MMC & SD development history

I. Overview

Both MMC and SD cards are mobile memory cards based on Nand Flash technology.

The MMC (MultiMediaCard) card was launched by Siemens and Sandisk in 1997, and the SD (Secure Digital Memory Card) card was first publicly released by Panasonic, Toshiba and Sandisk in 1999.

The SD card is based on the development of MMC. The initial dimensions of the two are also very similar. The SD card is 0.7mm thicker than the MMC card.

Early SD cards have strong compatibility with MMC cards. Most devices that support SD card slots can also support MMC cards. On the contrary, only devices with MMC card slots cannot support SD cards.

With the development and competition of MMC cards and SD cards, the difference between the two is getting bigger and bigger and moving in a completely different direction.

Today, the MMC card has basically withdrawn from the historical stage and turned to the embedded field, and launched the eMMC (embedded MMC) standard, which is widely used in embedded storage.

SD cards are becoming more and more stable in the field of mobile storage cards, and they are constantly developing towards large capacity and high speed. A series of interface standards have been introduced. The latest theoretical transmission rate has reached 985MB / s.

2. MMC development history

The full name of the MMC card is MultiMediaCard, which translates to "multimedia card" in Chinese.

The design goal of the MMC card is to provide a general low-cost data storage and communication medium "widely used in the fields of electronic toys, pda, cameras, smart phones, digital recorders, MP3 players, pagers, etc."

2.1 Major version evolution

2.1.1 MMC 1.x / 2.x version stage

From the currently retrievable MMC technical documents, the initial 1.0 version of MMC Spec has been formulated in 1996, and related products were launched in 1997.

The development stage of the 1.0 to 2.0 version was from 1996 to 2000. There was no major change in the shape and technical specifications of the MMC card at this stage, mainly due to the evolution of some internal functionality.

The main features of the MMC card at this stage:

• Size：24mm x 32mm x 1.4mm
• Pins：7pins
• Bus Width：1bit
• Bus Mode：MMC mode & SPI mode
• Voltage：2.7 - 3.6V
• Clock：0 – 20MHz

The appearance is probably like this (picture from the network):

2.1.2 MMC v3.x version stage

The v3.0 version is an important version in the development history of MMC. This version has major changes from the 1.0 / 2.0 version.

However, because the v3.0 version has serious errors in the internal register definition, the v3.0 version is quickly updated to v3.1, and the v3.0 version is abandoned.

The v3.1 version adds two important features :

1. Introduce Low Voltage specification, support 1.65-1.95V working voltage
2. Add multiple block read / write features to improve Performance

The v3.3 version is also an important version. This version adds RS-MMC (Reduced Size MMC) specifications, formulating 24mm x 18mm x 1.4mm size specifications, which reduces the size of the MMC card by nearly half.

RS-MMC is just the definition of physical size, the hardware interface has not changed, the appearance is probably like this (picture from the network):

2.1.3 MMC v4.x version stage

The v4.x series version has been released since 2004. v4.x is the most popular version of MMC and the longest-lasting version so far. It lasted 9 years from the release of v4.1 in 2004 to the release of v5.0 in 2013.

In the meantime, JEDEC (Joint Electron Device Engineering Council) adopted MMC4.1 standard as JEDEC's flash memory card standard;

Subsequently, MMCA (Multi Media Card Association Flash Memory Card Standard Organization) was formally incorporated into JEDEC, and the MMC standard was led by JEDEC.

Currently, the earliest version of MMC that can be downloaded from JEDEC official site is v4.1, which directly inherits from MMC v4.1 version of MMCA.

