As the subject says, I bought some ENC28J60 boards (MNI ENC28J60) and they are supposed to work at 3.3V but, they don't. The ProMini doesn't even see them as being attached.
When I switch the supply to 5V, they work fine. For clarification, this module DOESN'T have the 3.3/5V switch on it. It looks like this: https://aws.robu.in/wp-content/uploads/2017/09/IMG_0099.jpg
5V is supposed to blow these, what's going on?
SOLVED: The problem was that the USB to TTL converter I am using just doesn't put out enough power at 3.3V. When I power the ENC28J60 directly from it's own 3.3V source, it works fine.