Micro SD cards

Micro SD cards come on all kinds of flavours.  Sometimes one doesn't see the forest for the trees anymore.

Below is an overview of the most important characteristics of an SD card.

Speed

Speed is expressed in MB/s write speed.  There are 3 types of speed class:

• Speed Class (the standard speed class)

• UHS Speed Class

• Video Speed Class

• UHS Speed Class

Speed class

 The following speed classes are defined

• • Class 2: minimum speed is 2MB/s

  • Class 4: minimum speed is 4MB/s

  • Class 6: minimum speed is 6MB/s

  • Class 10: minimum speed is 10MB/s

The above mentioned numbers are minimum values.  In reality, the speed will be higher.

UHS speed class

Theres' also another way of indicating the write speed of an SD card: the UHS speed class.  This is the U-shaped container one can see on the SD card.

• UHS Speed Class 1: supports write speeds up to 10MB/s

• UHS Speed Class 2: supports write speeds up to 20MB/s

• UHS Speed Class 3: supports write speeds up to 30MB/s

Video speed class

This is a rather new speed class and is indicated with a V instead of an U.

The following video speeds are currently defined:

• V6: up to 6MB/s

• V10: up to 10MB/s

• V30: up to 30MB/s

• V60: up to 60MB/s

• V90: up to 90MB/s

UHS speed class

This is the next-level speed class and there are currently two ratings defined.  This is also more commonly used nowadays as hardware devices require write speeds that cannot be guaranteed anymore by the early standard speed classes.

The following ratings are defined:

• U1 (UHS Speed Class 1): minimum write speed of 10MB/s

• U3 (UHS Speed Class 3): minimum write speed of 30MB/s

UHS bus interfaces

To communicate with the hardware, bus interfaces are needed.  There are 2 types of bus interfaces defined for the moment.  The bus interfaces are denoted by a Roman numeral.  Below are the defined versions:

• UHS-I: supports write speeds up to 104MB/s

• UHS-II: supports write speeds up to 312MB/s

Capacity

Capacity is expressed in - mostly - GB.  The following capacities are defined:

• SD: Secure Digital, up to 2GB

• SDHC: Secure Digital High Capacity, between 2GB and 32GB

• SDXC: Secure Digital Extended Capacity, between 32GB and 2TB

• SDUC: Secure Digital Ultra Capacity, between 2TB and 128TB