CARDSPEED - Card Readers and Memory Cards

SD Compatibility

Actually, I wanted to start this page in November 2005, after the first 4GB SD cards became available. But at that time, there was no big demand for information, so I forgot about this issue. But now there are even more compatibility issues due to the SD 2.00 specification introducing SDHC.

Overview  �  SD Card Size  �  SD 2.00  �  SD/SDHC Overlap  �  Super SD  �  SDHC Players  �  Reader Compatibility

Quick Overview

SD 1.01

• One CSD register layout, CSD Version 1.0.
• 4bit transfers at up to 25MHz, good for about 10MB/s (66x).
• Technical limit of 4GB, more than 1GB requires special handling.
• Specified limit of 2GB to keep compatibility with FAT16 SD host devices.

SD 1.1

• 4bit transfers at up to 50MHz, good for about 22MB/s (150x).
• Full compatibility between cards and host devices, speed will be negotiated.

SD 2.00

• Additional CSD Register layout: CSD Version 2.0 for SDHC cards.
• SDHC standard for cards of more than 2GB and up to 32GB (technical limit 2TB).
• SDHC cards can only be used in SDHC compatible host devices.
• Speed Classes 2, 4 and 6 for guaranteed minimum sustained write rates of 2, 4 and 6MB/s.

SD Card Size Details

The first limit for regular SD cards is 4GB, due to the 32bit byte addressing scheme. In contrast to other standards, there is no block addressing mode defined.

The second limit is caused by the card size given in the CSD (Card Specific Data) Register of regular SD cards using CSD Version 1.0. There are two fields within this CSD that indicate the number of blocks available: C_SIZE and C_SIZE_MULT. You can interpret these two values as mantissa and exponent, where C_SIZE is a 12bit value with an offset of 1, and C_SIZE_MULT is a 3bit value with an offset of 2:
BLOCKNR = (C_SIZE+1)�2^{(C_SIZE_MULT+2)},   where C_SIZE_MULT < 8
The default block length is 512 bytes, but can also be specified as 1024 bytes or 2048 bytes. Actually, the block length is given through a third parameter in the CSD, the READ_BL_LEN. The block length is calculated as
BLOCK_LEN = 2^READ_BL_LEN,   where READ_BL_LEN = 9, 10 or 11
Note: Compared to the other two values, especially the C_SIZE_MULT with only three bit, the READ_BL_LEN is a 4bit value with only three different values allowed, and therefore a waste of register space.
Together, this leads to a total storage capacity of
BLOCKNR�BLOCK_LEN = (C_SIZE+1)�2^{C_SIZE_MULT+2}�2^READ_BL_LEN
When a block length of 1024 bytes is used, cards of up to 2GB can be specified; with a block length of 2048 bytes, cards of up to 4GB can be specified.
Devices that don't care about the READ_BL_LEN will assume a block length of 512 bytes and therefore be limited to a maximum of 1GB. Usually, these devices will detect cards of more than 1GB and up to 2GB as half their actual size, and cards of more than 2GB and up to 4GB as 1/4 of their actual size. Some devices even have hardcoded tables for interpreting C_SIZE_MULT values, and therefore might be limited to 512MB or 256MB.

The third limit was set by the SD Association: To ensure wider compatibility, FAT12 and FAT16 were defined as the standard fileystems for SD cards. According to the strict FAT16 rules, there can be a maximum of 65524 clusters available in a FAT16 filesystem, and the maximum cluster size is 32kB. This leads to a limit of 1.999GB for a FAT16 filesystem.
But the parameters of a FAT16 filesystem allow a number of up to 128 sectors per cluster, giving a cluster size of 64kB on a standard media with 512 bytes per sector. This allows FAT16 filesystems of up to 3.999GB. There are many devices with support for 64kB cluster size, but according to the FAT specification, this is not mandatory. Most likely, the 32kB limit was just set to have a defined limit for the cluster buffer in devices supporting FAT16.

SD 2.00: SDHC and SD Speed Class Rating

SD 2.00 is not a synonym for SDHC. SD 2.00 is a new SD specification, where SDHC is one of the features introduced with this specification. SDHC is not supposed to replace SD, it is an additional standard intended for SD cards larger than 2GB. This means that all SD cards of 2GB or less should still be implemented as regular SD cards, and all SD card of more than 2GB - in regular sizes 4GB or more - will be SDHC cards. This would be the same as the change from the regular Memory Stick Standard up to 128MB to Memory Stick PRO for more than 128MB, in practice 256MB or more.

