I have a project that uses an SD Card on the SPI interface. I also wanted to add a small amount of extra EEPROM. The EEPROMs come in SPI and TWI interfaces. I only have one free pin right now so TWI would mean sacrificing something else, shuffling things around to end up with two free TWI pins, and adding a pullup resistor on the SDA line. It would seem that SPI would save me a pin and a resistor because I would use the same SPI bus and just need any single GPIO for CS of the EEPROM.
I'm currently using Petit FatFS for SD-Card access. If I share the SPI bus with the EEPROM I presume I'll need to un-mount the SD-Card (if it was previously used) before switching to the EEPROM? I don't think Petit FatFS's pf_mount can un-mount, unlike full FatFS's f_mount which says it can 'Register/Unregister the work area of the volume.' Does de-asserting the SD-Card's CS pin accomplish the same thing, more or less? Does this even matter since I would re-initialize and mount the card if I need to access it again later? Neither the SD-Card or EEPROM will be accessed regularly, and sometimes never. Is there a reason I shouldn't rule out a TWI EEPROM?