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There are a large number of storage devices that you can use, such as M.2 SSD and DRAM memory. This post is mainly talking about the MMC card, but if you want to know other storage devices’ information, you can visit the MiniTool website.
The Definition of MMC Card
What is the MMC card? It is short for MultiMediaCard, which is a memory card standard used for solid-state storage. MMC was launched by SanDisk and Siemens AG in 1997. It is based on a surface-contact low-pin-count serial interface using a single memory stack substrate assembly.
Therefore, it is much smaller than earlier systems based on high-pin-count parallel interfaces using traditional surface-mount assembly, such as CompactFlash. Both products were initially released using SanDisk NOR-based flash technology.
The dimensions of the MMC card is 24 mm x 32 mm x 1.4 mm. MMC card originally used a 1-bit serial interface, but updated versions of the specification allow 4 or 8 bits to be transmitted at a time. MMC can be used in many devices that can use secure digital (SD) cards.
Typically, MMC is used as a storage medium for portable devices in a form that can be easily removed for PC access. Modern computers (laptops and desktops) usually have an SD slot, which can additionally read MMCs if the operating system drivers can.
As of 2018, few companies have built-in MMC slots (more commonly SD cards) in devices, but embedded MMC card (eMMC) is still widely used in consumer electronics products as the main means of integrated storage in portable devices.
It provides a low-cost flash memory system with a built-in controller, which can reside in an Android or Windows phone or a low-cost PC, and can be used as a bootable device on the host, rather than in a more expensive form, such as SSD.
The Variants of MMC Card
There are multiple variants of the MMC card, and some of them are listed below.
RS-MMC
In 2004, with dimensions of 24mm x 18mm x 1.4mm, RS-MMC (Reduced-Size MultiMediaCard) was released as a smaller form factor of the MMC card. It adopts a simple mechanical adapter to extend the card's length, so it can be used in any MMC (or SD) slot. Currently, the maximum capacity of RS-MMC is 2 GB.
The modern continuation of RS-MMC is often called MiniDrive (MD-MMC). MiniDrive is usually a microSD card adapter in the RS-MMC form factor. This enables users to take advantage of a wider range of modern MMCs to exceed the historical 2 GB limit of older chip technology.
In the past, both Nokia and Siemens were using RS-MMC, but now only Siemens continue to take advantage of MMC to continue using the MMC card for some PLC storage.
DV-MMC
DV-MMC (Dual-Voltage multimedia card) is one of the earliest acceptable changes in the MMC card, which can operate at 1.8 V in addition to 3.3 V. Operating at lower voltages reduces the power consumption of the card, which is important in mobile devices.
But simple dual-voltage parts were soon out of production and replaced by MMCplus and MMCmobile, which provided other features in addition to the dual-voltage feature.
MMCplus and MMCmobile
The version 4.x of the MMC standard (launched in 2005) brought two very important changes to compete with SD cards: the capability to run at higher speeds (26 MHz and 52 MHz) than the original MMC card (20 MHz) or SD (25 MHz, 50 MHz) and a four- or eight-bit-wide data bus.
Version 4.x full-size and reduced-size cards can be sold as MMCplus and MMCmobile, respectively.
Version 4.x cards are fully backward compatible with existing readers, but require updated hardware/software to use their new features; even though the 4-bit-wide bus and high-speed operating mode are intentionally electrically compatible with SD, the initialization protocol is different. Therefore, a firmware/software update is needed to take advantage of these capabilities in the SD reader.
MMCmicro
MMCmicro is a micro-size version of the MMC card. It measures 14mm x 12mm x 1.1mm, which is smaller and thinner than RS-MMC. Like MMCmobile, MMCmicro allows dual voltages, is backward compatible with MMC card, and can be used in full-size MMC and SD slots with a mechanical adapter.
The MMCmicro card possesses the high-speed and four-bit-bus features of the 4.x specification, but because there are no extra pins, there is no eight-bit bus.
Its predecessor was the S-Card when it was launched by Samsung on December 13, 2004. It was later adapted and adopted by the MultimediaCard Association (MMCA) in 2005 as the third form factor memory card in the MultimediaCard family.
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MMCmicro looks very similar to microSD, but both formats are physically incompatible and have incompatible pin assignments.
MiCard
MiCard is a backward-compatible extension of the MMC standard and was released on June 2, 2007. Its theoretical maximum size is 2048 GB (2 TB). The card is made up of two detachable parts, much like a microSD card with an SD adapter.
The small memory card can be installed directly in the USB port. It also has MMC-compatible electrical contacts, and the included electromechanical adapter can be installed in traditional MMC card and SD card readers. Now only one manufacturer (Pretec) has produced cards in this format.
eMMC
eMMC (embedded MMC) architecture puts MMC components (flash memory plus controller) in a small ball grid array (BGA) IC package, which can be used as an embedded non-volatile storage system on a circuit board.
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eMMC is available in 100, 153, 169 ball packages and is based on an 8-bit parallel interface. This is significantly different from other versions of the MMC card because it is not a user-removable card, but a permanent attachment to the circuit board. If the problem is caused by the memory or its controller, the PCB (printed circuit board) needs to be replaced or repaired.
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Until 2016, when UFS began to dominate the market, almost all phones and tablets used this form of flash memory as primary storage. The latest version of JEDEC's eMMC standard (JESD84-B51) is version 5.1A released in January 2019, and its speed is comparable to SATA-based discrete solid-state drives (400 MB/s). eMMC does not support the SPI-bus protocol.