Editing SD card (section)

== Bus speed ratings ==

SD card performance is commonly measured by its sequential read and write speeds. These metrics are especially relevant for applications involving large files, such as photos and videos, where data is accessed in contiguous blocks. In contrast, performance for smaller files—such as directory metadata or configuration files—depends more on random access speed, often measured in input/output operations per second (IOPS). Random access performance can vary significantly between cards and may be a limiting factor in some use cases.<ref name="necstudy">{{cite conference |last1=Kim |first1=H. |last2=Agrawal |first2=N. |last3=Ungureanu |first3=C. |title=Revisiting Storage for Smartphones |conference=USENIX Conference on File and Storage Technologies (FAST) |publisher=NEC Laboratories America |date=2012-01-30 |url=http://static.usenix.org/events/fast/tech/full_papers/Kim.pdf |archive-url=https://web.archive.org/web/20121010194100/http://static.usenix.org/events/fast/tech/full_papers/Kim.pdf |archive-date=2012-10-10 |access-date=2012-12-27 |quote=Speed class considered irrelevant: our benchmarking reveals that the "speed class" marking on SD cards is not necessarily indicative of application performance; although the class rating is meant for sequential performance, we find several cases in which higher-grade SD cards performed worse than lower-grade ones overall.}}</ref><ref name="Gough2014">{{cite web |last=Lui |first=Gough |title=SD Card Sequential, Medium & Small Block Performance Round-Up |url=http://goughlui.com/2014/01/16/testing-sd-card-performance-round-up/ |website=Gough's techzone |date=2014-01-16 |access-date=2015-11-29 |archive-url=https://web.archive.org/web/20151208112031/http://goughlui.com/2014/01/16/testing-sd-card-performance-round-up/ |archive-date=2015-12-08 |quote=Variations in 4k small block performance saw a difference of approximately 300-fold between the fastest and slowest cards. Distressingly, many of the tested cards were mediocre to poor on that metric, which may explain why running updates on Linux running off SD cards can take a very long time.}}</ref><ref>{{cite web |title=Raspberry Pi forum: SD card benchmarks |url=http://www.raspberrypi.org/forums/viewtopic.php?t=4076 |access-date=2014-08-12 |archive-url=https://web.archive.org/web/20140813011920/http://www.raspberrypi.org/forums/viewtopic.php?t=4076 |archive-date=2014-08-13 |url-status=live}}</ref>

Subsequent generations of SD cards have improved performance primarily by increasing the bus speed—the frequency of the clock signal used to transfer data between the card and the host device. Regardless of the bus rate, the card can signal that it is "busy" while completing a read or write operation. Compliance with higher speed ratings generally means that the card reduces its reliance on the "busy" signal, allowing for more efficient data transfers.

