using an external reference V for DAC

[Dimitree]

hi everyone,
I noticed this part on the guide for various M4 Adafruit boards:

On the Metro M4, at least for now, AREF is tied to 3.3V due to a silicon v0 bug that does not allow the DACs to work unless AREF is connected to 3.3V. You cut the bottom jumper if you need a different AREF voltage but note that this may change DAC range!

I wanted to provide a stable and accurate external reference voltage to the DAC, but then I read this.
Is this a hardware "bug" or something in the bootloader that has been/will be fixed?

I also have another question (assuming that bug has been fixed and AREF can be used) :
I read on the datasheet that

The DAC voltage reference must be below VDDANA

checking with my multimeter VDDANA/VDD, I get 3.25V. If I use a precise 3.3V voltage reference for AREF, this will be higher than VDDANA. So do you think I should use a lower voltage reference? Or maybe I can tie VDDANA to the voltage reference too?

thank you!
Dimitri

[adafruit_support_mike]

Is this a hardware "bug"

Yep.. a design flaw in the physical circuit that can only be fixed by changing the circuit design in the silicon.

That isn't as huge a problem as it sounds.. chip designers know things go wrong, and usually design in blocks of spare transistors or low-level circuits that they can use to fix things. Instead of having to lay out a whole new chip, they can just change some connections in the metal interconnect layers.

That still isn't trivial, and I don't know whether Microchip has been able to spin a new version of the silicon among all the supply chain and chip shortage issues we've seen in the last couple of years. I'll check, but for now it's safest to assume the bug is still there.

[Dimitree]

thank you! but does this mean that I can't connect a < 3.3V reference to AREF, but still can connect a stable 3.3V reference to that pin? Or I must connect AREF to the VDD pin? I'm a bit confused

[adafruit_support_mike]

You can connect an external 3.3V source to the AREF pin, but you'll have to cut the trace connecting AREF to the onboard 3.3V supply. You never want to put two voltage sources in parallel.

[Dimitree]

Last question: If I use a precise 3.3V voltage reference for AREF, this will be higher than my actual VDDANA (since the onboard regulator is making 3.25V. The datasheet says that AREF must be < VDDANA. Would it be within tolerance or should I connect the reference to VDDANA too (removing the current connection)?

[adafruit_support_mike]

You'd need to use a lower external reference voltage.

Table 54-23 in the SAMD51 datasheet says an external reference needs to be between 1.0V and AVDD-0.4V. That 400mV offset is probably related to some internal headroom value.