Hi all,
My first post here on the SF Forums.
I’m setting up a Rover-Base RTK rig using a pair of GPS-RTK-SMA ZED-F9P’s linked by Telemetry Serial Radios, basically following the SF tutorial here: https://learn.sparkfun.com/tutorials/se … rtk-system
I was planning on powering the GPS-RTK-SMA ZED-F9P boards over Qwiic from an Adafruit Feather STM32F405 or SF Artemis Thing Plus pulling from a 2500mAh LiPo, but I notice in the GPS-RTK-SMA board schematic that the 5V Terminal is powered by V_BUS from the USB-C connector, and thus wouldn’t have any power if the GPS-RTK-SMA board is powered only by Qwiic.
In my Rover, the USB-C connector will be connected to a Samsung tablet which will provide 5V V_BUS. I’m not happy about trying to power my Telemetry Serial Radio from the Samsung tablet. And this raises a back-powering question I will detail below.
My original fallback power design has always been to power both the Base and Rover units from Milwaukee M18 battery packs run through 5V buck converters or Milwaukee’s own M18 USB power tap. (I could probably run for days using 4Ah @ 18v M18 batteries!) For the base that’s not an issue, as the base’s overall weight doesn’t matter since it’s not moving, but I was trying to keep the overall weight of my rover down.
Another power option would be powering the Telemetry Serial Radio 5V via a separate LiPo battery and dedicated 5v boost converter.
So… a couple questions:
1.) BACKFEEDING / DOUBLE POWERING: How is the GPS-RTK-SMA ZED-F9P board regulator circuit going to like having both 3.3V coming over Qwiic and 5V coming over USB?
2.) Any other ideas on powering the 5V Telemetry Serial Radios if I don’t have a real USB power source? Or should I just go with the Milwaukee batts?
Thanks!
John
Hello John, thanks for posting!
1.) BACKFEEDING / DOUBLE POWERING: How is the GPS-RTK-SMA ZED-F9P board regulator circuit going to like having both 3.3V coming over Qwiic and 5V coming over USB?
It’s not designed to be powered from two different power sources, but if you’re connecting the GPS to a tablet with USB you could just cut the red wire on your Qwiic connector to keep power isolated between the GPS and your micro controller.
2.) Any other ideas on powering the 5V Telemetry Serial Radios if I don’t have a real USB power source? Or should I just go with the Milwaukee batts?
As long as you’re OK powering the radios from the tablet, you could connect those to the 5V pin on the GPS. Just cut that red Qwiic cable wire to prevent power going back into your micro controller. Alternatively you use your Milwaukee batteries along with a voltage regulator to power the radios on their own, but make sure you keep all the grounds between the various bits connected.
Thanks for the reply Chris.
Powering anything from the tablet is actually my least favorite option.
I am not familiar with the technical details of USB and OTC.
Question: Would it be possible to power the radios from 5V coming from the Milwaukee USB power supply via the V_USB pad on my microcontroller, and *** cut the 5V line in the cable between the GPS-RTK-SMA and the tablet *** so the tablet isn’t back powering the microcontroller or GPS-RTK-SMA?
My reason for this proposal is that I need the GPS system to work without a tablet, displaying its output on an OLED. See photo.
PS: note that the Qwiik +V line is 3.3v, sourced by the microcontroller.
FWIW, it’s not clear how Nate is powering his rig at https://learn.sparkfun.com/tutorials/se … rtk-system
There’s no explicit mention of power source connections in the tutorial, other than from the GPS-RTK-SMA board 5V (out) to the Telemetry Serial Radio (in).
There is no microcontroller or non-tablet display, and from the photo of the rig it would seem the GPS-RTK-SMA board is powered by the USB OTG cable to the Tablet.
Again, my question is, in my design with a microcontroller/OLED and dedicated 5v battery power source (Milwaukee M18 USB), could one cut the 5V wire in the USB OTG cable, so the tablet can’t source power to the GPS-RTK-SMA/microcontroller/OLED?
I’m a bit confused here.
Again, my question is, in my design with a microcontroller/OLED and dedicated 5v battery power source (Milwaukee M18 USB), could one cut the 5V wire in the USB OTG cable, so the tablet can’t source power to the GPS-RTK-SMA/microcontroller/OLED?
Are you using a micro controller, GPS, OLED, telemetry radio and a tablet all at the same time? I’m not seeing a tablet anywhere in your photo. I thought your goal was to not have a heavy Milwaukee battery on the rover?
You could cut the 5 volt wire in a USB OTG cable to disconnect power between a tablet and your GPS or micro controller while still allowing data communication via USB.
As far as Nate’s guide goes, he never shows how things are wired but it appears that he’s using a USB OTG cable to connect a phone to the GPS and the phone is powering the GPS and the telemetry radio. There’s no other devices connected to either USB or Qwiic.
I see a USB cable in your photo attached to the micro controller, is that connected to a tablet or is it connected to a 5 volt battery? If you’re able to supply a schematic showing all the parts you’re interconnecting, it might be easier for us to understand how you have things setup and what could be done to make things work correctly.
Thanks Chris!
Attached is a sketch of what my current thinking is.
There are three options:
-
as drawn, including yellow items
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as drawn without yellow items, USB cable between GPS-RTK-SMA board and tablet with 5v conductor connected
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as drawn without yellow items, USB cable between GPS-RTK-SMA board and tablet with 5v conductor DISconnected
I tested the USB splitter that I have on hand (yellow in my diagram) and the 5v and ground lines are connected between all three ends. (Not that everything you get on Amazon is correctly engineered.)
Thoughts about the three options?
I decided to build switched into the enclosures to switch the 5v manually as appropriate depending on whether the devices Base and Rover were powered via the USB-C port to the ZED-F9P board or the micro-B port to the Arduino, and whether or not the rover was powered by the tablet or by an external USB power source like a Milwaukee M18 USB.
I am still curious for a more systematic analysis of how best to route power to multiple subsystems that have multiple and various data/ground/5V/3.3V interconnects like this.
On your base diagram, just run TX,RX and ground (Not 5V!) between the GPS and the radio, and power the radio from the 5V pin on your STM32.
For your rover diagram, don’t connect 5V between the STM32 and the GPS, on the usb cable between the tablet and GPS, cut the 5 volt wire in the USB cable. Connect just RX,TX and ground between the radio and GPS and power the radio from the 5V pin on the STM32.