{| class="wikitable" style="text-align: center;"
|+ Comparison of bus speeds<ref>{{Cite web |date=2020-12-11 |title=Bus Speed (Default Speed/High Speed/UHS/SD Express) {{!}} SD Association |url=https://www.sdcard.org/developers/sd-standard-overview/bus-speed-default-speed-high-speed-uhs-sd-express/ |access-date=2025-01-07 |website=www.sdcard.org |language=en-US}}</ref>
! rowspan="2" scope="colgroup" |
!  rowspan="2" |Mark
! colspan="3" |Bus
! colspan="4" scope="colgroup" | Standards
! rowspan="2" scope="col" | {{Abbr|Spec|Specification}}
|-
! scope="col" | Speed
! scope="col" | [[PCI Express|PCIe]]
! scope="col" | [[Duplex (telecommunications)|Duplex]]
! scope="col" | SD
! scope="col" | SDHC
! scope="col" | SDXC
! scope="col" | SDUC
|-
! scope="row" | Default
| {{N/a}}
| {{Nowrap|12.5 MB/s}}
| rowspan="8" {{N/a}}
| Half
| rowspan="2" {{Yes}}
| rowspan="11" {{Yes}}
| rowspan="11" {{Yes}}
| rowspan="11" {{Yes}}
| 1.01
|-
! scope="row" | {{Nowrap|High Speed}}
| {{N/a}}
| {{Nowrap|25 MB/s}}
|Half
| 1.10
|-
! scope="row" rowspan="2" | UHS-I
| rowspan="2" style="background:white" | [[File:UHS-I.svg|x20px]]
| {{Nowrap|50 MB/s}}
| rowspan="2" |Half
| rowspan="9" {{No}}
| rowspan="2" | 3.01
|-
| {{Nowrap|104 MB/s}}
|-
! scope="row" rowspan="2" | UHS-II
| rowspan="2" style="background:white" | [[File:UHS-II.svg|x20px]]
| {{Nowrap|156 MB/s}}
| Full
| rowspan="2" | 4.00
|-
| {{Nowrap|312 MB/s}}
| Half
|-
! scope="row" rowspan="2" | UHS-III
| rowspan="2" style="background:white" | [[File:UHS-III.svg|x20px]]
| {{Nowrap|312 MB/s}}
| rowspan="2" | Full
| rowspan="2" | 6.00
|-
| {{Nowrap|624 MB/s}}
|-
! scope="row" rowspan="3" | SD Express
| rowspan="3" style="background:white" | [[File:SDex(short).svg|x20px]]
| {{Nowrap|985 MB/s}}
| {{Nowrap|3.1 ×1}}
| rowspan="3" {{N/a}}
| 7.00, 7.10
|-
| {{Nowrap|1,970 MB/s}}
| {{Nowrap|3.1 ×2}}, {{Nowrap|4.0 ×1}}
| rowspan="2"| 8.0
|-
| {{Nowrap|3,940 MB/s}}
| {{Nowrap|4.0 ×2}}
|}

{| class="wikitable collapsible collapsed" style="text-align: center;"
|+Bus speed of host and card combinations (in MB/s)<ref>{{Cite journal|date=June 2018|title=SD Express Cards with Pie and Name Interfaces|url=https://www.sdcard.org/downloads/pls/latest_whitepapers/SD_Express_Cards_with_PCIe_and_NVMe_Interfaces_White_Paper.pdf|journal=SD Association|pages=9|access-date=2018-06-27|archive-date=2020-11-12|archive-url=https://web.archive.org/web/20201112011753/https://www.sdcard.org/downloads/pls/latest_whitepapers/SD_Express_Cards_with_PCIe_and_NVMe_Interfaces_White_Paper.pdf|url-status=dead}}</ref>
! colspan="2" rowspan="2" {{Diagonal split header 2|Card|Host}}
! colspan="2" | UHS-I
! colspan="2" | UHS-II
! rowspan="2" | UHS-III
! rowspan="2" | Express
|-
! UHS50
! UHS104
! {{Abbr|Full|Full-duplex}}
! {{Abbr|Half|Half-duplex}}
|-
! rowspan="2" |UHS-I
! UHS50
|'''{{0|0}}50'''
|{{0|0}}50
|{{0|0}}50
|{{0|0}}50
|{{0|0}}50
|{{0|0}}50
|-
! UHS104
|{{0|0}}50
|'''104'''
|104
|104
|104
|104
|-
! rowspan="2" |UHS-II
! {{Abbr|Full|Full-duplex}}
|{{0|0}}50
|104
|'''156'''
|156
|156
|104
|-
! {{Abbr|Half|Half-duplex}}
|{{0|0}}50
|104
|156
|'''312'''
|312
|104
|-
! colspan="2" |UHS-III
|{{0|0}}50
|104
|156
|312
|'''624'''
|104
|-
! colspan="2" |Express
|{{0|0}}50
|104
|104
|104
|104
|'''985'''
|}

=== Default Speed ===
The original SD bus interface, introduced with version 1.00 of the SD specification, supported a maximum transfer rate of 12.5&nbsp;MB/s. This mode is referred to as '''Default Speed'''.

