The recent shopping spree has led us to great deals on microSD cards and with that has come the sad realization that many of us no longer have a phone with a memory expansion slot. This made us take a trip down memory lane to revisit the history of the microSD.

A few years ago we did a memory card retrospective, covering MMC, SD, Memory Stick and the like. Today we want to focus exclusively on the microSD because – for better or for worse – this is the card that won the format wars.

It’s ancient history now, we have an article from over a decade ago that chronicled the growing adoption of microSD. With very few exceptions, it was the memory card format of choice for most manufacturers. Looking back, it was an easy win: MMC and SD (and even the short-lived miniSD) were too big, and really only Sony was pushing Memory Stick.

The percentage of smartphone manufacturers adopting microSD by 2010
The percentage of smartphone manufacturers adopting microSD by 2010

microSD, sometimes referred to as “TransFlash”, was launched in 2004. The first phone to use the new card format was definitely a Motorola – there are a few models that came out in 2004, but evidence indicates that the Motorola E398 is the first.

The E396 was capable of playing MP3s and came with a 64MB card in the box. Even with heavy compression you couldn’t put many songs in it, but you could always take it out and replace it with a new card.

Flashback: The rise (and fall?) of the microSD card

This phone has an important place in history as it served as the basis for the Motorola ROKR E1, the first phone with iTunes support. Apple had a controlling 75% market share of digital music sales in 2005 and this was heavily dependent on the success of the iPod. However, Steve Jobs saw the danger that phones posed to his pocket music player and wanted to enter that market as well. The ROKR was a failure, but the phone that followed was, let’s just say, a huge success.

Flashback: The rise (and fall?) of the microSD card

microSD is a smaller version of the SD card. There are some minor differences (beyond size we mean), but they’re so small that a passive adapter can convert from microSD to full-size SD. This was handy for plugging the card into a computer to load it with songs, or for offloading the photos and videos you shot to your phone.

This relationship meant that microSD improvements went hand in hand with the evolution of the SD card. The first big change occurred in 2006 with the introduction of the SDHC – HC standard for “High Capacity”.

Previously, cards had a maximum capacity of 2GB. SDHC expanded it to 32GB and made support for FAT32 mandatory. This file system allowed not only large cards but also large files (up to 4GB).

Flashback: The rise (and fall?) of the microSD card

The next big leap was in 2009 with the SDXC format, “eXtended Capacity”. These have increased the limit to 2TB and switched to exFAT, an evolution of the FAT32 file system that allows files to grow beyond 4GB.

A few years ago the SD specification was updated with SDUC, “Ultra Capacity”, which supports cards up to 128TB. It will be a long, long time before that limit is reached. In fact, even the over ten year old SDXC format has yet to become a limiting factor as the largest microSD cards currently on the market have a capacity of 1TB.

The world's first 1TB microSD card arrived in 2019 with a price tag of $450

The world’s first 1TB microSD card arrived in 2019 with a price tag of $450

Capacity is the most important measurement of a microSD card, but there are a few others you should be aware of. The “speed class” is very important for some applications as it guarantees a minimum sequential write speed. The speed class is usually indicated on the card itself if you can read the icons.

The simplest rating is something like “class 2”, which is marked on the card as a 2 within a C. This means a guarantee that the card will never drop below 2MB/s. There are C2, C4, C6 and C10 classes. The faster the card, the faster you can copy files to it.

Flashback: The rise (and fall?) of the microSD card

Some real-time applications such as video recording are heavily dependent on sustained write speeds, so much so that there is a dedicated class for this. Ranging from V6 to V90, i.e. 6MB/s (enough for standard definition video) up to 90MB/s (needed for 8K footage).

Here’s a handy chart from the SD Association showing the relationship between sequential write speed and video resolution. Note that this is just a guideline as different cameras use different codecs at different bandwidths.

