Nucleo H745ZI-Q drawing current spikes from USB power port - producing noisy 5V

[MColl.3]

I have a Nucleo H745ZI-Q board powered from the CN1 STLINK USB port. I am observing bursts of noise on the 5V line.

The noise is 150mV p-p, burst of about 6 (80us) pulses, every 18ms.

The H745 is idle and the debugger is not active.

I have tried several different USB ports and adapters all with the same noise on the 5V.

Has anybody seen this? What could be causing the current draw?

[TDK]

What do you mean by "idle"? Is NRST pulled low?

It could also be something upstream of the USB connector. Does it still happen if you use a power supply?

[MColl.3]

The NRST line is high (not held in reset). By "idle" I mean the H745 MCU is not running a program (either core).

I have tried different USB power supplies, resulting in the same noise observations above. Later this week I will try a bench 5Vdc supply.

Thanks

[Tesla DeLorean]

Tight while(1) loops are not fuel efficient

[KMach.1]

It is the Ethernet PHY, it is active even without any MAC running in the MCU. To disable it, you need to remove SB177 and tie the concave pad to ground.