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3.6 SD Memory Card--Pins and Registers

The pin layouts differ between SD cards (9 pins), UHS-II cards (second
row of UHS-II pins), and SD Express cards (second row PCIe/NVMe pins).

Looking at a standard SD card, contacts at top, the diagonal inset is
in the upper left. The first pin, inset below it, on the left, is #9.
Then they proceed #1 -> #8 from left to right, with the write protect
switch if present on the very right edge.

The card has two modes, SD or SPI. The pins have different functions
in these modes.

Each pin can be any of power Supply, Input, Output, or i/o using
Push-Pull drivers.

Ground: Pins 3 & 6
V+: Pin 4
CLK: Pin 5, input only

SD mode

CMD: Pin 2
Data IO: Pins 7, 8, 9, 1 are Bits 0, 1, 2, 3 respectively

Notes:
Data Bit 3 is also Card Detect
Data Bits 1 and 2 are also used for signals in SDIO cards.

SPI Mode

Chip Select CS: Pin 1
Data In DI: Pin 2
Data Out DO: Pin 7

Notes:
Pins 8 & 9 are reserved in SPI mode.
CS has a 50KOhm pull up in the card at power up. The host can drive
the line high or let it be pulled high to select SD mode. If the host
wants SPI mode, it should drive Pin 1 low. To detect the card, the
host detects that the line is pulled high.

The Pin 1 Pull-up should be disconnected by the user, during regular
dta transfer, with SET_CLR_CARD_EDETECT(ACMD42)

That's page pdf page 32, physical page 12. The next page shows the
internals of a card and describes the information registers. A
description of the command protocol was in an earlier chapter; I
skipped it since voltage information is more satisfying to start with
when prototyping.