A clone of the Adafruit Ultimate GPS FeatherWing but with the u-blox NEO-M8T replacing the GlobalTop FGPMMOPA6H. The NEO-M8T can receive signals from GPS, Galileo and either GLONASS or BeiDou concurrently and supports both SBAS and QZSS. It also provides Multi-GNSS Raw Measurement (RAWX) Data which can be used for Post-Processed Kinematic (PPK) positioning.
See NEO-M8T_GNSS_FeatherWing.pdf for the schematic, layout and Bill Of Materials.
The Eagle directory contains the schematic and PCB design files.
The SMA_Antenna directory contains the schematic and PCB design files for an extended version of the PCB which incorporates a robust SMA connector for the antenna instead of uFL.
The Arduino directory contains code for the Adafruit Feather M0 Adalogger which will log RAWX data to SD card.
POST_PROCESS.md contains results of a first time successful attempt at post-processed kinematic positioning using rtkexplorer's demo5 version of RTKLIB. Please refer to the Precise Positioning Resources for more information.
This guide concentrates on the differences between the NEO-M8T FeatherWing and the Adafruit Ultimate GPS FeatherWing. Please refer to Lady Ada's excellent documentation to get started with the Feather family and GPS:
Like the Ultimate GPS FeatherWing, the NEO-M8T FeatherWing runs from +3.3V power and uses 3V logic. The NEO-M8T is powered directly from the 3V and GND pins on the bottom left of the Feather. Each Adafruit Feather has a regulator to provide clean 3V power from USB or battery power.
If you want to completely power down the Adalogger and the NEO-M8T, you can do this by pulling the Feather EN pin low (e.g. by connecting it to GND via a small slide switch).
The NEO-M8T FeatherWing, like the Ultimate GPS FeatherWing, can communicate over UART serial. It sends ASCII NMEA sentences from the TX pin to the microcontroller RX pin and can be controlled to change its data output from the microcontroller TX pin. Logic level is 3V for both.
The u-box M8 chipset also supports much more comprehensive UBX binary format messages (see Serial Protocol below).
The baud rate by default is 9600 baud, but you can configure the module to use a different baud rate if desired. For RAWX logging you will need to use at least 115200 baud.
The NEO-M8T Tx and Rx pins are wired directly to the Serial pins at the bottom right of the Feather. If you need to connect to a different set of pins, you can cut the RX and TX jumpers on the bottom of the board and connect instead to the TX and RX pads at the top of the board.
The ATSAMD21G18 ARM Cortex M0 chip on the Adalogger has multiple Sercom channels which can be used to provide additional serial ports, which is handy if you're already using Serial1 for something else:
- Adafruit: https://learn.adafruit.com/using-atsamd21-sercom-to-add-more-spi-i2c-serial-ports/creating-a-new-serial
- MartinL: https://forum.arduino.cc/index.php?topic=341054.msg2443086#msg2443086
The NEO-M8T also provides an I2C (DDC) interface. The SCL and SDA pins are broken out onto two pads near the top right corner of the board. Pull up resistors are provided (R5 & R6).
The SCL and SDA pads are not connected directly to SCL and SDA on the Adalogger. You will need to add your own jumper wires if you do want to make use of this interface.
The NEO-M8T SPI interface can be enabled by shorting the DSEL split pad:
- SDA becomes SPI CS
- SCL becomes SPI CLK
- TX becomes SPI MISO
- RX becomes SPI MOSI
- You may need to remove the SCL and SDA pull-up resistors (R5 & R6)
- You will need to cut the TX and RX jumpers to isolate the Adalogger Serial pins
There is a small button that will connect the microcontroller RESET pin to ground for resetting it. Handy to restart the Feather. Note that this is not connected to NEO-M8T reset unless you short the GRESET split pad on the rear of the PCB (see below).
GRST is connected to the NEO-M8T reset pin. You can reset the NEO-M8T by pulling this pin low. If you want to reset both the Feather and the NEO-M8T via the reset button, short the GRESET split pad on the rear of the PCB. Note that pulling GPS Reset low does not put the NEO-M8T into a low power state, you'll want to disable it instead (see En below).
