Module identification and Teensy 4.1 PSU


Well-known member
Hey all. Still designing this DIN module and a couple of questions.

Power supply. I have a 3.3v @ 1.2A and a 5v @ 1A available to power my Teensy 4.1v.

Normally I just pile the 5v onto the VIN (I know all the pins are only 3.3v).

Would it be better to apply 3.3v to the Teensy instead and not waste power in the onboard reg?
If yes, does this +3.3v external supply go in on the 3.3v pin?


I have several modules that connect to this main 4.1 Teensy. What would be the simplest way to identify these modules over a pair of bus terminals (that are common to all modules)?

Some of my modules are I2C with their own individual addresses. These modules should be detectable by searching the I2C bus for their presence.
But the dumb modules with no I2C, I was going to fit an ATtiny85 that does nothing but issue an I2C address (well, add a certain I2C address).

Originally I thought of using resistance, but that seems far too hit 'n' miss.
Power Teensy with 5V input. It's not designed for power applied to the 3.3V pin.

I2C is a good way to detect hardware. If you're willing to write slightly more complex code, you could use a 24C02 EEPROM and store info in its memory about the hardware.
Thank you Paul. That is why I asked, it didn't seem prudent to feed 3.3v to the 3.3v pin.
Nice idea regarding the 24C02. I will have a tinker.

Thank you for the advice!

Oh... what is the supply of 4.1's looking like? I spent FOREVER designing this module around a 3.2 Teensy a while back and then they went away.
Don't want to do that again!
Am i right in thinking that when I add a Micro USB for the USB host ability, I only really need to run D+ and D- to the USB port?
This is because I have obviously already got ground connected to the Host USB socket locally, Pin 4 is unused, and I plan to feed the +5v pin with 5v from my PSU directly so that it doesn't draw from the Teensy 4.1.
Oh... what is the supply of 4.1's looking like?

Plenty in stock and more in the production pipeline.

For the ones with pins soldered, we currently have low stock due to losing 1 week during the recent winder storm. Plenty of material, but soldering those pins is labor intensive, so expect a 2-3 week lead time for larger orders if you need the pins soldered.
Now.... just so that I am not going mad....

I want to add a couple of DIP switches (if I can find some decent rated ones) to control the +5v supply to the Host and main USB ports.
I believe the host USB port on the Teensy 4.1 is limited to 500mA?

So... I cut the trace on the Teensy 4.1 so it's not fed from the USB programming lead by default.

One switch allows me to select that either the Teensy is powered from the PCB mounted 5v PSU and not the 5v from the programming USB lead, or from the 5v of the USB programming lead (if the 5v PSU on the PCB is turned off). A diode stops it back-feeding to other items on the PSU 5v bus.

The other switch simply stops the Host USB never using the Teensy 4.1's 5v regulator and instead either uses the PCB's main 5v PSU, or leaves the USB with no 5v supply, so then it will need it's own external 5v supply.

Make sense?

Any suggestions for a diode that will work to stop the back-feed around but still power the Teensy 4.1 OK? It's a perfect +5v PSU on the PCB.

Sorry for the awful image...


  • PSU layout.png
    PSU layout.png
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If you take a look at the T4.1 schematic, you'll see that the 5VDC fed to the USBhost interface is switched under software control (when the call to myusb.begin(); is made, where the USBhost interface is defined as USBHost myusb;). Rather than cutting the trace & patching it to your 5VDC supply, couldn't you just use the existing control mechanism (unless you need the extra current) ??

Mark J Culross

The TPD3S014 chip provides more than just software control of the power. It also gives current limiting, which is an essential feature to support hot plugging if the power to Teensy 4.1 is USB or some other current limited source.
Connect a powered USB hub to your USBhost interface to provide enough power for your power-hungry devices. Make sure the define at least one USBHub device in your sketch.

Mark J Culross
The power switch isn't strictly required. If your power supply is good enough to handle the sudden current inrush of USB hot plugging, you could just connect it directly to the host connector. Best practice would be to at least put a PTC "self resetting" fuse inline, so a defective cable that shorts the power to GND won't short out your power supply.
I have a decent quality Vigortronix 5v PSU and yes.... I added PTC fuses on both the 3.3v and the 5v lines.

Thanks for the advice
Out of interest..... what trip and hold current would you specify for the 5v and 3.3v supplies?

I have:

5v @ 1000mA
5v @ 2000mA
3.3v @ 1272mA

I picked 1.5A Hold and 3A trip (in 500ms) for the 5v @ 1000mA and the 3.3v @ 1272mA
I picked 2.5A hold and 4A trip (in 500ms) for the 5v @ 2000mA

Just guessed