Teensy 4.1 powered from 3.3V pin not initializing USB communication

Hello,

I have a project where I need to power a Teensy 4.1 from 3.3V. My current setup is as follows:
- I have cut the VUSB/VIN pad jumper on the bottom of the Teensy
- I have connected a CR2032 to the VBAT pin to keep the time management circuitry alive before connecting USB/wall power
- I have connected the VUSB pin to a voltage regulator that feeds power to the 3.3V pin on the Teensy (through a diode so I can "or" two power sources together, either from the USB connector or from an external power source)

The problem I'm experiencing is that my computer doesn't recognize the Teensy for Serial or other communication. I did a little snooping and found my way to the schematic for the Teensy where I noticed that there are pins on the Teensy IC (USB1_VBUS and USB2_VBUS) that seem to get power from a number of places (including normally the USB connector). So to test I just used a header pin to apply power to the VIN pin on the Teensy which seemed to trigger the USB system to initialize and I was able to program the chip again (at least until I unplugged power and then I had to use the header pin again to trigger the USB to start again). I assume this is because it is trying to minimize power consumption when not plugged into USB, but my system can't run on battery power anyway so this feature doesn't help me.

My question:
Is it possible to trigger the USB part of the Teensy to initialize through code or fuse setting? I have already created a PCB with the circuitry and would love to not make another one just to connect the VIN pin to power.


P.S. Some other related questions:
- How long can a typical CR2032 last powering the time management circuitry of the Teensy 4.1? Or maybe a better question is how much current does the Teensy draw from the VBAT pin?
- Is it possible to measure the VBAT voltage to get a sense of when the battery might need to be replaced? Especially in this scenario where people would be powering it from 3.3V and VBAT is required to get the device to power on, it would be really nice to know why the device isn't powering on anymore when the battery is dead.
- Would it be possible to make it more clear on the power section of the Teensy 4.1 documentation about how to handle the VIN/VUSB/VBAT/3.3V pins so someone like myself in the future doesn't fall into the same trap?

Thanks!
 
P.S. Some other related questions:
- How long can a typical CR2032 last powering the time management circuitry of the Teensy 4.1? Or maybe a better question is how much current does the Teensy draw from the VBAT pin?
- Is it possible to measure the VBAT voltage to get a sense of when the battery might need to be replaced? Especially in this scenario where people would be powering it from 3.3V and VBAT is required to get the device to power on, it would be really nice to know why the device isn't powering on anymore when the battery is dead.
this thread has some helpful posts, including some current measurements
 
Maybe the Power Up Sequence info on this page can help?

https://www.pjrc.com/store/ic_mkl02_t4.html

I had looked through the power up sequence page previously but wasn't sure how much of a requirement everything was (it mentions a USB voltage regulator needing power but assumed that was to convert 5V to 3.3V for powering the chip, something I'm doing externally). I was reading through another thread that I found (here) where someone else was trying to implement powering specifically from 3.3V and there was no mention about the USB circuitry, again reinforcing the idea that I shouldn't need it when powering the 3.3V pin directly.

Looking back at it now, knowing that powering the VIN pin is required to get the USB communication to work, I can understand how you could interpret that from the power up sequence page. I guess I just wish it was clearer for those trying to power up from 3.3V. Maybe it would be useful to update the Power > 3.3V Power section on the main page to describe the full details of powering from 3.3V pin instead of just pointing to that incomplete "special modification" link to the forum post.

I want the documentation to be complete when describing how to power the Teensy with 3.3V for people like me in the future. Thanks!
 
I am running into this same problem when migrating from T3.6 to T4.1 - the only way I can get the USB interface up is to first power the T4.1 via the USB port, then switch on the 3.3V supply. This works ok for testing, but is risky and means you cannot plug in the USB after the system is already running.

If I understand this discussion correctly, if I add a 3V coin cell, that would solve the issue by keeping the power management system up all the time? Or am I not following that correctly?
 
I have some large (10uF) caps on the system 3.3v power rail which is driven by a DC-DC converter to clean it up. These seems to be a likely the cause of the issue I am seeing. Would adding a separate small 3.3v LDO without any filtering caps to drive the battery terminal work - as long as it turns on faster than the main supply? Or a 3V shunt reference like an ADR530?
 
Hi nathanielbelles,
according your question on current consumption of VDD_SNVS_in see IMXRT1060CEC Rev. 3, 03/2022 pages 27/28.
Absolute maximum comsumption is 250µA, in power saving modes much less ... but I would calculate with worst case always.
 
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I wanted to follow up on this - adding a constant 3v supply to VBatt does not seem to fix the issue with the USB port. Any other thoughts?
 
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