=== High Speed ===
With version 1.10 of the specification, the SD Association introduced '''High-Speed''' mode, which increased the maximum transfer rate to 25&nbsp;MB/s. This enhancement was designed to meet the growing performance requirements of devices such as digital cameras.<ref>{{cite web |url=https://www.sdcard.org/developers/overview/bus_speed/ |title=Bus Speed (Default Speed/High Speed/UHS/SD Express) |publisher=SD Association |access-date=2020-04-18 |archive-url=https://web.archive.org/web/20161004053633/https://www.sdcard.org/developers/overview/bus_speed/ |archive-date=2016-10-04 |url-status=live }}</ref>

=== {{Anchor|Ultra High Speed|UHS}} UHS (Ultra High Speed) ===

The '''Ultra High Speed''' ('''UHS''') bus is a type of interface used by some [[Secure Digital#SDHC|SDHC]] and [[Secure Digital#SDXC|SDXC]] cards to enable faster data transfer between the card and a host device.<ref name="sda_bus_speed">{{cite web |url=https://www.sdcard.org/developers/overview/bus_speed/ |title=Bus Speed (Default Speed/ High Speed/ UHS) |publisher=SD Association |access-date=2013-11-13 |archive-date=2016-10-04 |archive-url=https://web.archive.org/web/20161004053633/https://www.sdcard.org/developers/overview/bus_speed/ |url-status=live }}</ref><ref>{{cite web |url=https://www.engadget.com/2010/06/24/sd-cards-branded-with-an-upper-case-i-are-faster-yo/ |title=SD cards branded with an upper-case 'I' are faster, yo |publisher=Engadget |date=2010-06-24 |access-date=2010-08-22 |archive-date=2010-08-28 |archive-url=https://web.archive.org/web/20100828145021/http://www.engadget.com/2010/06/24/sd-cards-branded-with-an-upper-case-i-are-faster-yo/ |url-status=live }}</ref>

UHS-compatible cards are marked with Roman numerals next to the SD logo, indicating the version of the UHS standard they support.<ref name="sda_bus_speed"/><ref name="sda_speed_class">{{cite web |title=SD Speed Class |url=https://www.sdcard.org/developers/overview/speed_class |url-status=dead |archive-url=https://web.archive.org/web/20201221172542/https://www.sdcard.org/developers/overview/speed_class/ |archive-date=2020-12-21 |access-date=2013-11-13 |publisher=SD Association}}</ref> These cards offer significantly faster read and write speeds than earlier SD card types, making them well suited for high-resolution video, burst photography, and other data-intensive applications.

To achieve higher transfer speeds, UHS cards and devices use specialized electrical signaling and hardware interfaces. UHS-I cards operate at 1.8&nbsp;V instead of the standard 3.3&nbsp;V and use a four-bit transfer mode. UHS-II and UHS-III introduce a second row of interface pins and use [[low-voltage differential signaling]] (LVDS) at 0.4&nbsp;V to increase speed and reduce power consumption and electromagnetic interference (EMI).<ref>{{cite web |title=SD Standard Overview |url=https://www.sdcard.org/developers/sd-standard-overview/ |website=SD Association |date=2020-12-11 |access-date=2023-06-19}}</ref>

Each LVDS lane can transfer up to 156&nbsp;MB/s. In full-duplex mode, one lane is used for sending data and the other for receiving. In half-duplex mode, both lanes operate in the same direction, effectively doubling the data rate at the same clock speed.

