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

The recent shopping spree has us searching for great deals on microSD cards and with that came the sad realization that many of us no longer have a phone with a memory expansion slot. This sent us for a spin down memory lane revisiting the history of the microSD card.

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

That’s ancient history now, we have an article from over a decade ago that documents the growing adoption of the microSD card. With very few exceptions, memory card formatting has been the preferred choice for most manufacturers. In retrospect, it was an easy win – MMC and SD (and even the short-lived miniSD) were too big and only Sony was pushing the Memory Stick.

Percentage of smartphone makers that have adopted microSD by 2010
Percentage of smartphone makers that have adopted microSD by 2010

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

The E396 was capable of playing MP3 and came with a 64MB card in the box. Even with heavy compression, you can’t fit many songs on it, but you can always take it out and replace it with a new one.

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

This phone holds an important place in history as it served as the basis for the Motorola ROKR E1 – the first phone to support iTunes. Apple had 75% of the market share of digital music sales in 2005 and that was based heavily on the success of the iPod. However, Steve Jobs saw the danger phones posed to the pocket music player and wanted to get into that market as well. The ROKR was a failure, but the phone that followed it 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 (other than the size we mean), but they’re so small that a passive adapter can convert from a microSD to a full-size SD. This was useful for connecting the card to a computer to load it up with songs, or to download photos and videos you’ve taken to your phone.

This relationship means that microSD improvements are steadily increasing as the SD card evolves. The first big change came in 2006 with the introduction of SDHC – HC for “High Capacity” – the standard.

Previously, the capacity of cards was limited to 2 GB. SDHC expanded that to 32GB and made FAT32 support mandatory. This file system allows not only large cards but large files as well (up to 4GB).

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

The next big leap was in 2009 in the SDXC format, “eXtended Capacity”. These maxed out at 2TB and switched to exFAT, an evolution of the FAT32 file system that allows files to be larger than 4GB.

A few years ago, the SD specification was updated with SDUC, “Super Capacity”, which supports cards up to 128TB. It will be a long time before this limit is reached. In fact, even a decade plus the old SDXC format has not yet become a limiting factor because the largest microSD cards currently on the market have a capacity of 1TB.


The world's first 1TB microSD card arrived in 2019 at $450

The world’s first 1TB microSD card arrived in 2019 at $450

Capacity is the most important measurement for a microSD card, but there are a few other data you should be aware of. The Speed ​​Class is very important for some applications because it guarantees a minimum sequential write speed. The speed class is usually indicated on the card itself if you know how to read codes.

The simplest rating is just something like “Class 2”, which is marked on the card as the number 2 within C. This means a guarantee that the card will never fall short of 2MB/s. There are classes C2, C4, C6 and C10. The faster the card, the faster the files will be copied to it.

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

Some real-time applications such as video recording depend so heavily on sustained write speed, that there is a category dedicated to it. It goes from the V6 to the V90, which means from 6MB/s (enough for standard video) all the way up to 90MB/s (which you need for 8K footage).

Here is a handy chart from the SD Association showing the relationship between sequential write speed and video resolution. Note that this is just a guide as different cameras use different codecs with different frequency ranges.


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 provisions for transfer speeds of up to 12.5MB/s, which were later increased to 25MB/s. The data bus has been further upgraded with UHS-I (“Ultra High Speed”), which raises the speed limit to 104MB/s.


Complete overview of SD Speed ​​Classes

Complete overview of SD Speed ​​Classes

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

UHS-II microSD cards do exist, but they are rare and expensive. Devices that support UHS-II microSD cards seem to be much rarer. Even without UHS-II, the cards are good enough to capture HD video, but the advent of smartphones has presented a new challenge.

Greater speed requires more pins—enter UHS-II and SD Express
Greater speed requires more pins—enter UHS-II and SD Express

So far, we have talked about a memory card as a multimedia storage – MP3 files and videos. These are still the most popular uses. Its most common use is to store apps and games, which have grown in size and complexity over time.

However, these aren’t good card apps, because they’re slow in another way. The video is recorded in sequence, so only sequential speeds matter. Apps and games need fast random access and most cards are not designed for it.

Some are better than others – SD Association provided the Application Performance category. Both describe speed in terms of random I/O operations per second. The first category is called A1 and it guarantees 1500 IOPS for reads and 500 IOPS for writing. A few years later came the A2, which increased the targets to 4000 IOPS for reads and 2000 IOPS for writes.

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

The latest development is SD Express, which only follows the lead of NVMe SSDs and adopts 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 one PCIe 4.0 lane (or two PCIe 3.0 lanes) with a speed of up to 1,970MB/s. The highest possible speed is now achieved with two PCIe 4.0 lanes – a whopping 3940MB/s.

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

The Steam Deck can play games from a microSD card, however, Valve has only provided it with a UHS-I slot. This means that transfer speeds aren’t much higher than a spinning hard disk (better seek times, but nowhere near as good as an SSD). The Nintendo Switch also only has a UHS-I slot.


The emergence of SD and microSD cards

The emergence of SD and microSD cards

microSD cards are still fairly common, and their small form factor has earned them a place in action cameras, drones, etc. And they’ve found use in handheld consoles, even if larger SD cards (especially the SD Express type) are a better choice.

However, its popularity on smartphones is declining. How is that? We want to partially blame streaming services – how many MP3s and video files do you have on your phone? What about your friends? With fast 4G and now faster 5G and falling mobile data costs, streaming has gone from viable to preferred. Spotify, Netflix, YouTube, and the like mean you don’t need all that storage space on your phone.

Mobile games are now bigger than PC and console games combined, but this will not boost microSD adoption for the reasons discussed above. A game large enough to have trouble fitting into internal storage will be too demanding to get off the card.

The other reason is to increase the capacity of the built-in storage. 128GB seems to be the average at the moment and most people say they need 128-256GB. With that there is not much need for expandable storage.

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

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