Secure Digital standard was originally developed by Matsushita (Panasonic), SanDisk and Toshiba in 2001 and is based on MultiMediaCard standard.

Figure 6: Secure Digital (SD) card.

Secure Digital cards were originally intended to protect the music industry by using rights management and copy-protection scheme from a group called SDMI (Secure Digital Music Initiative), hence its name. Funny enough, the SDMI specification was cracked shortly after it was released and the SDMI group no longer exists.

There is a smaller SD card called MiniSD, developed to be used on cell phones, which can be installed on SD slots using an adapter.

Figure 7: MiniSD card.

On SD slots you can use other kind of devices besides memory cards. These devices, called SDIO, can be modems, Bluetooth antennas, Wi-Fi (IEEE 802.11) antennas, GPS, etc.

As it occurs with CompactFlash, Secure Digital cards can be found in several transfer speed grades. An "1x" card has a transfer speed of 150 KB/s, the same unit used by CF cards, so you can compare SD speeds to CF speeds directly. A 32x card has a transfer speed of 4.8 MB/s on both SD and CF standards, for example. The same way as it occurs to CF, SD cards manufacturers don't specify if this speed is for reading or writing. Since write speeds are lower than read speeds, we think the transfer rate refers to the card read speed.

Main specs

• Available Capacities: 32 MB, 64 MB, 128 MB, 256 MB, 512 MB and 1 GB
• Voltage: 2.7 V to 3.6 V
• Power Consumption: < 33mA (read), < 35mA (write) and 100µA (standby)
• Read speed: 12 MB/s
• Write speed: 1x = 150 KB/s, 32x = 4.8 MB/s, 40x = 6.0 MB/s
• Length (SD): 1.26 in (32 mm)
• Width (SD): 0.94 in (24 mm)
• Height (SD): 0.082 in (2.1 mm)
• Weight (SD): 0.070 oz max (2 g max)
• Length (MiniSD): 0.85 in (21.5 mm)
• Width (MiniSD): 0.79 in (20 mm)
• Height (MiniSD): 0.055 in (1.4 mm)
• Weight (MiniSD): 0.035 oz max (1 g max)
• Operating temperature: -25º C to 85º C  (-13º F to 185º F)
• Storage  temperature: -40º C to 85º C  (-40º F to 185º F)
• Shock: 1000G max 
• Vibration: 15G peak-to-peak max 
• Humidity: 8% to 95% 
• For more information: http://www.sdcard.org