how do sd cards work

How do SD Cards Work

Whether it’s on our cellphones, cameras, gaming consoles, or laptops, all of us use Secure Digital (SD) playing cards now and again for storage functions. Well, everybody prefers them, due to their small dimension, sturdiness, and non-volatile reminiscence.

If you will have a knack for expertise, youve most likely questioned how do SD playing cards work. Technically talking, they’re semiconductor-based flash reminiscence playing cards that use NAND flash reminiscence for storing information.

Moreover, they use the Write Protection mechanism to forestall unintended information loss. If you want to discover extra, this text will information you with all the pieces it is best to find out about SD playing cards and the way they work.

SD Cards Use NAND Flash Memory

The very first thing to know is that SD playing cards embed NAND flash reminiscence chips that shops blocks of information within the type of costs. Basically, the expertise adopts management and floating gates that assist management information circulate. Thus, if youre transferring information, paperwork, and even images to SD playing cards, they’re all saved electronically.

Moreover, NAND is predicated on Electronically Programmable Read Only Memory (EEPROM), that means it may be programmed and erased electronically. 

Besides SD playing cards, NAND flash reminiscence can be utilized in USB flash drives, SSDs, and different reminiscence gadgets. Given under are 5 of its varieties:

  • Single-Level Cells (SLC): This NAND shops just one bit of knowledge within the type of 0 or 1 in every cell, making the information circulate sooner. Although they’ve the best endurance, they’re comparatively costly.
  • Multi-Level Cells (MLC): This NAND shops two bits of knowledge in every cell. Thus, they’re slower than the SLCs however are comparatively economical.
  • Triple-Level Cells (TLC): As the identify suggests, this NAND shops three bits of knowledge in every cell. So, they’re even much less highly effective than SLCs and MLCs.
  • Quad-Level Cells (QLC): This NAND shops 4 bits of information per cell. Hence, they’re believed to be the slowest, least expensive, and with the least life expectancy among the many 4 varieties.
  • 3D NAND: Unlike the aforementioned 2D NANDs, they’re multi-layered and retailer information in reminiscence cells stacked one above the opposite. This ensures higher endurance and improved efficiency over the previous.

Basically, these NAND varieties are one of many many components that decide the lifespan of SSDs, SD playing cards, and different reminiscence drives. Hence, you probably have an SD card supporting 3D NAND, it typically lasts longer.

Although most producers declare that their SD playing cards final ten years, it is best to know that there are nonetheless different issues that decide the lifetime. For instance, utilizing the reminiscence playing cards at excessive temperatures, breaking the pins, or holding them close to a powerful magnet (electromagnetic discipline) could negatively affect their lives.

How Do SD Cards Store & Retrieve Data?

letter e in ascii code
The ASCII Value of E is Stored in SD Card

In earlier days, we closely relied on different storage drives, like Memory Sticks, Compact Flash playing cards, Smart Media playing cards, and so forth. Eventually, they have been changed by SD playing cards as they’re comparatively lighter in dimension and moreover assist giant storage.

Looking on the SD card carefully, youll probably discover a row of terminals (pins) on the prime part. Basically, some pins listed here are liable for powering the cardboard, whereas others work to assist customers retailer or get better information, like images, movies, paperwork, and different information.

Like different storage gadgets, these flash reminiscence playing cards additionally include transistors (that work like electrical switches) to assist retailer information in ASCII code (a mix of 0 and 1). Hence, if the swap is turned on, the worth 1 is saved within the SD card, and if off, 0 is saved.

For instance, should you attempt storing the phrase Example, it is going to be saved as 01000101 01111000 01100001 01101101 01110000 01101100 01100101

Well, the utilization of normal transistors solely aids in storing information quickly. Hence, the saved information get misplaced, and we cant retrieve them once more. Thus, the NAND expertise adopted by the SD playing cards makes use of MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) that makes use of management and floating gates to retailer information completely.

how data stores in single gate transistor
Illustration of How Data is Stored in a Single Gate Transistor

As proven within the illustration above, a typical transistor consists of a single gate. Here, when some optimistic voltage is handed from the supply (worth is 1), the gate is open, and information is saved. Likewise, when there is no such thing as a voltage (worth set to 0), the gate is closed, and data is just not saved.

In such a case, if we take away our drive from the facility supply, no voltage is handed, and even the saved information might be misplaced. Thus, utilizing a single-gate transistor isnt relevant to SD playing cards.

Data Stored as Charges Above the Floating Gate in a MOSFET

Now, lets take a look at the illustration of MOSFET, as proven above. Here, when the swap is turned on and the optimistic voltage is handed, some voltage will get saved on the prime of the floating gate.

This ensures that no matter information is saved stays even after we take away the SD card from its slot. Now, we will simply retrieve this data from some other gadget. Interestingly, you’ll be able to even use SD card readers and adapters in case your PC has no SD card slot.

Basically, the variety of transistors on an SD card determines the full reminiscence capability in SD playing cards. This signifies that the larger the variety of MOSFETs, the extra data might be saved. For instance, a 32 GB reminiscence card has a extra vital variety of transistors than a 16 GB, which is why we will retailer extra data within the former.

Lastly, if you delete the saved information in your SD card, this time, a detrimental voltage is handed within the management gate. This removes the optimistic voltage, inflicting the information to be deleted.

