Chapter 13 - Mass storage

Abstract

Embedded-control applications need a larger nonvolatile data storage space, well beyond the capabilities of the common serial EEPROM devices. This chapter discusses how to interface one of the most common and inexpensive mass-storage device types to a PIC24 microcontroller using the smallest amount of processor resources. The Secure Digital (SD) standard is one of the most commonly adopted mass-storage media for digital cameras and many other multimedia consumer applications. The SD card specifications represent an evolution of a previous technology known as Multi Media Card (MMC), with which they are still partially compatible both electrically and mechanically. The Secure Digital Card Association (SDCA) owns and controls the technical specification standards for the SD memory card and they require that all companies who plan to actively engage in the design, development, manufacture or sale of products that utilize the SD specifications must become members of the association. SD cards require only nine electrical contacts, and an SD/MMC-compatible connector, which can be purchased on most online catalogs for less than a couple of dollars, requires only a couple of pins more to account for insertion detection and write-protection switch sensing. There are two main modes of communication available: the first one (known as the SD bus) is original to the SD/MMC standard and it requires a nibble (4-bit) wide bus interface; the second mode is serial and is based on the popular SPI bus standard.