EMMC MMC mode with SPI mode

MMC memory cards can be divided into MMC and SPI two operating modes, MMC mode is the standard default mode, with all the features of MMC. The SPI mode is the second mode of MMC memory card, which is a subset of the MMC protocol, which is mainly used for systems that require only a small number of cards (usually 1) and a low data transfer rate (compared to the MMC protocol), which minimizes the design cost, but does not perform as well as MMC.

The SD/MMC Card's SD mode and SPI mode SD/MMC card reading and writing mode are divided into two kinds. One is to use the SD bus for data transmission, which is called SD mode, and the other is to use the SPI Bus for data transmission, which is called the SPI mode.

When the initial contact with SD/MMC card, Hanny always feel very strange, why SD/MMC card to provide two modes to access it? Compare the differences between the two models, analysis of the use of the occasion, you will understand the answer.

The SD card works primarily in 4BIT SD mode (MMC may work in 8BIT SD mode). Its read-write maximum clock can reach 50MHz, therefore, the market card reading speed can reach more than 20mb/s, has been calculated limit. SD mode is also the main operating mode of SD card.

The SPI mode has a bandwidth of only 1BIT, and the clock can be up to 25MHz, so the read speed is usually less than 3mb/s. However, the SPI mode has low hardware requirements, and is generally used for low-end applications such as MP3, which require less speed to read cards.

Hanny There was another question: Does SD mode also support 1BIT mode? Why not use SD 1BIT mode directly?

Later understand: 1, SPI Bus is a general-purpose bus, most of the chips are hardware modules, 2, SPI mode support without CRC transmission mode, you can reduce the hardware requirements, 3, the SD line between the CMD and the data line may be generated at the same time, It is more difficult to support a host without an SD hardware module.

SD (Secure Digital) and MMC (multimedia Card)
SD is a standard for flash memory Card, which is generally common SD memory card, and MMC is an earlier memory card standard, has now been replaced by SD standard. There are fairly detailed SD/MMC specifications on Wikipedia: [Http://zh.wikipedia.org/wiki/Secure_Digital].
SDIO (Secure Digital I/O)
SDIO is the current technology we are concerned about, SDIO the name of meaning, is the SD I/O Interface (interface), but this explanation may still be a bit like. More specifically, SD is the standard memory card, but now also can be used to plug the SD into the use of some peripheral interfaces, such technology is SDIO.
So the SDIO itself is a fairly simple technology, through the SD I/O pin to connect the external periphery, and through the I/O data on the SD connection with these peripheral transmission data, and SD Association members also launched a very complete SDIO stack driver, so that SDIO peripheral (i Known as SDIO cards) have become quite popular in their development and application. The
now has a very large number of mobile phones or handheld devices that support SDIO (the SD standard was originally designed for mobile device), and many SDIO peripherals have been developed to make the external peripheral of the mobile phone easier and more resilient to development (no need for built-in perimeter). The current common SDIO peripheral (SDIO card) is:
Wi-Fi card (wireless network cards)  
CMOS sensor Card (camera module)  
GPS card 
Gsm/gprs Modem card 
Bluetooth card 
Radio/tv Card (very fun)

• SDIO's application will be one of the most important interface technologies for future embedded systems, and will also replace the current GPIO-style SPI interface.

Sd/sdio the transport mode
The SD transfer mode has the following 3 types:
SPI Mode (required)
1-bit mode
4-bit mode

• The SDIO also supports the above 3 modes of transmission. According to SD Standard, all SD (memory card) and SDIO (peripheral) must support SPI mode, so SPI mode is "required". In addition, early MMC cards (with SPI transmission) can also receive SD plug-ins (SD slots) and are read using SPI mode or 1-bit mode.

SD the MMC Mode
SD can also read MMC memory, although MMC standard mentions that MMC memory does not necessarily support SPI mode (but must support 1-bit mode), but the market can see the MMC card actually supports SPI mode. Therefore, we can set the SD to the SPI mode transmission mode to read the MMC memory card.
The SD MMC mode is a transfer mode used to read the MMC card. However, although the SD MMC mode uses SPI mode, its physical characteristics are still different:
The SPI mode of MMC has a maximum transmission rate of-Mbit/s;
The SD SPI mode has a maximum transmission rate of-Mbit/s.

• to avoid confusion, it is sometimes used to make a clear distinction between SPI/MMC mode and SPI/SD mode.

EMMC MMC mode with SPI mode