Memory card interfaces keep pace with the internal bus evolution race: Part 1

Clock speeds get faster. Per-cycle (and per-clock edge) address and data dollops get larger. And protocols get more efficient. But here we’re talking about external, not internal, buses.

Back in 2023, I devoted two blog posts’ worth of content to comparing various memory card technologies, products and speed bin options, initially in March (identifying a fake card in the process) and more in-depth in July. Since then, I’ve come across numerous examples of both evolutionary and revolutionary successors to the devices discussed in that two-part series, not to mention those covered in even more distant-past writeups (themed, for example, around the cameras, digital audio recorders and other devices that leverage such storage).

I’ve had this follow-up piece in my to-do list for a while now, and I’ve finally decided to actualize my longstanding aspiration before the dust pile accumulating on this specific list entry gets any deeper. Not every technology to be discussed in the paragraphs to follow will likely achieve high-volume market success, mind you, with any sooner-or-later failures not necessarily the result of implementation shortcomings, either. Note, for example, that today’s (and past) industry supply constraints encourage manufacturers to “double down” on maximizing the output and profitability of existing approaches, versus devoting scarce capacity to dubious bets.

That said, win or lose there’s usually an interesting story behind each approach. Without further ado…and with the upfront qualifier that I’ll be intentionally delaying any discussion of USB-interface memory devices until later, since their connector locations compel them to be fully external to the system, either sticking straight out of it or cable-tethered to it…and that for related reasons, I won’t be covering eMMC and other fully internal formats, either…and lastly, that I’ll be skipping over legacy formats that were proprietary and/or otherwise non-impactful…

Historical precedents

A short writeup, “History Repeating” at Virginia Tech’s website, begins as follows:

Variations on the repeating-history theme appear alongside debates about attribution. Irish statesman Edmund Burke is often misquoted as having said, “Those who don’t know history are destined to repeat it.” Spanish philosopher George Santayana is credited with the aphorism, “Those who cannot remember the past are condemned to repeat it,” while British statesman Winston Churchill wrote, “Those that fail to learn from history are doomed to repeat it.”

Long-time readers may recall that I’ve referenced variants of this same quote theme in several past writeups, consistently with a negative connotation involving the downsides of ignorance to the past. That said, excessive dependence on history lessons can also be problematic, resulting in evolutionary, overly constraining baby-steps that suppress alternative more revolutionary strides, which may lead to failure but may also dramatically leap beyond traditional approaches.

I’ll leave you to decide for yourselves what to conclude from this first case study, admittedly too personal to likely allow me to be completely arms-length about it! Embedded within the tuple (card identifier) data structures reported by Intel’s Series 2 flash memory cards were the initials of the small team of developers, myself among them, who designed their ASIC (30 years ago…yikes!). I subsequently led the technical marketing launch of the 28F008SA 8 Mbit flash memories inside those same cards, followed by the definition, development and introduction of 16 and 32 Mbit component successors and cards based on them, all in the early-to-mid-1990s.

Products such as these, representing the industry’s first removable and high capacity (for the era, at least) memory cards, added these tuple structures and other enhancements in order to deliver full Personal Computer Memory Card International Association (PCMCIA, later known as PC Card) compatibility, in contrast to Series 1 precursors which were more elementary multi-component arrays along with address decode and chip select logic. Intel’s and others’ similar products were specifically referred to as linear flash memory PC Cards, both to differentiate them from other PCMCIA card types—modems, ISDN and SCSI, for example, and living on (at least to a degree) with CableCARDs—and from alternative ATA-interface flash memory cards.

The key difference between the two memory card types centered on where the flash media management intelligence was located: in the card itself for ATA flash PC Cards, thereby presenting a standardized hardware and software interface to the system regardless of what (and whose) media was inside, versus in the system, implemented as software and/or dedicated hardware, for the linear flash PC card approach. Proponents of the latter scheme touted its claimed reduced media bill-of-materials cost, not to mention the potential ability to direct-execute code out of it (acting as a big parallel-interface chip), but it was inherently relevant for only NOR (vs NAND) memory suppliers, along with being a “heavier lift” for system developers. For these and other reasons, the ATA approach eventually won out in the marketplace.