The following UHS speed classes are defined:

==== UHS-I ====

Support for the Ultra High Speed interface was first specified in SD version 3.01, released in May 2010.<ref name="SDHCspec">{{cite web |publisher=SD Association |url=http://www.sdcard.org/downloads/pls/simplified_specs/archive/part1_301.pdf |title=SD Part 1, Physical Layer Simplified Specification, Version 3.01 |date=2010-05-18 |access-date=2013-11-25 |archive-url=https://web.archive.org/web/20131205014133/https://www.sdcard.org/downloads/pls/simplified_specs/archive/part1_301.pdf |archive-date=2013-12-05 |url-status=dead }}</ref> This version introduced support for a 100&nbsp;MHz clock frequency—four times the rate of the original "Default Speed"—which enabled transfer rates up to 50&nbsp;MB/s using four-bit Single Data Rate (SDR) transfers, designated as '''SDR50'''. An extended mode called '''SDR104''' (also part of UHS-I) further increased the clock frequency to 208&nbsp;MHz, enabling data rates up to 104&nbsp;MB/s.

Version 3.01 also introduced '''DDR50''', a [[double data rate]] mode that transmits data on both the rising and falling edges of the clock signal. At 50&nbsp;MHz, DDR50 can achieve 50&nbsp;MB/s transfer rates by sending four bits per edge, or eight bits (one byte) per full clock cycle. This mode is mandatory for microSDHC and microSDXC cards labeled as UHS-I.

A proprietary extension of UHS-I, known as '''DDR200''', was developed by SanDisk to further increase transfer speeds without requiring additional pins. It combines double data rate transfers with the 208&nbsp;MHz clock of SDR104 to reach speeds of up to 170&nbsp;MB/s.<ref>{{cite web |title=SanDisk Extreme microSDXC datasheet |url=https://documents.westerndigital.com/content/dam/doc-library/en_us/assets/public/sandisk/product/memory-cards/extreme-uhs-i-microsd/data-sheet-extreme-uhs-i-microsd.pdf |website=Western Digital |access-date=2021-02-04 |archive-date=2021-01-08 |archive-url=https://web.archive.org/web/20210108210349/https://documents.westerndigital.com/content/dam/doc-library/en_us/assets/public/sandisk/product/memory-cards/extreme-uhs-i-microsd/data-sheet-extreme-uhs-i-microsd.pdf |url-status=live }}</ref><ref>{{cite web |title=GL3232 |url=http://www.genesyslogic.com/en/product_view.php?show=83 |website=Genesys Logic |access-date=2021-02-04 |archive-date=2020-09-21 |archive-url=https://web.archive.org/web/20200921071215/http://www.genesyslogic.com/en/product_view.php?show=83 |url-status=live }}</ref> Although not part of the official SD specification, DDR200 has been adopted by several manufacturers, including Lexar (1066x series, up to 160&nbsp;MB/s), Kingston (Canvas Go Plus, up to 170&nbsp;MB/s), and MyMemory (PRO SD card, up to 180&nbsp;MB/s).

==== UHS-II ====
[[File:Lexar 1000x MicroSDHC UHS-II U3 Class 10 - Back.jpg|thumb|upright|Back side of a Lexar UHS-II microSDHC card, showing the additional row of UHS-II connections]]

Specified in version 4.0, further raises the data transfer rate to a theoretical maximum of 156&nbsp;MB/s (full-[[Duplex (telecommunications)|duplex]]) or 312&nbsp;MB/s (half-duplex) using an additional row of pins for LVDS signalling<ref name="firstsdxc">{{cite web | url = https://www.sdcard.org/press/SD_Association_Announces_UHS-II_eBOOK_Jan_5_2011_ENGLISH.PDF | publisher = SD Card | title = Association Triples Speeds with UHS-II | date = 5 January 2011 | access-date = 2011-08-09 | archive-date = 2011-03-21 | archive-url = https://web.archive.org/web/20110321195143/http://www.sdcard.org/press/SD_Association_Announces_UHS-II_eBOOK_Jan_5_2011_ENGLISH.PDF | url-status = dead }}</ref> (a total of 17 pins for full-size and 16 pins for micro-size cards).<ref name="sda_bus_speed"/> While first implementations in compact system cameras were seen three years after specification (2014), it took many more years until UHS-II was implemented on a regular basis. At the beginning of 2025, 100 DSLR and mirrorless cameras support UHS-II.<ref>{{cite web |url=https://www.memorycard-lab.com/-Article/UHS-II-Cameras | publisher = memorycard-lab.com | title = UHS-II camera list | access-date=2025-01-04}}</ref>

