Editing SD card (section)

====Electrical interface====
All SD card families initially use a 3.3&nbsp;[[volt]] electrical interface. On command, SDHC and SDXC cards can switch to 1.8&nbsp;V operation.<ref name="SDHCspec"/>

At power-up or card insertion, the voltage on pin 1 selects either the Serial Peripheral Interface (SPI) bus or the SD bus. The SD bus starts in one-bit mode, but the host device may issue a command to switch to the four-bit mode, if the SD card supports it. For various card types, support for the four-bit SD bus is either optional or mandatory.<ref name="SDHCspec"/>

After determining that the SD card supports it, the host device can also command the SD card to switch to a [[#Speed|higher transfer speed]]. Until determining the card's capabilities, the host device should not use a clock speed faster than 400&nbsp;kHz. SD cards other than SDIO (see below) have a "Default Speed" clock rate of 25&nbsp;MHz. The host device is not required to use the maximum clock speed that the card supports. It may operate at less than the maximum clock speed to conserve power.<ref name="SDHCspec"/> Between commands, the host device can stop the clock entirely.