To solve the card size problem, a new CSD register layout, CSD Version 2.0 was introduced. SD Devices are supposed to check the CSD version given in the CSD_STRUCTURE field first, and reject cards using an unknown CSD version. In CSD Version 2.0, the C_SIZE field was extended to 22bit. The C_SIZE_MULT was dropped, assuming a preset multiplier of 2¹⁰. The READ_BL_LEN was kept, but the only value allowed is 9, indicating a block length of 512 bytes. The calculation is the same, but due to the two fixed values, the formula shrinks down to
Size = (C_SIZE+1)�2¹⁹
The smallest card size would be 512kB, but SDHC is supposed to be used for cards larger than 2GB. The new register layout would allow cards of up to 2TB, which would also supported by the new 32bit block addressing scheme. But the specification defines an upper limit of 32GB, where only 16bit of C_SIZE are used and the upper 6bit remain unused.

Another feature introduced in the SD 2.00 Specification is the SD Speed Class Rating. These SD Speed Classes 2, 4 and 6 with a guaranteed minimum sustained write rate of 2, 4 or 6MB/s might seem very low for todays SD cards and their advertized speeds of up to 22.5MB/s (in most cases the maximum peak read rate). But for the SD Speed Class Rating, there are most likely several prerequisites - meaning that the card must be able to accept data at the given rate under certain conditions, e.g. filesystem fragmentation, bank switching, wear-levelling and defect mapping. But the section about the SD Speed Class Rating is not available in the public version of the SD 2.00 specification.

Together, this means that there will be, for example, 2GB SD 2.00 Class 2 "SD Cards" and 4GB SD 2.00 Class 2 "SDHC Cards".

SD and SDHC Card Size Overlap

The SD Association tried to keep the limit for regular "Standard Capacity" SD cards at 2GB. But there was the technical possibility to build 4GB SD cards and a huge demand for larger SD cards, well before the SD 2.00 specification was finalized.

Now we are dealing with the situation that there are regular SD cards of more than 2GB and up to 4GB that are officially called "non-compliant", and "compliant" SDHC cards of more than 2GB. In real-life, the overlap applies to 4GB cards. 4GB "non-compliant" SD cards will work in many SD host devices, either formatted to FAT16 with a non-standard cluster size of 64kB, or formatted to FAT32. However, these cards might not work in SDHC compatible host devices. On the other hand, 4GB SDHC cards will definitely not work in host devices that are not prepared for SDHC cards (either by default or through a firmware update).

Super SD Cards

There is a new type of SD card available, the Super SD card. This is a card that looks like an MMCplus card, but also supports the SD interface standard. The card is supposed to detect the host interface and either act as SD or as MMCplus card, in order to avoid the slow SPIO mode when using MMCplus cards in hosts that don't support the faster MMC 4 modes. The card size is compliant to the SD specification that allows a 1.4mm "Thin SD" form factor without write protect slider.

There is only very little public information available, including confusion about the number of pads, the supported modes, and especially the resulting speed with the same figures stated as Mbit/s and MB/s. At least the count of pads was easily to verify after unpacking the the card: There are 13 pads indicating that the card could support 8bit MMC 4.

While the hybrid SD/MMC concept sounds like an excellent idea, first tests showed that the host detection is not as good as it should be. The card seems to prefer running in SD mode, even if the host would also support the faster MMC 4 modes. In SD mode, the maximum read rate is about 10.67MB/s, and the maximum write rate is about 3.47MB/s. So it looks like the SD mode is SD 1.01.
In several SD 1.01 and even SD 1.1 / MMC 4 card readers, the card only runs in the slow MMC 3.31 mode, leading to exactly the situation that was supposed to be avoided by inventing the Super SD. Due to this behaviour, the Super SD qualified for a column in the compatibility table.

SDHC Players

This is a list of currently announced or maybe even available SDHC cards.

Card Readers: SDHC, SD and Super SD Compatibility

Information about 8GB SDHC compatibility will be added as soon as I get the first card. The table entries are explained below.
The sort order is by controller manufacturer, controller type and controller firmware or manufacturing week.
If you want to learn more about your card reader, see: How To Identify Your USB Card Reader.