Miniaturization

That said, Intel and several of its NOR flash memory partner/competitors had also taken a stab at miniaturizing the linear flash PC Card with the creatively named (ha!) Miniature Card format:

Other flash memory suppliers countered with the ultimately much more popular CompactFlash card, now maintained by the aptly named CompactFlash Association (CFA), whose hardware interface was similarly PCMCIA-derived albeit instead (as with the ATA flash PC Card precursor) focused on the IDE/ATA (and later, UDMA) command set:

Amid this “where is the media management intelligence best located” debate, two other notable contending approaches of the same timeframe also bear mentioning. The first, SmartMedia, was championed by Toshiba (as well as, later, by its primary competitor, Samsung):

SmartMedia was essentially a single (although a few variants embedded multiple) NAND flash memory die embedded within a thin plastic membrane, plus a multi-contact metallic interface that wirebond-direct-connected to the die with no intervening media controller intelligence.

Conceptually sounds like linear flash PC Cards and their derivatives, doesn’t it? Yes…and no. For one thing, SmartMedia was much smaller than either Miniature Card or Compact Flash. For another, it was based on NAND flash memory, which was more HDD-like in its core attributes (notably erase block size and speed) than NOR, simplifying system-side media management development. And then there was the fact that Toshiba wasn’t just a semiconductor supplier; its various systems divisions were potential SmartMedia implementers, and the company also did a good job of cultivating business from other Japanese and broader Asian systems manufacturers.

Finally, near the end of the last century (in 1997, to be exact), Sandisk and systems partners Siemens and Nokia unveiled the MultiMediaCard (MMC), which ultimately came in multiple dimension options, as well as in both standard and clock-boosted performance variants:

MMC is best known today in its aforementioned non-removable eMMC form, which itself is being slowly supplanted by the embedded variant of the MIPI- and SCSI-based Universal Flash Storage (UFS) (an organization whose own removable-version standard ironically has conversely been underwhelmingly adopted by the industry). Today’s generational successor to MMC is the Secure Digital (SD) card, originally referred to as SecureMMC:

which built on the MMC foundation with “enhancements including a digital rights management (DRM) feature, a more durable physical casing, and a mechanical write-protect switch.” The SD standard’s successive iterations have expanded the available clock speed, protocol and electrical contact count options in a backwards-compatible fashion to keep pace with flash memory performance gains, such as in this high-end V90 card from OWC:

The microSD Card derivative tackled substantive dimensional decreases with notable success; here’s one alongside the SmartMedia card I showed you earlier:

One interesting newer SD (and microSD) card specification variation that I became aware of recently when shopping for storage media for a couple of new Raspberry Pi cards is the Application Performance Class. Quoting from Kingston Technology documentation:

A new classification has been presented with the introduction of Android’s Adopted Storage Device feature. The App Performance Class assures minimum random and sequential performance speeds to meet both run and store execution time requirements under given conditions. It does this simultaneously while providing storage for pictures, videos, music, files and other important data. Basically, they’re ideal for use in smartphones and mobile gaming devices that run applications at random read and write speeds while also being used for storage.

There are two ratings for the App Performance Class which are known as A1 and A2. A1 has a minimum random read of 1500 IOPS and a minimum random write of 500 IOPS while A2 has a minimum random read of 4000 IOPS and a minimum random write of 2000 IOPS. Both A1 and A2 have a minimum sustained write speed of 10MB/s. The App Performance Class is something to consider [editor: for example] when planning on installing Android apps on a microSD card.

And, by the way, unlike the SmartMedia competitor of the day, both MMC and successor SD Cards notably also embed (despite their smaller sizes) media management intelligence that simplifies and standardizes the system implementation. Moore’s Law strikes again, eh?

Hang tight; I’ll be right back

Believe it or not, I originally envisioned this being, and wrote it as, a single unified blog post. However, as thought of more (and more…and more…) things to include, the wordcount grew (and grew…and grew…), transforming it into something resembling a small book (I exaggerate, but you get my drift). Having passed through 1,500 words at the beginning of this paragraph, I’m instead going to pause for now, intending (God willing) to share the other half of this now-two-part series with you next week. Until then, please share in the comments your thoughts on what I’ve covered so far!

—Brian Dipert is the associate editor, as well as a contributing editor, at EDN.

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