change directory structure to match index

Signed-off-by: Michel-FK <michel.stempin@funkey-project.com>

docs/developer_guide/hardware_reference/sd-card.md

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+The **FunKey S** game console uses the SD Card both as its boot device
+and its only storage device, so a good operation of this interface is
+absolutely mandatory.
+
+The Allwinner V3s provides 2x 4-bit MMC / SD Card / SDIO
+interfaces. In the FunKey, only interface #0 is used.
+
+If you look on the Web, you will find many contradictory SD Card
+interface designs, with a combination of pull-up / pull-down
+resistors, ESD devices and power supply filtering, with all pins wired
+or not, such that it is very difficult to know what is really
+required. To better understand the situation, we need to go back to
+the specifications.
+
+## Specifications
+
+The SD Card physical interface is provided in the "<i>SD
+specifications, part 1, Physical Layer Specification version 2.00, May
+9, 2006</i>", for which a simplified version is available [here][1].
+
+The MMC phyiscal interface can be found in the "<i>Multi Media Card
+System Specification version 4.3, JESD84-A43, November 2007</i>",
+available [here][2] (registration required).
+
+But a good summary of the requirements is given in the "_[AN10911
+SD(HC)-memory card and MMC Interface conditioning][3]_" application
+note from NXP, from which this schematic is taken:
+
+![SD MMC Interface](/assets/images/SD_MMC_Interface.png){.lightbox}
+
+!!! Warning
+    This schematic does not include details concerning card-supply and
+    typical power-supply decoupling capacitors.
+
+## Write Protect (WP)
+
+A mechanical write protect switch is provided in the full-size SD
+Card, but not in the mini or micro SD Card form factor. As we plan to
+use a micro SD Card only, it is not used for the FunKey, along with
+its pull-up resistor and ESD protection.
+
+## Card Detection (CD)
+
+As the SD Card is mandatory to boot the **FunKey S**, is always
+inserted and opening the device is required for its removal, we don't
+need the optional card detect mechanical switch feature (even if the
+chosen connector provides it) and its related pull-up resistor and ESD
+protection.
+
+The SD Card specification provides another mean to detect the card
+using a card built-in pull-up resistor on its DAT3 signal, that can be
+later disconnected during normal operation using he
+**SET_CLR_CARD_DETECT** (**ACMD42**) command. In order to correctly
+detect if the card is inserted, a high value external pull-down
+resistor (> 270 kΩ) is required to drive the detect signal low when no
+card is inserted, while the card built-in 10-50 kΩ resistor will drive
+this signal high when inserted.
+
+However, this feature is not compatible with MMC cards, so its usage
+should be avoided and the mechanical detection is preferred.
+
+## Pull-Up Resistors
+
+Both the SD Card and MMC specifications require not to leave the
+interface signals floating, except for the CLK signal, where a pull
+resistor would cause significant signal distortion because of the
+required high speed and short rise/fall times. However, it is
+recommended to add a series resistor on this CLK signal as close as
+possible to the clock source (the CPU) to avoid ringing, as we already
+discussed it in the page about the [CPU][4].
+
+Fortunately, the Allwinner V3s CPU provides internal pull-up resistors
+for all these signals, so we don't have to add external pull-up
+resistors. These resistors are given with a typical value of 100 kΩ
+(50 min, 150 max). Unfortunately, the CMD signal for MMC card features
+an open-drain output mode, and its value should be undercut (down to
+4.7 kΩ) to guarantee a sufficiently short rise time in this mode.
+
+## Schematics
+
+The FunKey SD Card interface schematic is the following:
+
+![SD Card Schematics](/assets/images/SD_Card_Schematics.png){.lightbox}
+
+![TF-110](/assets/images/TF-110.png){: align=left }
+
+As can be expected, the main component is the [Micro SD (TF Card)
+Push/Pull connector][5] **J4**, which has been selected for its
+low-profile (1.3 mm height) characteristic and overall minimal
+dimensions.
+
+!!! Tip
+
+    "Push-Pull" means that you have to "Push" to insert the card and
+    to "Pull" to remove it, i.e. the operation is completely manual,
+    whereas "Push/Push" implies that you have a bistable mechanism to
+    remove the card.
+
+Even if in the FunKey device the SD Card and its connector are not
+accessible without opening the enclosure, there may be some situations
+where the user may decide to do so. We thus attach an ESD protection
+TVS diode (**D16**, **D17**, **D18**, **D25**, **D26**, **D27**,
+**D28**) on each signal to avoid any ESD hazard.
+
+As discussed above, a single pull-up resistor **R10** is used on the CMD
+signal for MMC compatibility.
+
+The micro SD Card connector built-in card detection switch is not
+used, since the card must always be inserted for the **FunKey S** to
+boot, and the corresponding pins are thus connected to GND.
+
+The SD Card power supply is applied through an RC low-pass filter
+**R9** / **C11** in order to provide a soft-start operation, as the
+card built-in large bulk capacitor on its power rail may collapse the
+supply voltage when initially powered up.
+
+[1]: https://www.sdcard.org/downloads/pls/click.php?p=Part1_Physical_Layer_Simplified_Specification_Ver6.00.jpg&f=Part1_Physical_Layer_Simplified_Specification_Ver6.00.pdf&e=EN_SS1
+[2]: https://www.jedec.org/document_search/field_keywords/emmc-3147
+[3]: https://www.mouser.com/catalog/specsheets/an10911.pdf
+[4]: /developers/hardware/cpu
+[5]: https://github.com/FunKey-Project/FunKey-S-Hardware/blob/master/Datasheets/C266613_%E6%8A%BD%E6%8B%89%E5%BC%8FTF%E5%8D%A1%E5%BA%A7_2018-08-20.PDF
+
+--8<--
+includes/glossary.md
+--8<--