3.3-volt interface. This is the only bus type that does not require a host license. One-bit SD bus mode: Separate command and data channels and a proprietary transfer format. Four-bit SD bus mode: Uses extra pins plus some reassigned pins. This is the same protocol as the one-bit SD bus mode which uses one command and four data lines for faster data transfer. All SD cards support this mode. UHS-I and UHS-II require this bus type. Two differential lines SD UHS-II mode: Uses two low-voltage differential interfaces to transfer commands and data. UHS-II cards include this interface in addition to the SD bus modes. The physical interface comprises 9 pins, except that the miniSD card adds two unconnected pins in the center and the microSD card omits one of the two VSS (Ground) pins. Notes: Direction is relative to card. I = Input, O = Output. PP = Push-Pull logic, OD = Open-Drain logic. S = Power Supply, NC = Not Connected (or logical high). Interface Command interface SD cards and host devices initially communicate through a synchronous one-bit interface, where the host device provides a clock signal that strobes single bits in and out of the SD card. The host device thereby sends 48-bit commands and receives responses. The card can signal that a response will be delayed, but the host device can abort the dialogue. Through issuing various commands, the host device can: Determine the type, memory capacity, and capabilities of the SD card Command the card to use a different voltage, different clock speed, or advanced electrical interface Prepare the card to receive a block to write to the flash memory, or read and reply with the contents of a specified block. The command interface is an extension of the MultiMediaCard (MMC) interface. SD cards dropped support for some of the commands in the MMC protocol, but added commands related to copy protection. By using only commands supported by both standards until determining the type of card inserted, a host device can accommodate both SD and MMC cards. Electrical interface All SD card families initially use a 3.3 volt electrical interface. On command, SDHC and SDXC cards can switch to 1.8 V operation. At initial power-up or card insertion, the host device selects either the Serial Peripheral Interface (SPI) bus or the one-bit SD bus by the voltage level present on Pin 1. Thereafter, the host device