2.1.3.1 MMC v4.1 version

In view of the poor performance of the previous version of the MMC card, the v4.1 version adds the following important features to improve performance and achieve backward compatibility :

1. The operating frequency supports three modes: 0-20MHz, 0-26MHz, 0-52MHz,
2. Bus Width supports three modes: 1/4 / 8bits
3. Defined the minimum Performance standard: 2.4MB / s
4. Backwards compatible with v3.x version MMC (1 bit data bus, multicard systems)
5. MMC mode only supports one card per MMC bus
6. SPI mode supports MMC Chip Select Signal, which can realize multiple cards per MMC bus
7. Increase the storage capacity of the MMC card

The MMC card conforming to the v4.1 version specification is called HS MMC (High Speed ​​MMC). Due to the change in bus width, the interface of the HS MMC card has been increased by 6pins to 13pins

The v4.1 version clearly divides MMC products on the market and defines two types of MMC cards: MMC plus and MMC mobile . Only cards that meet the corresponding specifications can use MMC plus or MMC mobile logo.

MMC plus and MMC mobile are used in different usage scenarios and are backward compatible with the working mode of v3.x 20MHz clock

2.1.3.1.1 MMC plus specifications

MMC plus specifications:

• size: 24mm x 32mm x 1.4mm, full size
• Voltage：2.7 - 3.6V
• Pins：13pins
• bus width：1/4/8bits
• Bus Mode：MMC mode & SPI mode
• Clock: 26MHz (52MHz optional)
• Performance: not less than 2.4MB / s

Its appearance is shown below:

2.1.3.1.2 MMC mobile specifications

MMC mobile specifications:

• size: 24mm x 18mm x 1.4mm, comply with RS-MMC standard
• Voltage: 2.7-3.6V / 1.65-1.95V, support Low Voltage mode
• Pins：13pins
• bus width：1/4/8bits
• Bus Mode：MMC mode & SPI mode
• Clock: 26MHz (52MHz optional)
• Performance: not less than 2.4MB / s

Its appearance is shown below:

2.1.3.1.3 MMC mirco specifications

Samsung released an MMC mirco card at the end of 2004. This card is different from the MMC plus and MMC mobile specifications defined in the MMC Spec. Its size is reduced to: 12mm x 14mm x 1.1mm, which is about 1/3 of RS-MMC.

MMC micro is a third-party specification released by Samsung. It was not included in the MMC standard at the beginning. It relies on Samsung's own influence and its compact size to occupy a certain market position.

Subsequently, the MMCA Association also officially released the technical specifications of MMCmicro. At the MMCA Summer Conference in Zurich, Switzerland, at the end of June 2005, all MMCA participants unanimously agreed to establish the MMCmicro card as the new generation of MMCA standards.

Following the release of MMC plus card and MMC mobile card by MMCA at the end of 2004 , the new micro-sized MMC micro card is a new generation standard of MMC technology.

The appearance of the MMC micro card is shown below:

2.1.3.2 MMC v4.2 version

The addressing mode defined in the v4.1 version is Byte addressing, which can theoretically support a maximum capacity of 4GB

The v4.2 version adds sector addressing mode, each sector is 512Byte, the card with less than 2GB capacity adopts Byte addressing mode, and the card with more than 2GB capacity adopts sector addressing mode

At the same time, the v4.2 version changes the Low Voltage working voltage range to 1.7-1.95V

2.1.3.3 MMC v4.3 version

The v4.3 version is also a landmark version in the history of MMC development. This version introduces eMMC specification definition , supports eMMC boot function, and enters the embedded field.

The format of CID register has been redefined to distinguish whether Device is eMMC or MMC Card.

Although the MMC Spec does not officially include the specifications of the MMC micro card, the v4.3 Spec adds the definition of the signal input capacitance of the MMC micro (signal input capacitance standard).

2.1.3.4 MMC v4.4x version

An important change in the v4.4 version is the addition of DDR (Dual Data Rate) mode, that is, bilateral sampling of the signal, and data will be sampled once on the rising and falling edges of the clock.

DDR mode doubles the theoretical transfer rate to 104MB / s.

The v4.4 version also adds RPMB (Replay Protected Memory Block) function, which is used to encrypt and read and write data. RPMB is mainly used for the storage of some key private data of the system.

The next v4.41 version mainly adds two optional functions, Background Operations and High Priority Interrupt, which are not required to be implemented.

