USB protection required when slave is powered, but master (i.MX8M) isn't

[arnemaathuis]

Hello,

Our application is placed on a card which is mounted on a carrier PCB, out application is USB Master.

during certain situations the Carrier which contains the USB slave can be powered but the i.MX8M is still unpowered.

The slave can then provide power to the i.MX8M via the USB lines, which is undesired behaviour.

should we employ any safety precautions(eg with FSUSB104 buffers/switches) or can the USB peripheral handle this.

note: the USB1_VBUS is connected to the board 3V3 rail since the USB 5V power pin is not connected from slave to master.

regards Arne Maathuis

[arnemaathuis]

Hello Igor,

thank you for your answer, we'll add protection for this behaviour

Regards Arne Maathuis

[igorpadykov · Accepted solution]

Hi Arne

>during certain situations the Carrier which contains the USB slave can be

>powered but the i.MX8M is still unpowered.

I am afraid it is not allowed by sect.3.2.3 Power supplies usage

i.MX 8M Dual / 8M QuadLite / 8M Quad Applications Processors Data Sheet for Consumer Products

"I/O pins should not be externally driven while the I/O power supply for the pin (NVCC_xxx) is OFF. This
can cause internal latch-up and malfunctions due to reverse current flows."

Please note some processors datasheets explicitly mention this situation,

for example from sect.4.2.1 Power-Up Sequence

i.MX 6Dual/6Quad Applications Processors for Consumer Products - Data Sheet

"NOTE
USB_OTG_VBUS and USB_H1_VBUS are not part of the power supply
sequence and can be powered at any time."

Best regards
igor
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