SD Card Architecture

sd card architecture
SD Card Architecture

An SD card features with these six parts – Contact Pins, Registers, Card Interface Controller, Memory Core Interface, Power on Detection, and Memory Core. In this part, we are going to focus on every of them briefly.

Contact Pins

The 9 contact pins or pads in an SD card guarantee the entire physique is seated accurately on the devoted slots within the supported gadgets. Otherwise, your pc will probably not learn the SD card.

Basically, the reminiscence playing cards have information, energy, and slot pins that work on both SPI (Serial Peripheral Interface) bus mode or SD bus mode. Moreover, they perform solely after interfacing with the Microcontroller Unit (MCU). 

Below are every of the pins and the person features they maintain:

  • Chip Select (CS): This pad or pin selects one or a set of built-in circuits. When energetic, it responds to enter modifications, and when inactive, it ignores any messages acquired.
  • Command (CMD): This is the command pin that helps the SD card work together with the microcontroller for the trade of important data.
  • VSS1 or GND: VVS1 is the primary voltage supply pin the place the bottom is to be related. 
  • VCC: VCC or the Voltage Collector is one other energy pin utilized in SD playing cards that connects to the optimistic energy in SD playing cards. Often, they’re additionally known as VDD or voltage drain. 
  • Clock (CLK): The clock pin is liable for getting the clock alerts or pulses from the microcontroller.
  • VSS2: Like the VSS1, they connect with the bottom and are additionally known as the GND pin.
  • DAT0 or DO: Basically, the Data Out or the DAT0 pin sends serial information to the MCU. In the SPI mode, that is utilized by the SPI Slave.
  • DAT1 and DAT2: The ultimate two pins are associated to information and are liable for the bi-directional communication between the SD card and MCU.

Like an SD card, the miniSD card additionally has 9 contact pins, however the microSD card has solely eight pins, the place DAT2 is lacking.


SD card registers are important parts that management numerous reminiscence and storage options. Moreover, in addition they maintain very important data, just like the reminiscence playing cards id quantity, serial quantity, manufacture date, and so forth. Below is a desk consisting of all six registers with their size and performance:

Register Function Length (in bits)
OCR (Operating Conditions Register) Stores voltage vary (2 – 3.6V) 32
CID (Card Identification Register) Stores the distinctive SD card identification quantity 128
RCA (Relative Card Address) (Only in SD mode) Selects SD card by the tackle 16
DSR (Driver Stage Register) Responsible for drive outputs 16
CSD (Card-Specific Data Register) Stores SD card particular configuration data 128
SCR (SD Card Configuration Register) Provides particular configuration options 64
Six Registers With Their Functions and Length (in bits)

Card Interface Controller

The SD Card Interface Controller communicates with the contact pins, registers, and reminiscence core interface to handle the general inner storage of the reminiscence card. Basically, the controller receives instructions from an finish person, and it responds accordingly. 

Well, an SD card is an enter gadget, and after we mount it on a PC, the CPU has to carry different operations and provides precedence to its instructions. However, due to the cardboard interface controller, these system interruptions are significantly minimized.

Moreover, the Card Interface Controller performs a number of operations, together with studying, writing, and erasing saved data on the SD playing cards.

Memory Core Interface

Memory Core Interface interacts with the Card Interface Controller to supply a easy and responsive interface to an SD card.

Basically, the part is liable for the total implementation of the reminiscence card. Also, it instructions and sends essential data to the Card Interface Controller for performing information operations.

Power on Detection

Power on Detection, situated on the appropriate facet, detects the SD card after we now have mounted it on a supporting gadget. 

Furthermore, it supplies energy to the Card Interface Controller and Memory Core Interface for correct functioning. Also, you want this part if youre planning to reset the SD playing cards reminiscence interface and controller.

Memory Core

The ultimate part of SD playing cards is the Memory Core. Basically, that is the core space for information storage.

However, the storage space depends upon the SD card household youre utilizing. Indeed, the working of an SD card is massively influenced by its kind. In brief, SDUCs have the best storage capability (2 TB to 128 TB) and likewise work sooner than the opposite SD households (SDSC, SDHC, and SDXC).

The under determine ought to assist you to determine the dimensions and reminiscence capability of various SD card varieties.

types of sd cards
SD Card Families With Their Size and Memory Capacity

How Secure are SD Cards?

One of the prime causes finish customers desire SD playing cards is their CPRM (Content Protection for Recordable Media) characteristic. This ensures copyright safety; therefore, others can not copy your content material illegally.

To allow CPRM, it’s essential encode your content material with Media Identifier and Media Key Block. This manner, even when one other person copies the information, they wont be capable of learn it. Moreover, to entry the CPRM-encoded information, you require a CPRM-enabled gadget and use the Media Identifier and Media Key Block to decrypt it.

Furthermore, some SD, SDHC, SDUC, and SDXC even have an added bodily safety mechanism inside the card. This is the write-protection lock and is current on the left a part of the reminiscence card.

Write Protection Lock
Write Protection Lock in an SD Card

Basically, sliding the lock downwards will activate write safety, that means you cant modify any information within the SD Card. Similarly, pushing it upwards will unlock your card, disabling the write-protection mechanism. This manner, it prevents an SD card from corruption.

Moving on, most storage gadgets are prone to bodily injury after falling from a sure peak. On the opposite hand, an SD playing cards wedge-shaped design, notch, and information grooves present extra safety. Moreover, they be certain that the cardboard is accurately inserted within the devoted slot.

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