A MicroSD is the most used type of memory card today, stores have been filled with them in packs with an adapter that allows them to be used in a standard SD card reader. If we add to this that they are used in mobile phones to expand their capacity, then we are faced with what has made both the classic SD cards and even more the MiniSD have completely disappeared from the map.
However, they have been with us for so long and surely more than one reader has an SD card in a drawer, inside a digital camera or smartphone from a few years ago. When observing them you will have seen how in many of them abbreviations appear on their label that do not appear in the rest. Acronyms such as SD, SDHC, SDXC or SDUC, refer to different capacities and speeds of SD cards of different types.
The different types of SD cards that exist
It is a type of memory card that comes in three different versions in terms of size, but apart from this the capacities do not vary from one type of size to another, which is why currently and as we have explained in the introduction, this has made MicroSD the current standard.
- SD: its size is 32 x 24 x 2.1 mm.
- MiniSD: they are no longer seen in stores, their size is 21.5 x 20 x 1.4 mm
- Micro SD: the most common today, with the following dimensions: 15 x 11 x 1 mm
However, it is wrong to talk about these types of cards, since the standard for this type of Memory Cards is currently version 8.0 and has had several improvements over time in terms of capacity and bandwidth. Saying that a MicroSDXC is a MicroSD is like saying the same thing about an NVMe SSD under PCI Express 3.0 and another under Gen 4. The interface and even the shape may be the same, but the performance is different.
That is why we are going to take a trip through the different generations of MicroSD cards until we reach the present so that you can see how they have evolved and so that you think twice before using an old card of this type in your smartphone more new. Or you simply wonder how it is that the memory card that you have just asked yourself does not work with that device from a few years ago that you have recently rescued.
SD, SDHC, SDXC, SDUC and SDIO, what do they mean?
They are different generations of SD cards that have been appearing from the year 2000 to the present, in short, the differences between them are the following:
- SD: the first card of all, makes use of the system of FAT32 files and because of this it cannot pass from the 2GB of storageare currently withdrawn from the market.
- SDHC; They have capabilities ranging from 2GB to 32GB, also out of the current market due to its low storage capacity. He was the first to use the exFAT files.
- SDXC: the most used today, since it can reach up to 2TB of storage.
- SDUC: If 2 TB of capacity seems too little, then you have the option of SDUCs that can reach up to 128TB. They are rare and expensive at the moment, but will replace SDXC when the time comes.
Then we have a category called SDIOwhich are cards not for storage, but rather function as interface for peripherals. In this way we can add a Bluetooth module, a WiFi, a GPS. They are not at all common in home computing, but they are widely used in robotics, especially to integrate communication and positioning into robots.
Classes and speeds of SD, MiniSD and MicroSD cards
Another thing that you will have noticed is that apart from the type of MicroSD, which indicates its generation, it is the so-called Speed Class, which is marked on each of the cards. Well, if it is not a UHS Speed Class then the number tells us the number of megabytes per second of sequential writing that we can achieve. For the Class 10 we need a MicroSDHC card, which requires a somewhat more powerful interface than the classic MicroSD.
In contrast, in the UHS Speed Class the number indicates the number of megabytes per second measured in tens. UHS-1 type cards can reach 10MB/sthe UHS-II all 30MB and for the UHS-III can be reached at speeds of 60 and 90 MB/s speed in sequential writing.
SD Express and Micro SD Express
The SD Express and the MicroSD Express are memory cards that replace the classic interface of their predecessors with one based on the third generation PCI Express, but using a single line. The latest version of the standard allows you to create cards with a PCIe 4.0 and not only one line, but up to 2 lines.
They are usually the rarest to see next to the UHS-III, due to the fact that they are the type of interface that consumes the most and this is counterproductive for any device that depends on a battery to function.
The interface for SD, MicroSD and MiniSD cards
In every memory card there is an interface that allows the content of the card to be communicated with the device that wants to access it. It should be noted that all SD, MiniSD or MicroSD cards are backward compatible, but not forward compatible, this is because later generations have included pins for additional data, but have kept the position of the previous ones.
| bus type | Peak speed (MB/s) | SD cards that use it |
|---|---|---|
| Default Speed (SD) | 12.5 | All |
| High Speed (HS) | 25 | All |
| Ultra High Speed I (UHS-I) | 104 | SDHC, SDXC, SDUC |
| Ultra High Speed II (UHS-II) | 312 | SDHC, SDXC, SDUC |
| Ultra High Speed III (UHS-III) | 624 | SDHC, SDXC, SDUC |
| SD Express (PCIe 3.0) | 1969 | SDHC, SDXC, SDUC |
| SD Express (PCIe 4.0) | 3984 | SDHC, SDXC, SDUC |
Thus, the transfer speed that we can obtain will not only depend on the type of card, but also on the device. For example, if we use a standard SD card in a reader for SD Express, it does not work at its maximum speed, but at a maximum of 50 MB/s and vice versa. Therefore, try to match each type of card with its corresponding interface to get the most out of both and that the interface is not a bottleneck for the card or vice versa.
