Hi Charles,
I tried your breakout board with the same setup I used for mine, and it works. So it's not the software, and not the Arduino Feather board. Must be the difference in setup of the breakout board.

I solved an issue with the ground pin, so DVDD and VDD now both show 3.3V.
Further, I connected Force2 with Force+ instead of Ground (different to the diagram above), to adapt it to yours. Which doesn't change anything. It shouldn't be the difference in packaging, as SSOP takes less power than your TQFN. So the only difference I see are the two ferrite beads, which I don't use.
Which ones do you use? Or would you see a mistake I made with the assembly shown above?

Thanks!
Stefan

Alright, I ordered one of your boards. That way it's easier to see if it's a problem of the "simpler" arduino boards, or an assembly problem / change of sketch.
Did you check if VDD/DVDD shows 2.8V or 3.3V when connected with a "proper" Arduino?

No, M0 Adalogger, so it's the atsamd21, should run on 48MHz.

But even with a clock divider of 16 it wouldn't work.
I think there must be a hardware issue somewhere in the sketch.

Hm, ESP? Ah, no, the picture on the right is the realization of the schematic on the left. The SSOP packaging of the MAX31865 is less common than the TQFN which you're used to, so this might look like an ESP. But I'm more confident when it comes to soldering SSOP, so I chose this one.
Even though I use an 3.3V arduino, your comment might be right that I should reduce the clock frequency as the MAX31865 only accepts 5MHz. Will check this out later on.

Dear Charles,
it's been a while, but still, thanks for sharing this project!

I tried to modify this for a compact arduino embedded system with multiple breakout boards for the Adafruit Feather (as the Feather runs on 3.3V), but failed so far in getting any output.
Any recommendation for debugging, like simple SPI inputs which must generate a simple output? Something like "byte0 = SPI.transfer(0xFF); readValue = SPI.transfer(0xFF);" ? I guess there's a problem with the hardware, so I attached my setup:

With your

rtd.configure( true, false, false, USE_3WIRES, MAX31865_FAULT_DETECTION_NONE,
true, true, FAULT_LOW_THRESHOLD, FAULT_HIGH_THRESHOLD );
status = rtd.read_all();
I get "No communication, is breakout board plugged ?"

I thought it could be the soldering, but I measured all lines and pins, and remade the breakout board with the same result.

Also, I wonder if it's rather the Feather. 3.3 voltage regulator max is 400mA, pin max is 10mA, which should be sufficient as there's only an additional OLED board taking 20mA.
But DVDD/VDD I'd only measure 2.8V with the multimeter when running, so it might be that the voltage regulator is not strong enough. Or would I really need a ferrite bead between them?

Thanks again for sharing this project! It has been inspiring already.