Swan switched 3.3V output resets the STM32L4R5


This seemed like the best place to report the issue I am having with the Swan.

My hardware setup:

I have a Swan programmed as a USB device powered from the USB bus (no battery). I have a second board providing the USB host which has a 500mA current limit on VBUS supplied to the Swan. The Swan is driving a NFC Reader chip (ST25R3916) powered from the Swan 3.3V Vout. The ST25R3916 requires around 100-200 mA.

I am using STMCubeIDE and an STLink to programme the Swan.

My problem:

When my code enables the 3.3V output to power the ST25R3916 the STLink disconnects (I think the VIO bus suffes a short glitch resetting the CPU).

(As a separate test I have a STM ST25R3916 Discovery board (with STM32L476) running the same software which I can connect to the same USB Host and it powers up without resseting the STLink.)

My workaround:

I hardwired the nEN_3V3_OUT line to 1.8V to permanently enable the 3.3V output and all is good.

My theory:

I suspect that the 200uF worth of capacitance on the 3.3Vout is causing a current inrush that is triping the USB Host 500mA current limit. A TPS53020 only requires around 60uF on its output at 3.3V so removing one of the 100uF capacitors might help but it looked like there was a risk damaging the board if I try this.

My suggestion

It would perhaps be nice to nave a zero-ohm option on the Swan PCB to enable the 3.3V output.


Hey abstractsig,

Thanks for the feedback. Why don’t you give it a shot pulling off the 100uF caps and see if you can reproduce. DM me if you end up damaging the board doing the rework. But before that.

When you refer to the swan powering the NFC reader chip is this on a board of your own design or the STM discovery board? Or both?

Did you try running your code with the STLink attached while the swan is attached to a USB power adapter that can source more than 2.5W? Like a 2.4A charger?

Can you confirm the hardware devices you have attached to your host computer?
Are you using the STLINK-V3MINI?
Are you plugging the Swan into a USB hub (self powered or adapter powered) or a root port on your computer?
Are you plugging the STLinkV3mini into a USB hub?

For the discovery board + STM ST25R3916 are you attaching both USB ports to a root port on your machine or onto

When you use this device in your deployment how are you powering the swan?

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Short answer is I pulled one of the 100uF caps off the PCB (no damage to PCB) and it now works.

Long answer

The NFC reader hardware I am using is a ST X-NUCLEO-NFC06A1.
The host is a i.MXRT1050-EVB; it has 5V/3A power but its USB host/OTG ports have the 500mA current limiters.

To programme the Swan I use the STLink on a random STM Nucleo board.

The Swan will switch on the 3.3V output to the NFC reader without disconnecting the STLink if I connect the Swan to a powered USB hub.

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