STUSB4500 Unexpectedly Resets.

[segfault]

Good day. Thank you for taking the time to review this query.

 

We have recently developed a USB PD board using the STUSB4500. Most features seem to be in working order: PDO negotiation and NVM programming are functional and reliable. We have added a SMPS on the board (at the output of the STUSB4500 power path) for a 3v3 output in addition to USB PD.

 

There is a problem when probing the STUSB4500, however. When the board powers on, everything seems fine, until the board's GND is probed with the GND of a battery-powered multi-meter or oscilloscope connected to mains (without connecting the positive connection) - when this happens, it seems like the chip resets, because power is cut to the SMPS. We are using a 10K pull-down that is 2 mm from the RESET pin (and we did also try 1K and 100K, but this did not resolve the resetting issue).

 

After a decent amount of debugging, I noticed that there is improvement under these conditions:

1. The GND probe is connected before the board is plugged in and powered on. This has been a seemingly reliable fix so far. (This would lead me to suspect ESD issues, but proper protection has been put in place on the board).
2. A capacitor (~100 nF) is placed in parallel to the pull-down resistor. This fix still has the odd glitch, but is improved a lot.
3. The GND of the board is connected to the GND of a desktop USB port (without powering the board from the USB port). This has been a seemingly reliable fix so far.
4. The board is powered from a desktop. This has been a seemingly reliable fix so far.
5. A human touches (without releasing) the GND of the board before powering it on. This has been a seemingly reliable fix so far.
6. Powering the board from a power bank. This has been a seemingly reliable fix so far.

 

Even with the above mentioned fixes, I noticed that the RESET pin seems to be hyper sensitive. When these are implemented, the GND probes can reliably be connected without problems, but then probing the RESET pin will cause a reset of the chip. The slightest touch (even with a finger) seemingly causes a reset.

 

The total routed track length to the reset pin on the header is ~87 mm.

Here’s to hoping there is a simple fix for this.

 

Kind regards.

[Didier HERROUIN]

Dear Segfault,

The reset track is very long indeed. 
You can try to connect the Reset pin directly to GND and check the result.
A 100nF decoupling capacitor in parallel with the resistor may also be useful.

Best regards,

Didier Herrouin

[segfault]

Hi Didier,

Thank you for your response.

Connecting the RESET pin to GND directly is a reliable correction, but you then lose the ability to externally reset the chip - which isn’t preferable.

Using the 100 nF capacitor in parallel to the pull-down resistor is indeed useful, since it eliminates almost all of the reset glitches. However, I have been able to recreate the issue once. This, in addition to shortening the reset line, is the current approach to correct the problems for the next revision.

This does not seem unique to our layout. A user of SparkFun’s implementation reports a similar symptom; SparkFun Forum. Unfortunately, that thread is unresolved, but the behaviour looks related. It is a different board and it uses the minimum implementation, however.

Thank you for your time.

Kind regards.

[Didier HERROUIN]

Dear Segfault,

Indeed, touching a pcb (referenced to a “floating ground”) with a wire or connecting a probe will generate high spurious. For STUSB4500, this can trigger the chipset reset or the CC overvoltage protection.

Except if you really need to reset the chipset from the Host as you mentioned, our last recommendation is to connect the Reset pin directly to GND as in the ref design SCS001V1 : https://www.st.com/resource/en/schematic_pack/stref-scs001v1_schematic.pdf

Thank you for sharing your discoveries and experiences !

Best regards,