Because many sites operate a scheme to connect an SD/MMC card to a ATmega microcontroller that is non functional, I mean scheme with dividing resistive to the ground, I decided to post an overall scheme real functional and will post and parameters operation of a card at different voltages, everything after more probe:
We know that most cards SD/MMC operates in a voltage between 2.7 and 3.6 V, I will recommend one to use the minimum operating voltage.
However such a card works at 5V supply voltage without any risk of failure, but a voltage across the card manufacturer's specifications will work only read data and read CID etc. Write delete functions will not work
SD/MMC card and ATmega at diferent voltages:
SD/MMC card and ATmega at same voltages: