The basic SD Memory Card communication is based on 9-pin interface (Clock, Command, 4xData and 3xPower lines) designed to operate in at maximum operating frequency of 208 MHz and low voltage range. Additional communication methods, based on differential signaling interface (UHS-II and PCIe/NVMe) are specified as optional. The communication protocol is defined as a part of this specification unless specified otherwise (i.e. PCIe and NVMe cases).
It lists specifications: - audio specification - file system specification - security specification - physical layer specification (includes basic command protocol) the mechanical section describes a "mini" SD between micro and SD. didn't know about that one. - mc-ex interface specification mobile commerce - sdio specification sdio sd cards are sd cards that have IO peripherals, and can only be engaged with sdio-capable interfaces