SD/MMC Drivers

SD (Secure Digital) cards and MMCs (MultiMedia Cards) are portable, low-cost media often used for storage in consumer devices. Six variants, as shown in Table - SD/MMC devices, are widely available to electronic retail outlets, all supported by SD/MMC driver. The MMCplus and SD or SDHC are offered in compatible large card formats. Adapters are offered for the remaining devices so that these can fit in standard SD/MMC card slots.

Two further products incorporating SD/MMC technology are emerging. First, some cards now integrate both USB and SD/MMC connectivity, for increased ease-of-access in both PCs and embedded devices. The second are embedded MMC (trademarked eMMC), fixed flash-based media addressed like MMC cards.

Table - SD/MMC devices

Card	Size	Pin Count	Description
MMCPlus	32 x 24 x 1.4 mm	13	Most current MMC cards can operate with 1, 4 or 8 data lines, though legacy media were limited to a single data line. The maximum clock frequency is 20 MHz, providing for maximum theoretical transfer speeds of 20 MB/s, 80 MB/s and 160 MB/s for the three possible bus widths.
MMCmobile	18 x 24 x 1.4 mm	13	Most current MMC cards can operate with 1, 4 or 8 data lines, though legacy media were limited to a single data line. The maximum clock frequency is 20 MHz, providing for maximum theoretical transfer speeds of 20 MB/s, 80 MB/s and 160 MB/s for the three possible bus widths.
MMCmicro	14 x 12 x 1.1 mm	13	Most current MMC cards can operate with 1, 4 or 8 data lines, though legacy media were limited to a single data line. The maximum clock frequency is 20 MHz, providing for maximum theoretical transfer speeds of 20 MB/s, 80 MB/s and 160 MB/s for the three possible bus widths.
SD or SDHC	32 x 24 x 1.4 mm	9	SD cards can operate in cardmode with 1 or 4 data lines or in SPI mode. The maximum clock frequency is 25 MHz, providing for maximum theoretical transfer speeds of 25 MHz and 50 MHz for the two possible bus widths.
SDmini	21.5 x 20 x 1.4 mm	11	SD cards can operate in cardmode with 1 or 4 data lines or in SPI mode. The maximum clock frequency is 25 MHz, providing for maximum theoretical transfer speeds of 25 MHz and 50 MHz for the two possible bus widths.
SDmicro	15 x 11 x 1.0 mm	8	SD cards can operate in cardmode with 1 or 4 data lines or in SPI mode. The maximum clock frequency is 25 MHz, providing for maximum theoretical transfer speeds of 25 MHz and 50 MHz for the two possible bus widths.

SD/MMC cards can be used in two modes: card mode (also referred to as MMC mode and SD mode) and SPI mode. The former offers up to 8 data lines (depending on the type of card); the latter, only one data line, but the accessibility of a communication bus common on many MCUs/MPUs. Because these modes involve different command protocols, they require different drivers.