Teensy 4.1 with 2.5v io

[cirthix]

Hi,
I've got a teensy 4.1 and need to interface with a 2.5v system. Could use a level shifter, but it would be easy to replace the tlv74733p with a tlv74725p. This would set the 3.3v rail of the teensy to 2.5v, including the NVCC_GPIO rail. Would this break other aspects of the board? Would the usb2 host port still function?

Thanks

[PaulS]

I would be seriously surprised when the chip is able to run at 2V5 supplied from the regulator.
Please have a look at this page for the rather complex power-up sequence.
A quick scan of the datasheet shows that the DCDC_IN pins[L1, L2] need to be powered by 3.0V at the minimum. Same for VDD_HIGH_IN and other pins.



Probably better to use a levelshifter...
Paul

[PaulStoffregen]

it would be easy to replace the tlv74733p with a tlv74725p.
...
Would this break other aspects of the board?

This would definitely break the power up sequence at step #7. Details here: (scroll down to "Power Up Sequence")

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

The bootloader chip waits until the 3.3V power reaches about 2.9 to 3.0 volts before it sends the enable signal to turn on the DCDC which makes the CPU voltage. If you solder a 2.5V regulator, it will just sit there waiting forever for the power to come up.

Must admit it's been years since I tested, but as I recall the 2.9V threshold was chosen because the DCDC can shut off under extreme conditions if the input voltage is too low. If you worked around the enable issue, you might need more capacitance at the DCDC input. Or you might need an external step-down converter and not use the one built into the chip.

Would the usb2 host port still function?

This too is supposed to be at least 3.0 volts, but that's mostly needed for the 1.5K presence resistor and 12 Mbit speed. If you run at 480 Mbit, pretty likely it will work, if your PC (or USB hub) is able to see 1.5K resistor to start the USB enumeration process.

[cirthix]

Thanks for the quick feedback, level shifter it is then.