INT is connected to the NEO-M8T EXTINT external interrupt pin. It can be used for control of the receiver or for aiding. See the u-blox documentation links below for further details.
TP is connected to the NEO-M8T TP time pulse pin. It can be used to output pulse trains synchronized with GPS or UTC time grid with intervals configurable over a wide frequency range. Thus it may be used as a low frequency time synchronization pulse or as a high frequency reference signal (up to 10 MHz). By default, the time pulse signal is configured to 1 pulse per second.
TP is also connected to an LED via a buffer transistor. By default it will flash once per second once the receiver is synchronised to GNSS time.
En is a true 'power disable' control line you can use to completely cut power to the NEO-M8T. This is good if you need to run at ultra-low-power modes. By default this is pulled low (enabled). So pull high (to 3V) to disable the NEO-M8T.
The NEO-M8T SAFEBOOT pin is broken out on a test pad on the rear of the PCB. You will need access to this to update the firmware.
Like the Ultimate GPS FeatherWing, the NEO-M8T FeatherWing includes a holder for a CR1220 back-up battery which will keep the NEO-M8T's internal clock going if the power is removed or disabled, providing a much quicker "warm start" when the power is reconnected.
If you don't want to use the backup battery, you can instead draw backup power from the 3V pin by shorting the 3V BACKUP split pad. But be careful! Only do this if you won't be using the backup battery. If you install the battery and have the split pad shorted BAD THINGS WILL HAPPEN! You might want to put some tape over the battery holder if you have shorted the split pad.
The NEO-M8T needs to be connected to an external antenna via a uFL connector. 3V power for an active antenna is provided.
The SMA_Antenna directory contains the schematic and PCB design files for an extended version of the PCB which incorporates a robust SMA connector for the antenna instead of uFL. The connector is Johnson part number 142-0711-821 (Farnell / Element 14 order code 1019328).
The NEO-M8T is based on u-blox's sophisticated M8 chipset. By default it will output standard NMEA format navigation messages, but also supports much more comprehensive UBX binary format messages. As the NEO-M8T is able to provide location information from GPS, GLONASS, Galileo and BeiDou you will find that the prefix of the NMEA messages changes to: $GP for GPS, SBAS and QZSS; $GL for GLONASS; $GA for Galileo; $GB for BeiDou; or $GN if it is using a mix of satellites (which is usually the case). Having the messages prefixed with $GN will confuse the Adafruit GPS Library and Mikal Hart's TinyGPS. To get round this, you can force the "Talker ID" to GP by sending the following UBX binary format message:
- 0xb5, 0x62, 0x06, 0x17, 0x14, 0x00, 0x20, 0x40, 0x00, 0x02, 0x00, 0x00, 0x00, 0x00, 0x00, 0x01, 0x01, 0x01, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x96, 0xd9
The M8 chipset supports several different navigation modes including: static, portable, pedestrian, automotive, sea and "Airborne <1G" (which is really useful for high altitude ballooning!).
By default, the NEO-M8T will receive signals from GPS, QZSS and GLONASS. If you want to enable Galileo and/or BeiDou reception, you'll need to do this with another UBX message (see below).
You can find the messages to change the navigation mode and GNSS configuration in the RAWX_Logger Arduino code.
RAWX_Logger is Arduino code written to run on the Adafruit Feather M0 Adalogger.
Rover data is logged to a file called r_hhmmss.ubx, base (static) data is logged to a file called b_hhmmss.ubx. The code logs RAWX data continuously to SD card until the stop button is pressed. Since the code is logging a large amount of data, it is necessary to keep the log file open throughout. If the power is removed or the board is reset before the stop button is pressed, the logged data will be lost. Connect a normally-open push-to-close switch between swPin and GND. By default, swPin is Digital Pin 15 (0.2" away from the GND pin on the Adalogger). The pin can be changed by editing the code.
RAWX_Logger_2 is an improved version of the logger. A new log file is automatically opened every INTERVAL minutes to: minimise data loss in the event of a power failure; and keep the file size manageable. RAWX_Logger_2 pauses the GNSS RAWX messages while one file is closed and another opened, incurring a data loss of one or two RAWX messages.
