Bizarre behavior detected in a Raspberry Pi Pico based design and partially understood

ORIENTATION OF THE PRODUCT CAN CAUSE THE PI TO FREEZE AT STARTUP

This particular product with a Raspberry Pi Pico would not boot up when sitting flat on the table in its normal orientation, but if tilted or turned it would come right up. When I hooked up a USB serial connection to the debug header, it worked fine even flat on the table. 

INVESTIGATION BY DESIGNER AS WELL AS ME

I first assumed that I had introduced this problem in the product, but happened to describe it to the maker who was able to reproduce it reliably. I continued to try variations to determine whether it was pressure on a board or component that caused it. I also searched the web for any reports that might cause Pi Picos to fail to boot based on orientation, but found nothing. 

The designer found signals introduced on a serial connection trace when sitting in the normal (failing) orientation but not injected when rotated or otherwise turned. Traces were routed from the UART0 on the Pico to a header, for use in debugging. A voltage shifter for the RX line involved a considerable trace distance for that line, which is the one that picked up voltages. 

The exact mechanism that causes these spurious signals on UART RX to freeze the boot of the Pico aren't fully known, but likely it interrupted the boot loader. The fix is a pullup resistor on the line to absorb the phantom pulses, which the designer already tested. I will add it to my product. 

The source of signals that are coupled when flat on the table but not when rotated are not absolutely identified, but I suspect that the switching power supply sitting nearby is radiating like an antenna and the trace just happens to be resonant with the switching frequency. When the trace, a receiving antenna, is oriented at angles from the source, the signal received is weaker than when it is pointing directly at the source. 

I am intellectually curious about the coupling source and the mechanism of the freeze, but not enough to invest large amounts of time tracking them down. Just thought I would share this oddity for others who might be interested. 

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