SD Cards

The package for a SD card may say Ultra High Speed Pro, but the markings on the card tell the truth. These markings are defined by the SD Card association as shown below:

Size markings
SD up to 2 Gbytes
SDHC (High capacity) 4G to 32G
SDXC (Extended capacity) 64G to 2 Terabytes
Standard SD Cards, write speed markings

  • circle 2 – 2 Mbytes/sec
  • circle 4 – 4 Mbytes/sec
  • circle 6 – 6 Mbytes/sec
  • circle 8 – 8 Mbytes/sec
  • circle 10 – 10 Mbytes/sec

Interface speeds
(1) Default Speed mode: 3.3V signaling, Frequency up to 25 MHz, up to 12.5 MB/sec
(2) High Speed mode: 3.3V signaling, Frequency up to 50 MHz, up to 25 MB/sec

UHS SD Cards
There are new devices out call ultra-high speed (UHS). In fact the version 2 spec is finalized, but there are no parts yet. These devices add faster interface speeds. The UHS interface runs at up to 104 Mbytes/sec, and the UHS-II up to 312Mbyte/sec.

Write speed markings
U1 – 10 Mbytes/sec write speed
U3 – 30 Mbytes/sec write speed

Interface speeds supported
(1) Default Speed mode: 3.3V signaling, Frequency up to 25 MHz, up to 12.5 MB/sec
(2) High Speed mode: 3.3V signaling, Frequency up to 50 MHz, up to 25 MB/sec
(3) SDR12: UHS-I 1.8V signaling, Frequency up to 25 MHz, up to 12.5MB/sec
(4) SDR25: UHS-I 1.8V signaling, Frequency up to 50 MHz, up to 25MB/sec
(5) SDR50: UHS-I 1.8V signaling, Frequency up to 100 MHz, up to 50MB/sec
(6) SDR104: UHS-I 1.8V signaling, Frequency up to 208 MHz, up to 104MB/sec
(7) DDR50: UHS-I 1.8V signaling, Frequency up to 50 MHz, sampled on both clock edges, up to 50MB/sec
(8) UHS156: UHS-II RCLK Frequency Range 26MHz – 52MHz, up to 1.56Gbps per lane.
(9) UHS312: UHS-II RCLK Frequency Range 26MHz – 52MHz, up to 1.56Gbps two lanes half duplex

The UHS class 3 devices are available from a few vendors for about the same price, why would you buy anything else?

Link to the SD cards tested with a Raspberry PI.

http://elinux.org/RPi_SD_cards#SD_card_performance

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