SD Speed ​​Classes Required for a Given Video Resolution (and Frame Rate)

SD Speed ​​Classes Required for a Given Video Resolution (and Frame Rate)

The original SD format had transfer speeds of up to 12.5MB/s, later increased to 25MB/s. The data bus was further upgraded to UHS-I (“Ultra High Speed”), which increased the speed limit to 104MB/s.

A comprehensive overview of SD speed classes

A comprehensive overview of SD speed classes

UHS-II is a major departure from the original format as it adds an extra row of pins. This further increased the transfer speed to 156 MB/s in full duplex mode and 312 MB/s in half duplex (ie data flows in both directions or only one direction, respectively). Putting an extra row of pins on large SD cards was easy enough, however, the size of the microSD posed a challenge.

UHS-II microSD cards exist, but they are rare and expensive. Even rarer appear to be devices that actually support UHS-II microSD cards. Even without UHS-II cards are good enough for high-resolution video capture, but the rise of smartphones has introduced a new challenge.

Higher speed requires more pins - insert UHS-II and SD Express
Higher speed requires more pins – insert UHS-II and SD Express

So far we have talked about the memory card as a storage for multimedia files: MP3 and video. These remain its most popular uses. A more interactive use is to archive apps and games, which have grown in size and complexity over time.

These aren’t good card applications, however, as they’re slow in another way. Video is recorded sequentially, so only sequential speeds count. Apps and games require quick random access, and most cards aren’t designed for that.

Some are better than others, though: The SD Association introduced the Application Performance Class. Both describe speed in terms of IOPS, random input/output operations per second. The first class is called A1 and guarantees 1,500 read IOPS and 500 write IOPS. A few years later came the A2, which raised the targets to 4,000 IOPS for reads and 2,000 IOPS for writes.

Flashback: The rise (and fall?) of the microSD card

The latest development is SD Express, which simply follows the example of NVMe SSDs and adopts the PCIe data bus. The original specification allowed for a single PCIe 3.0 lane and transfer speeds of up to 985MB/s. Then came support for a single PCIe 4.0 lane (or two PCIe 3.0 lanes) at up to 1,970MB/s. The maximum speed possible right now is achieved with two PCIe 4.0 lanes: a good 3,940 MB/s.

SD Express requires extra pins similar to UHS, which hinders adoption on tiny microSD cards. And as we said, devices that support the extra pins are rare.

The Steam Deck can run games from a microSD card, however Valve has only equipped it with a UHS-I slot. That means transfer speeds not much faster than a spinning hard drive (better seek times, but nowhere near as good as an SSD). The Nintendo Switch also only has one UHS-I slot.

The rise of SD and microSD cards

The rise of SD and microSD cards

MicroSD cards are still quite popular, their tiny form factor earning them a place in action cameras, drones, and so on. And they’ve found use in portable consoles, although larger SD cards (particularly the SD Express type) would have been a better choice.

However, their popularity on smartphones is declining. How come? We want to place partial blame on streaming services: How many MP3 and video files do you have on your phone? And your friends? With fast 4G and now faster 5G and falling mobile data costs, streaming has gone from being viable to the preferred option. Spotify, Netflix, YouTube, and the like mean you don’t need all that storage space on your phone.

Mobile gaming is now bigger than PC and console gaming combined, but that won’t increase microSD adoption for the reasons discussed above. A game large enough to have trouble fitting into internal memory will also be too challenging to escape from the card.

Another culprit is the growing built-in storage capacity. 128GB seems to be the average right now, and most people say they need 128-256GB. With that there isn’t much need for expandable storage.

Flashback: The rise (and fall?) of the microSD card

We know some of you absolutely hate that most manufacturers have stopped equipping their phones with microSD slots, especially in the flagship class. Unfortunately, the average consumer seems to care about the card slot as much as they do about compact phones. The same goes for the average smartphone maker.

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Philip Owell

Professional blogger, here to bring you new and interesting content every time you visit our blog.