sicco

80 C ambient temperature is a challenge. The Teensy CPU easily adds 20 C when it’s working at 528 MHz. It has internal temperature sensors that shut it down when it sees it’s getting too hot. The trigger point for that can be increased a little...

PS this is not your level shifter i hope? https://tinkering.vandoeselaar.com/wp-content/uploads/2019/04/cdj-shifter.jpg (this will push 5V high levels onto the 3V power rail... the hysteresis will not work as intended… caps on the CLK and DATA...

Looks like it reports many more bytes received than what your master sent. Strong hint that your SPI clock signal at the slave input really sees more than 27*8 edges. And thus registers more bytes received while the CS was low. As it depends on...

Rezo, a diagram plus a photo, and the code that you use might help here. What does ‘things get messy’ look like? Are things not messy when SPI and GND wires are all short (< 5 cm)?

Then you probably have a resonating LC circuit on the SCK line. So not so much an echoing transmission line. Just LC that's ringing. 100pF... 12cm wires = 24 cm wire length = about 1 uH f0 = just a couple of MHz...

So how long is the SCK wire (PCB track) from Teensy pin to zener? And how long a copper track is GND-Teensy to same zener?

Maybe capacitance from the zener diodes. Note that the zeners are 3.3V and the logic high voltage level is also 3.3V, so already in normal operation these zeners start zenering already - with parasitic capacitance going sky-high. But you...

On “There is one other thing thing. If I change the SPI to SPI1.transfer(buffer, sizeof(buffer)); (and remove if(_spiDone)) the code only loops once and crashes after that.” - as mentioned above, echos on the Teensy SPI clock output AND INPUT pin...

The SPI clock speed in Hz has no impact. What matters are the ramps. So how fast the signal rises and falls. Unfortunately these are very fast transients and despite the many pin property settings i think there is no way to adjust any of that...

Looks to me like signal reflections on your SPI clock line. If the SPI clock signals wire gets longer then the problem starts to bite at some point. I can see the issue emerging already here in your scope image. If you had a better scope (higher...