It is the card detection switch but it isn't connected to anything on any of the Pi boards. The firmware detects card presence by trying to talk to it.
FCBGA5903 isn't a socket...MysticWhiteDragon wrote: ↑Thu Jan 23, 2020 7:13 pmNot enough pins for connections? How about 5093? Use this socket form for a CM.
https://www.anandtech.com/show/14182/ha ... ckage-ever