I've been using a Teensy 2.0 for several projects and have really enjoyed it. I wanted to play around with RFID so I bought the following RC522 based module. http://www.banggood.com/MIFAIR-RC522...r-p-81067.html I didn't really read the specs before I purchased and it runs off 3.3v. Can I just use a 5v-3.3v regulator/buck to power the module and feed the SPI directly? Is the SPI going to be off going from 3.3V to a 5V system and need some conversion? Do I need to get something like this? http://www.ebay.com/itm/331073612421...84.m1423.l2649 Thanks for any information.