2024-01-09 10:04 PM
I want to run MCU with VDD 1.8V (in order to have all I/O-signals as 1.8V). I changed to use a LQFP64 package which provides a dedicated pin for VDDUSB.
I power VDDUSB with 3.3V (for sure not below 2.7V) but all other VDDs on MCU are 1.8V:
MCU seems to run with VDD a bit below 1.8V, e.g. 1.794V. But USB is not working anymore!
For USB I need at least 1.804V: then boot is OK, USB comes up and later VDD can drop to 1.793V and still working (but not booting up properly with USB on 1.793V: no USB enumeration anymore).
All my 1.8V ordered and soldered LDOs do not work - because they provide just 1.79xV: USB does not come up.
And the debugger (external ST-Link) becomes flaky with VDD a bit below 1.8V (50% of the tries it does not find the target).
So,
with "off-the-shelf" LDOs (ordered) for 1.8V, even USBVDD is 3.3V - it does not work (with USB, but MCU seems to be running still).
BTW:
With a VDD a bit below 1.8V and VDDUSB 3.3V - I can shortcut very briefly (but not permanently) VDDSMPS with VDD11: then the USB comes up and keeps running, even VDD is a bit below 1.8V (e.g. 1.794V).
It tells me: the SMPS might not start working if below 1.8V and: I would be running anyway "right at the corner of the voltage (specification)" - not very reliable even I could get it to work with finding an LDO providing 1.804 instead of 1.79xV.
Or: Am I doing something wrong?
I am out of ideas why 1.8V VDD and 3.3V VDDUSB should not work, or why the MCU is so "sensitive" on VDD as 1.8V (and a tiny bit below).
BTW: all works fine when I power VDD with greater 1.8V: VDD as 1.8V seems to be right at the edge of spec. I think I am STILL running in the spec. range - but it fails. Is the spec. wrong?
Solved! Go to Solution.
2024-01-17 06:11 PM
I use SMPS (see comments below).
Schematics are attached here.
Comments:
All fine for me. I want to go with U5A5 anyway (larger memories) and same board works fine with this version (just U575 seemed to have this issue).
BTW: my VDD is above 1V (1V8 used). And I do not have a specific power-up-scheme on board.
STM32U5A5/STM32U575 MCU board (with LQFP64 package)
2024-01-10 06:15 AM - edited 2024-01-10 06:16 AM
Is VDDUSB at/below VDD during power-up?
Can you clarify if you're using SMPS or not?
A schematic would be insightful, if you can provide.
2024-01-17 06:11 PM
I use SMPS (see comments below).
Schematics are attached here.
Comments:
All fine for me. I want to go with U5A5 anyway (larger memories) and same board works fine with this version (just U575 seemed to have this issue).
BTW: my VDD is above 1V (1V8 used). And I do not have a specific power-up-scheme on board.
STM32U5A5/STM32U575 MCU board (with LQFP64 package)