• Background Operations gives MMC / eMMC the ability to perform background operations
• High Priority Interrupt allows certain commands of MMC / eMMC to be executed and is interrupted by higher priority tasks

2.1.3.5 eMMC v4.5x version

The v4.5 version is another milestone version in the development history of MMC. The cover of this version has officially removed the MMCA logo, and only the JEDEC logo has been retained.

At the same time, the v4.5 version removes the support of MMC Card, and only retains the eMMC specification definition. MMC Card has also withdrawn from the historical stage and entered the era of eMMC standards.

Other important improvements in v4.5 include:

Increase HS200 mode, increase the operating frequency to 200MHz, and the theoretical transmission rate can reach 200MB / s

Add Cache function to further improve eMMC performance.

2.1.4 eMMC v5.x version stage

The main direction of eMMC development at this stage is to improve performance.

The v5.0 version released in 2013 added HS400 mode, and added DDR mode (bilateral signal sampling) on ​​the basis of HS200 to increase the theoretical transmission rate to 400MB / s.

The v5.1 version released in 2015 added the Command Queue function to optimize the operation efficiency of eMMC and improve the overall performance of eMMC.

The v5.1 version also released a Command Queue Host Controller Interface (CQHCI) standard for the Host side to implement the design standard for Command Queue hardware support.

In addition, the v5.1 version adds the HS400ES mode, which improves the reliability of CMD Responce reception based on the HS400 mode.

2.2 eMMC development trend

MMC has developed to the present day, leaving only the eMMC standard for embedded applications, but after the release of the v5.1 version in 2015, there have been no updated standards for 4 years.

If eMMC wants to further improve performance, it needs a higher frequency clock. For the parallel interface used by eMMC, it may encounter bottlenecks in signal integrity and other aspects.

In 2016, the UFS storage standard released by JEDEC adopted a differential serial bus to replace the parallel interface of eMMC, which can achieve far better performance than eMMC and has a tendency to replace eMMC.

At present, UFS is gradually replacing eMMC in the field of high-end handheld mobile devices; eMMC also occupies a relatively high proportion of applications in vehicle machines, low-cost / low-end handheld devices, and some other embedded devices.

Both eMMC and UFS are currently maintained and developed by JEDEC, and there is no competition. From the current situation, it is unlikely that eMMC will launch a revolutionary version upgrade.

Therefore, one can guess the future development trend:

eMMC may be gradually replaced by UFS and become an outdated technology; eMMC will also exist for a long time on some low-cost or low-performance storage devices.

 3. SD development history

SD (Secure Digital Memory Card) card is developed on the basis of MMC card, the Chinese name is: secure digital memory card.

At the beginning of the SD card release, the biggest difference with the MMC card was in security. It supports the SDMI standard and provides functions such as protecting the copyright of music stored on the SD card.

The SD card was developed in 1999, and Panasonic, Sandisk, and Toshiba established the SDA (SD Association) association in 2000 to lead the SD card standard development.

So far, the SD card standard has been developed to v7.0, which has occupied a dominant position in the field of mobile memory cards.

3.1 Evolution of important versions

3.1.1 v1.xx version stage

The v1.xx version phase began in 2000 and continued until the v2.xx version was released in 2006.

Perhaps benefiting from the advantage of latecomers, the SD card supports dual-voltage operation mode and 1 / 4bits bus width in the initial v1.00 version. At the beginning of the market, it was accepted by the market more than the MMC card in the same period.

The main features of the v1.00 version of the SD card initially released are:

• Size：24mm x 32mm x 2.1mm（Normal）/24mm x 32mm x 1.4mm（Thin）
• Pins：9pins
• Bus Width：1/4bits
• Bus Mode：SD mode & SPI mode
• Voltage：2.0 - 3.6V/1.6 - 3.6V
• Clock：0 – 25MHz
• Performance：10MB/s

In order to cope with the market competition of RS-MMC, SanDisk took the lead in launching miniSD in 2003, with a size of 21.5mm x 20mm x 1.4mm, known as the smallest Nand memory card at that time.

SD cards and MMC cards are moving forward in competition. The main competitor of SD v1.10 released in 2004 is the MMC v4.x standard.

