TLDR: A long trace to the
EN pin may pick up enough noise from the cellular radio to continuously reset a Boron.
I had an issue where most of my Boron devices would not connect reliably. They typically were in a state where they flashed green as if they were trying to establish a connection and then would reset (LED goes off, white, then flashes green again). This issue happened most often when my Boron was plugged into the Adafruit Terminal Block Breakout Feather Wing.
Thanks to @ParticleD, the particle engineering team theorized that the switch on that FeatherWing was picking up noise from the antenna, specifically while sending / receiving data. This noise was causing the
EN pin to go low enough to reset the power switch (U2) and consequently reset the Boron.
You can see that the
EN pin on the power switch is pulled up by a 100K resistor. Apparently this is not strong enough to overcome some noise on a long trace to the
EN pin as is the case on the FeatherWing. Particle recommended using a smaller pull-up resistor (10K) to keep the
EN pin high in the presence of noise.
Below are some screenshots of my scope connected to the
EN line. All of these tests were done with the Boron in the FeatherWing. The first test was with no additional pull-up resistor and the Boron was in a continuous reset cycle.
In almost all cases, the drop to around 1.7V on the
EN line was enough to reset the device.
Next I used a 22K resistor from
BATT (I was powering with battery only).
The drop to around 2.5V during cellular communication was not always enough to reset the device however there were cases where enough noise was picked up that the device would reset.
Finally I tested a 10K pull-up as Particle suggested.
There is still a measurable drop (600mV) but the
EN line is effectively kept high and the device works reliably. I suspect that if the battery were extremely low, it may be possible to experience reset events but I have not yet tested this.