CH32V003 warning - Don't leave SWIO/PD1 floating!

[mikeselectricstuff]

Just did a small production batch (300) and had about 5 units randomly resetting after a few secs to a few mins.
Turns out the cause was leaving SWDIO/PD1 floating.
Fix is to either add a pullup/pulldown R, or set bit 26 of AFIO->PCFR1 to make the pin I/O, and enable pullup/pulldown I/O mode on PD1.
Latter may disable debug functionality but it seems to still allow reprogramming without access to the reset pin.

[rhodges]

It sounds like a temporary workaround would be a jumper on the 3-pin programming header.

[HwAoRrDk]

That's weird. I haven't encountered such a problem myself. Not even with devices running continuously for many hours on end. I've never put any external pull-up/down on the SWDIO, and in my code although I am always configuring that pin explicitly as an input, it is without internal pull-up/down.

Maybe configuring PD1 as input is all that is necessary, regardless of internal pull-up/down setting. The datasheet is ambiguous as to whether SWDIO is active or not by default on that pin at reset. In section 1.4.16 it says: "The default debug interface pin function is turned on after system power on or reset", but then the pin-out table states that "Main function (after reset)" for PD1 is "PD1" (i.e. GPIO). I suspect it's the former.

Are you absolutely sure it wasn't a problem with NRST/PD7? I have had problems with the reset pin being overly sensitive and causing random resets, fixed by adding a 100nF capacitor there.

[mikeselectricstuff]

Yes, certain - PD7 has an internal pullup, but PD1 floats, which is never a good thing, even on pins that aren't connected to debug circuitry.
Had 3 units that reliably would reset after 10-20 seconds running. Grounding PD1 fixed them reliably.

Enabling pullup/down didn't actually enable pullup/down until I set bit 26 of the AFIO->PCFR1 register.

The SPI port was routed to a header alongside PD1, and it always reset at some random time within a SPI access, so I suspect it was noise coupling.

[DavidAlfa]

Ubused pins should he configured as analog to turn all its circuitry off.