==== UHS-III ====
Version 6.0, released in February 2017, added two new data rates to the standard. FD312 provides 312&nbsp;MB/s while FD624 doubles that. Both are full-duplex. The physical interface and pin-layout are the same as with UHS-II, retaining backward compatibility.<ref>{{Cite web|url=https://www.sdcard.org/press/DoublesTransferSpeeds_with_UHS3_2_9_2017.pdf|title=SD Association Doubles Bus Interface Speeds with UHS-III|date=23 February 2017|access-date=23 February 2017|archive-date=24 February 2017|archive-url=https://web.archive.org/web/20170224211349/https://www.sdcard.org/press/DoublesTransferSpeeds_with_UHS3_2_9_2017.pdf|url-status=dead}}</ref>

=== {{anchor|Express}} SD Express ===
[[File:SD Express Highest Speed Card Standard.jpg|thumb|Front and back of an SD Express card]]

The SD Express bus was introduced in June 2018 with the SD 7.0 specification. By incorporating a single [[PCI Express 3.0]] (PCIe) lane and supporting the [[NVM Express]] (NVMe) storage protocol, SD Express enables full-duplex transfer speeds of up to 985&nbsp;MB/s. Compatible cards must support both PCIe and NVMe, and may be formatted as SDHC, SDXC, or SDUC. For backward compatibility, they are also required to support the High-Speed and UHS-I buses. The interface reuses the UHS-II pin layout and reserves space for two additional pins for future use.<ref>{{cite web |date=June 27, 2018 |title=SD Express Card Spec Announced – PCIe + NVMe Up To 985 MB/s |url=https://www.anandtech.com/show/12902/sd-express-card-spec-announced-pcie-nvme-up-to-985mbs |website=AnandTech |access-date=12 April 2025 |archive-date=12 April 2025 |archive-url=https://web.archive.org/web/20250412032606/https://www.anandtech.com/show/12902/sd-express-card-spec-announced-pcie-nvme-up-to-985mbs |url-status=live }}</ref> In February 2019, the SD Association announced microSD Express,<ref>{{cite news |last=Gartenberg |first=Chain |date=25 February 2019 |title=Memory cards are about to get much faster with new microSD Express spec |url=https://www.theverge.com/circuitbreaker/2019/2/25/18239558/microsd-express-sd-association-new-format-speed-faster-mwc-2019-data-transfer |url-status=live |archive-url=https://web.archive.org/web/20190315212414/https://www.theverge.com/circuitbreaker/2019/2/25/18239558/microsd-express-sd-association-new-format-speed-faster-mwc-2019-data-transfer |archive-date=15 March 2019 |access-date=18 March 2019 |work=The Verge}}</ref> along with new visual marks to help users identify compatible cards and hosts.<ref>{{cite news |last=Henchman |first=Mark |date=25 February 2019 |title=The microSD Express standard combines PCI Express speeds, microSD convenience |url=https://www.pcworld.com/article/3343067/the-microsd-express-standard-combines-pci-express-speeds-microsd-convenience.html |url-status=live |archive-url=https://web.archive.org/web/20190808085536/https://www.pcworld.com/article/3343067/the-microsd-express-standard-combines-pci-express-speeds-microsd-convenience.html |archive-date=8 August 2019 |access-date=18 March 2019}}</ref>