In RAWX_Logger_3, a new log file is automatically opened every INTERVAL minutes, but the GNSS RAWX messages are not paused while one file is closed and another opened, to allow contiguous data capture with no missing messages. This technique is completely reliant on the serial receive buffer being large enough to store the incoming RAWX data between files. In the unlikely event of a UBX checksum error or a loss of sync: the RAWX messages are paused, a new log file is opened, and then the RAWX messages are restarted.
In RAWX_Logger_4, serial data is automatically diverted into a large RingBufferN by a timer interrupt, which means you do not need to increase the size of the serial receive buffer (by editing RingBuffer.h). Version 4 also includes NeoPixel support making it easier to tell what the code is doing. The NeoPixel and LEDs can be disabled if the need the logger to be blacked out.
By default, RAWX data is logged every 250 msec (4Hz). This can be slowed down by selecting an alternate CFG-RATE message.
By default, the code will log raw measurements from GPS + Galileo + GLONASS + SBAS. This can be changed to GPS + Galileo + BeiDou + SBAS by commenting out the line which says #define GLONASS
By default, the code will use the static navigation mode. The chosen mobile (roving) mode can be selected by commenting out the line which says #define STATIC
The code now calculates the UBX checksum bytes for you, so it is much easier to make changes to the GNSS configuration. See sendUBX() in the code.
For RAWX_Logger, RAWX_Logger_2 and RAWX_Logger_3, you will need to increase the size of the serial receive buffer to avoid data overruns while data is being written to the SD card. See this post by MartinL:
For the Adafruit Feather M0 Adalogger (SAMD), under Windows, edit:
- C:\Users\ ...your_user... \AppData\Local\Arduino15\packages\adafruit\hardware\samd\1.0.19\cores\arduino\RingBuffer.h
and change this line:
- #define SERIAL_BUFFER_SIZE 164
For RAWX_Logger, change it to:
- #define SERIAL_BUFFER_SIZE 2048
For RAWX_Logger_2 and RAWX_Logger_3, change it to:
- #define SERIAL_BUFFER_SIZE 8192
RAWX_Logger_4 uses a large RingBufferN to store the data. You do not need to edit RingBuffer.h if you use RAWX_Logger_4.
If you do edit RingBuffer.h, check the reported freeMemory before and after to make sure the change has been successful. (You should find that you've lost twice as much memory as expected!)
The code uses: the Adafruit GPS Library; Bill Greiman's SdFat; and Michael P. Flaga's MemoryFree. For RAWX_Logger_2, RAWX_Logger_3 and RAWX_Logger_4, you will also need the Arduino Real Time Clock library for the M0 (Arduino Zero). See below for the download links.
To log the RAWX data to file on a PC (instead of the Adalogger SD card):
- The UBX_Echo directory contains Arduino code for the Adalogger which will change the NEO-M8T Baud rate to 115200 and then echo all data to the PC.
- NEO-M8T_GNSS_RAWX_Logger.py is Python code which configures the NEO-M8T and then logs the RAWX data to file on a PC.
- UBX_Checker.py is Python code which can be used to check the integrity of the RAWX file (to make sure no data has been lost).
Hidden in NEO-M8T_GNSS_RAWX_Logger.py is code which calculates the UBX message checksums.
Useful precise positioning resources can be found at:
- https://rtklibexplorer.wordpress.com
- http://rtkexplorer.com/
- http://www.rtklib.com
- https://github.com/tomojitakasu/RTKLIB
Useful documentation about the NEO-M8T and its protocol specification can be found at:
- NEO-LEA-M8T-FW3_ProductSummary
- NEO-LEA-M8T-FW3_DataSheet
- NEO-8Q-NEO-M8-FW3_HardwareIntegrationManual
- u-blox8-M8_ReceiverDescrProtSpec
- CadSoft-Eagle-Library
The NEO-M8T GNSS FeatherWing is based extensively on an original design by Adafruit Industries:
Distributed under a Creative Commons Attribution, Share-Alike license
Adafruit invests time and resources providing this open source design, please support Adafruit and open-source hardware by purchasing products from Adafruit!
Enjoy!