SD v1.10 introduces High Speed ​​mode, which increases the Clock frequency to 0-50MHz, the maximum theoretical performance is 25MB / s, and is compatible with MMC v2.11 standard.

At the same time, TransFlash Card (also known as T-Flash or TF card) was launched, which was later unified into a microSD card.

The size of microSD is only: 11mm x 15mm x 1mm.

At this point, there are three physical sizes of SD : SD, miniSD, and microSD . In the subsequent market screening, only the two standards of SD and mircoSD were retained, that is, the two SD cards that can be purchased now (as shown in the figure below) .

3.1.2 v2.xx version stage

SD cards are similar to MMC cards, and have evolved in three directions: speed, capacity, and size.

In the v1.xx version stage, the size and specifications of the SD card have been basically finalized. The v2.00 standard introduced in 2006 expanded the capacity of the SD card to 32GB, and divided the SD card capacity into two specifications: SD / SDHC:

• SD: Standard Capacity SD Memory Card, refers to SD card with capacity of 2GB and below
• SDHC: High Capacity SD Memory Card, refers to SD card with 2GB-32GB capacity

3.1.3 v3.xx version stage

The v3.xx version released in 2009 is a very important stage in the history of SD card development. Although the SD card standard has been developed to v7.xx, v3.xx is still very popular.

Based on v2.xx version, v3.xx has the following major updates:

• Added SDXC capacity standard, SDXC standard SD card capacity is 32GB – 2TB
• Introduce UHS-I (Ultra High Speed) bus standard to raise Clock to 208MHz, the highest theoretical transmission rate is 104MB / s

To v3.xx version, the basic classification of SD card:

• Capacity: SD (0-2GB), SDHC (2-32GB), SDXC (32GB-2TB)
• 速度：Default（0 - 25MHz），High Speed mode （0 - 50MHz），UHS - I（0 -208MHz）

3.1.4 v4.xx version stage

From v3.xx version to v4.xx version, it can be counted as an SD card development to a watershed.

In the v3.xx version, the bus specifications supported by the SD card are all parallel buses with a width of 4 bits, with a maximum operating frequency of 208 MHz and a theoretical transmission rate of 104 MB / s.

Like the problems encountered by eMMC, if the SD standard of v3.xx version wants to further increase the transmission rate, the 4bits parallel bus will become its resistance.

Therefore, the v4.xx version introduces the UHS-II mode, which uses a low-voltage serial differential bus technology to increase the theoretical transmission rate to 312MB / s, supports full-duplex mode, and the single theoretical rate is 1.56Gbps.

3.1.5 Stage of rapid development

After the v4.20 version, the SD standard has entered a stage of rapid development. The following SD technology evolution diagram can well show its development.

v5.xx version unifies the classification specifications of SD card capacity and speed

The v6.xx version introduces a new speed mode, UHS-III, which increases the theoretical transmission rate to 524MB / s

The v7.xx version has been upgraded in speed and capacity, and introduced the SDUC capacity specification. The bus transmission supports PCIe, and the theoretical transmission rate is increased to 940MB / s.

3.2 SD development trend

Judging from the technical development status of the SD standard and the existing SD card products in the market, the formulation of the standard has been far ahead of the development of product technology.

The current development direction of SD cards is nothing more than larger capacity and faster speed. Both of these development directions depend on the technological progress of solid-state storage.

The SD standard has made the transmission channel thick enough. Next, let's see how each SD card manufacturer fills this thick channel and effectively uses its transmission performance.

4. Summary

MMC and SD have undergone more than 20 years of development as two homologous storage technologies, and today are moving towards two different technical routes.

MMC has evolved into the eMMC standard for the embedded storage field, and SD is deeply involved in the field of mobile storage.

From the current development trends of the two, eMMC is being replaced by more advanced UFS technical standards, and the development prospect of SD seems to be broader.

With the rapid development of technology, with the development of 5G and cloud storage technology, ubiquitous high-speed network connection and lower and lower cloud storage costs will be an inevitable trend.

We will wait and see what impact these technological advancements will have on UFS / eMMC / SD and other local storage technologies.