SD Express cards can perform [[direct memory access]] (DMA), boosting performance but also increasing the host system’s attack surface in the event of a malicious or compromised card.<ref>{{cite web |date=May 19, 2022 |title=When microSD Cards Turn Against You |url=https://research.nccgroup.com/2022/05/19/when-microsd-cards-turn-against-you/ |website=NCC Group}}</ref>

The SD 8.0 specification, announced on 19 May 2020, expanded the bus interface to support [[PCIe 4.0]] on all cards and dual lanes on full-size cards. With dual lane PCIe 4.0, this update raised theoretical maximum transfer speeds to 3,938&nbsp;MB/s using an additional row of contacts.<ref>{{Cite press release |title=SDExpress Delivers New Gigabtye Speeds For SDMemory Cards |publisher=SD association |url=https://www.sdcard.org/press/whatsnew/SDExpressDeliversNewGigabtyeSpeedsForSDMemoryCards.pdf |access-date=2020-05-19 |url-status=live |archive-url=https://web.archive.org/web/20200520193348/https://www.sdcard.org/press/whatsnew/SDExpressDeliversNewGigabtyeSpeedsForSDMemoryCards.pdf |archive-date=2020-05-20 |work=SD card}}</ref> Revisions continued with version 9.0 in February 2022<ref>{{Cite web |title=sd 9.0 |url=https://www.sdcard.org/cms/wp-content/uploads/2022/05/SD-SPECIFICATION-9-%E2%80%93NEW-OPPORTUNITIES-FOR-SD-MEMORY-CARDS_EN.pdf |url-status=live |archive-url=https://web.archive.org/web/20240802052922/https://www.sdcard.org/cms/wp-content/uploads/2022/05/SD-SPECIFICATION-9-%E2%80%93NEW-OPPORTUNITIES-FOR-SD-MEMORY-CARDS_EN.pdf |archive-date=August 2, 2024 |accessdate=July 31, 2024}}</ref> and version 9.1 in October 2023,<ref name="SDver9.1">{{Cite web |date=2023-10-27 |title=New SD Express Specifications Introduce New Speed Classes and Next-Level Performance Features {{!}} SD Association |url=https://www.sdcard.org/press/thoughtleadership/sd-9-1-specification-introduces-new-speed-classes-and-next-level-performance-features-2/ |url-status=live |archive-url=https://web.archive.org/web/20240401075430/https://www.sdcard.org/press/thoughtleadership/sd-9-1-specification-introduces-new-speed-classes-and-next-level-performance-features-2/ |archive-date=2024-04-01 |access-date=2024-04-01 |website=www.sdcard.org |language=en-US}}</ref> further refining the standard.

Adoption has been gradual. In February 2024, [[Samsung]] announced it was sampling its first microSD Express cards,<ref>{{Cite web |last=Cunningham |first=Andrew |date=2024-02-28 |title=Speedy "SD Express" cards have gone nowhere for years, but Samsung could change that |url=https://arstechnica.com/gadgets/2024/02/sd-express-cards-from-samsung-promise-faster-than-sata-speeds-for-microsd-devices/ |access-date=2025-04-09 |website=Ars Technica |language=en-US |archive-date=9 April 2025 |archive-url=https://web.archive.org/web/20250409140340/https://arstechnica.com/gadgets/2024/02/sd-express-cards-from-samsung-promise-faster-than-sata-speeds-for-microsd-devices/ |url-status=live }}</ref> though commercial availability remained limited. Interest increased when Nintendo confirmed in April 2025 that the upcoming [[Nintendo Switch 2|Switch 2]] would only support microSD Express cards, without backwards compatibility for UHS-I cards.<ref>{{Cite web |last=Anderson |first=Robert |date=2025-04-04 |title=You Can Already Buy Switch 2 Compatible MicroSD Express Cards |url=https://www.ign.com/articles/nintendo-switch-2-microsd-express-cards-where-to-buy |access-date=2025-04-09 |website=IGN |language=en}}</ref>