided. Note:

U.FL connectors are rated for only a few mating cycles (about 30) so we recommend you set it and forget it. A

U.FL to SMA cable threaded through the mounting hole provides a robust connection that is also easy to

disconnect at the SMA connection if needed. Low-cost magnetic GPS/GNSS antennas can be used (checkout the

ublox white paper) but a 4” / 10cm metal disc is required to be placed under the antenna as a ground plane.

LEDs

The board includes four status LEDs as indicated in the image below.

The power (PWR) LED will illuminate when 3.3V is activated either over USB or via the Qwiic bus.

The pulse per second (PPS) LED will illuminate with each successful update once a position lock has been

achieved.

The RTK LED will be illuminated constantly upon power up. Once RTCM data has been successfully received it

will begin to blink. This is a good way to see if the NEO-M8P is getting RTCM from various sources.

The FENCE LED can be configured to turn on/off for geofencing applications.

Jumpers

There are four jumpers located on the back of the board to configure the GPS-RTK.

Closing DSEL enables the SPI interface and disables the UART and I2C interfaces. USB will still function.

Cutting the I2C jumper will remove the 2.2k Ohm jumpers from the I2C bus. If you have many devices on your I2C

bus you may want to remove these jumpers. Not sure how to cut a jumper? Read here!

Jumpers JP1, JP2, JP3, are provided on the rear of the board to allow isolation of the various status LEDs.

Backup Battery

The MS621FE rechargeable battery maintains the battery backed RAM (BBR) on the NEO-M8P. This allows for

much faster position locks. The BBR is also used for module configuration retention. The battery is automatically

trickle charged when power is applied and should maintain settings and GNSS orbit data for up to two weeks

without power.

Connecting an Antenna

U.FL connectors are very good but they are a designed to be implemented inside a small embedded application

like a laptop. Exposing a U.FL connector to the wild risks it getting damaged. To prevent damaging the U.FL

connection we recommend threading the U.FL cable through the stand-off hole, then attach the U.FL connectors.

This will provide a great stress relief for the antenna connection. Now attach your SMA antenna of choice.

Be Careful! U.FL connectors are easily damaged. Make sure the connectors are aligned, flush face to

face (not at an angle), then press down using a rigid blunt edge such as the edge of a PCB or point of a small

flat head screwdriver.

If you’re indoors you must run a SMA extension cable long enough to locate the antenna where it has a clear view

of the sky. That means no trees, buildings, walls, vehicles, or concrete metally things between the antenna and the

sky. Be sure to mount the antenna on a 4”/10cm metal ground plate to increase reception.

Connecting the GPS-RTK to a Correction Source

Before you go out into the field it’s good to understand how to get RTCM data and how to pipe it to the GPS-RTK.

For this example we will show how to get correction data from the UNAVCO network, pull that data in using

RTKLIB, and pipe it down over serial to the GPS-RTK.

Required Materials

1x GPS-RTK

1x GPS or GNSS Antenna

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Metal Plate of 4” or larger
1x SMA extension cable (if needed to get a clear view of the sky)
2x micro-B USB cable
1x Serial Basic
A few jumper wires
Required Software
Credentials with a free RTCM provider such as UNAVCO
U-Center
Download and unzip RTKLIB. We will be using 2.4.2.
UNAVCO has fairly good coverage in the USA. Using their interactive map find a station that is near your location.
It’s ok if it is more than 10km (6 miles) away, we’re just practicing.
Site P041 is pretty close to SparkFun HQ. We’ll be using it. To access UNAVCO data feeds you will need to send
an email to [email protected] to request credentials. Let UNAVCO know if you are affiliated with any business,
school, or organization and if you are using the account for personal use. Access to UNAVCO is free; I believe
they need this information for reporting on their grant funding.
Once you have your UNAVCO credentials open RTKLIB (in Windows run rtklaunch.exe). This small window allows
you to launch the various RTK programs. For this tutorial we’ll be using RTKNAVI, the second button from the
right.
RTKNAVI allows you to connect to RTCM feeds from various providers, including UNAVCO. Click on the small “I”
button.