Voting resources, early voting, and poll worker information - VOTE. ... Adafruit is open and shipping.
0

Stemma QT Voltage Levels
Moderators: adafruit_support_bill, adafruit

Please be positive and constructive with your questions and comments.

Stemma QT Voltage Levels

by LinkArcher on Mon Jun 29, 2020 9:29 pm

I plan to use the PA1010D GPS, BNO055 IMU, DPS310 pressure sensor and an XBee 915 MHz module with Adafruit's USB adapter board, all controlled by a Teensy 4.1. Power will come from a 2S lipo battery. My goal is to run all of the electronics using a single regulator. I'll need everything running on 3.3V logic as well. With that in mind I have some questions:

All three STEMMA boards indicate that they can be powered by 5V or 3.3V. Can I power them with 5V through the STEMMA connector or do I have to supply 3.3V if I use that connector? Does it matter whether I use the STEMMA connector versus the breakout pins?

These boards indicate that they are 5V logic tolerant using level-shifting resistors. If I use 5V to power them will that cause them to output 5V logic on their serial connections or do they always output 3.3V regardless of which supply pin I use?

The XBee USB adapter has an onboard 5V to 3.3V regulator. Can I safely bypass this regulator by applying 3.3V directly to the 3.3 pin?

How much current can I pull from the XBee adapter board's regulator?

How much current can I pull from the Teensy 4.1's onboard regulator?

LinkArcher
 
Posts: 2
Joined: Mon Jun 29, 2020 8:21 pm

Re: Stemma QT Voltage Levels

by dastels on Mon Jun 29, 2020 9:51 pm

The V+ on the Stemma connectors is connected to Vin header pin, so pre-regulator. So you can supply 5v via the stemma cable. Do note that both stemma connectors are fully in parallel, so if you put 5v on one, it will be on the other as well.

Generally speaking, the external logic signals are at the same level as Vin. So if you power it with 5v, you get 5v logic. Power it with 3.3v and you get 3.3v logic. Internally the boards use 3.3v.

The XBee adapter gets powered by the USB connector, so applying 3.3v to the 3.3v pin while USB is connected is likely not a good idea. It's meant as an output.

Going by the part number on the XBee adapter's schematic https://cdn-shop.adafruit.com/product-files/247/247+schematic.PDF (and looking up the datasheet https://www.diodes.com/assets/Datasheets/AP1117_R21.pdf) it seems to be able to supply 1A from it's 3.3v regulator. The adapter circuitry and the XBee require some of that current.

The Teensy's 3.3v pin can supply 250mA max according to it's data https://www.pjrc.com/store/teensy41.html.

Dave

dastels
 
Posts: 3519
Joined: Tue Oct 20, 2015 3:22 pm

Re: Stemma QT Voltage Levels

by LinkArcher on Mon Jun 29, 2020 10:46 pm

Thanks for the help!

Sounds like the best approach would be to use the Teensy's regulator to power all three sensors while using a 5V regulator to power the Teensy itself plus the XBee. That just means I'll need a level shifter for the XBee board to set it back down to 3.3V.

EDIT: Actually from the schematic it looks like the breakout pins on the adapter are directly connected to the XBee pins, not through a level shifter. Looks like I don't need a level shifter after all.

LinkArcher
 
Posts: 2
Joined: Mon Jun 29, 2020 8:21 pm

Please be positive and constructive with your questions and comments.