Question on Microcontroller Voltage

[Freshman]

I have this Microcontroller, https://ww1.microchip.com/downloads/aemDocuments/documents/MCU32/ProductDocuments/DataSheets/SAM-D5x-E5x-Family-Data-Sheet-DS60001507.pdf and using the USB interface of the MCU (PA24 & PA25).

On page 44 of the datasheet, it is mentioned like, the acceptable voltage for the MCU for the power rails would be 1.7V to 3.63V.

I am working on a Low Power USB device. So, I am using 1.8V for the Microcontroller Power Supply rails.
The micro is powered from 1.8V, and uses the micro's internal linear regulator for the 1.2V VCore (not using SW).

Laptop connected to the USB device. USB Type A to USB Type C cable. When I try to bootload via USB, I have issues.
When VDD (of MCU) drops below 1.75V, USB drops out. When VDD is > 2V, then no issues. Note that the micro doesn't reboot when USB drops out, and scoping the USB data lines seems to show the host still pinging every 1ms.

While debugging, I checked page 1884 on datasheet, an it mentions for USB, All operating voltage should be 3.3V.

Can someone please confirm what this means? Our board is designed for 1.8V. Does it mean for USB to work we need to power the micro from 3.3V?

The datasheet earlier says it can run down to 1.71? Which is correct?

Also is USB the only peripheral that won't work at 1.8V, or should we be concerned about other parts of the micro that won't work correctly when running the micro at what seems within the recommended voltage range?

[Jeroen3]

For USB to work the operating voltages must be 3.3V (Min. 3.0V, Max. 3.6V).
You cannot use the USB tranceiver with less. Because USB has a Voh of 2.8V to 3.6V, it's not possible to run it with only 1.8V.
USB also has clock requirements. Check those.

The rest of the chip seems fine at 1.8V.

Lucky for you, USB provides it's own 5V, so you can switch over 

Sometimes the analog peripherals also have similar constraints.
1.8V looks nice, but all logic on the board needs to support it, or you'll be buying level converters.

[ataradov]

Also is USB the only peripheral that won't work at 1.8V

It will work, it will just produce incompatible voltage levels, so it would be useless for working with other devices. This is not an MCU issue, it is a USB standard "issue".

This is pretty standard. I don't think there is an MCU out there that can work at 1.8 V and support USB.

[Freshman]

Thank you. But I am able to program sometimes with the same 1.8V power levels.

Sometimes, it fails.
When I use a shorter cable (cable length) between the laptop and the device, It works fine.

So, when you say it will not work, I have some cases, where it was working with 1.8V levels itself. Can it happen?

[Freshman]

When you mention, "USB provides it's own 5V, so you can switch over  ", how to switch over?

The MCU has only 1.8V. And no 5V. How to switch?

[ataradov]

Thank you. But I am able to program sometimes with the same 1.8V power levels.
Sometimes, it fails.

Well, this is how marginal things work.  It will also depend on the host, some will have higher threshold voltage and will not work at all. Others will have lower threshold and may appear to work fine.
Strange and unpredictable things may happen if you violate specifications.

The MCU has only 1.8V. And no 5V. How to switch?

Design your system to switch to 3.3 V operation when USB is plugged in. Derive 3.3 V from USB 5 V. You don't need to worry about low power when plugged in. I would not say it is easy, especially if you are also using peripheral devices that may have issues with that.

It may not be worth the power savings you get from going down to 1.8 V.

On the other hand, you have VDDIOB that can be permanently powered by 1.8 V and if you have enough pins in that domain, you can move all the peripherals there.