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MColl.3
MColl.3
Associate
September 18, 2020
Question

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

  • September 18, 2020
  • 4 replies
  • 1428 views

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?

This topic has been closed for replies.

4 replies

TDK
TDK
September 18, 2020

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?

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MColl.3
MColl.3Author
Associate
September 21, 2020

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
Tesla DeLorean
Guru
April 15, 2021

Tight while(1) loops are not fuel efficient

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KMach.1
KMach.1
Associate
April 15, 2021

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.

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