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emFile - MultiMedia & SD card driver

emFile supports the use of MultiMedia & SecureDigital (SD) cards. Two optional generic drivers for MultiMedia & SecureDigital (SD) cards are available. MultiMedia & SecureDigital (SD) cards can be accessed though two different modes:

SPI MODE
MMC/SD card mode.

For both modes drivers are available. To use one of these drivers, you need to configure the MMC driver and provide basic I/O functions for accessing your card reader hardware. This section describes how to enable one of these drivers and all hardware access functions required by emFile's for either the MultiMedia & SD card SPI mode or Multi- Media & SD card mode driver.

Supported hardware

MultiMedia Cards (MMC) and SecureDigital Cards (SD card) are small size factored mass storage devices.

The main design goal of these devices are to provide a very low cost mass storage product, implemented as a card with a simple controlling unit, and a compact, easy-to-implement interface. These requirements lead to a reduction of the functionality of each card to an absolute minimum.
In order to have a flexible design, MMC and SD cards are designed to be used in different I/O modes:

MMC/SD card mode
SPI mode
Emulated SPI mode, using port pins.

The difference between MMC and SD cards are that SD cards can operate with a higher clock frequency. The clock range can be between 0 - 25MHz, whereas MMCs can only operate up to 20MHz. Additionally the initialization of these cards differs. They need to be initialized differently, but after initialization they behave the same way.
MMC and SD cards also differ in the count of pins. SD cards have more pins than MMCs. Depending in which mode they are used the pins are used or not. Additionally SD cards have a write protect switch, which can be used to lock the data on the card.

In MMC/SD card mode
MultiMedia Cards use a seven pin interface in MultiMedia card mode (Command, Clock, Data and 3x Power lines). In contrast to the MultiMedia cards SD cards use a 9 pin interface (Command, Clock, 1 or 4 Data and 3 Power lines).

In SPI mode
Both card systems use the same the pin interface. (ChipSelect (CS), DataIn, DataOut, Clock and 3 Power lines).

Pin description for MMC/SD card in Card mode

Pin No.	Name	Type	Explanation
1	CD/DAT3	Input/Output using push pull drivers	After power up this line is input with 50 kOhm pull-up. This can be used for card detection. Relevant only for SD cards, pull-up resistor is disabled after initialization procedure for using this line as DATA3 line for data transfer.
2	CMD	Push Pull	This is a bi-directional line. It is a bidirectional command channel used for card initialization and data transfer commands. The CMD signal has two operation modes: open-drain for initialization mode and push-pull for fast command transfer. Commands are sent from the MultiMediaCard bus master (card host controller) to the card and responses from the cards to the host.
3	Vss	Power supply	Supply voltage ground.
4	Vdd	Power supply	Supply voltage.
5	CLK	Input	With each cycle of this signal an one bit transfer on the command and data lines is done. The frequency may vary between zero and the maximum clock frequency.
6	Vss2	Power supply	Supply voltage ground.
7	DAT[0]	Input/Output using push pull driver	DAT is a bidirectional data channel. The DAT signal operates in push-pull mode. Only one card or the host is driving this signal at a time. Relevant only for SD cards: On data transfer, this line is DATA 0.
8	DAT[1]	Input/Output using push pull drivers	On MMC card this line does not exist. Relevant only for SD cards: On data transfer, this line is DATA 1.
9	DAT[2]	Input/Output using push pull drivers	On MMC card this line does not exist. Relevant only for SD cards: On data transfer, this line is DATA 2.

Pin description for MMC/SD card in SPI mode

Pin No.	Name	Type	Explanation
1	CS	Input	Chip Select sets the card active at low level and inactive at high level.
2	Data In	Input	Data In is seen from the card, therefore data transmitted to the card will be received from this line.
3	Vss	Supply ground	Supply voltage ground.
4	Vdd	Supply voltage	Supply voltage.
5	SCLK	Input	Clock signal must be generated by the target system. The card is always in slave mode.
6	Vss2	Supply ground	Supply voltage ground.
7	Data Out	Output	Data Out is seen from the card. Data transferred to host will be sent by card from this line.
8	Reserved	Not used	-
9	Reserved	Not used	-

Additional Information

Data transfer width is 8-bit.
Data should be output on the falling edge and must remain valid until the next period. Rising edge means data is sampled (i.e. read).
Bit order requires most significant bit (MSB) to be sent out first.
Data polarity is normal, which means a logical "1" is represented with a high level on the data line and a logical "0" is represented with low level.
MultiMedia & SD cards support different voltage ranges. Initial voltage should be 3.3V.

Power control should be considered when creating designs using the MultiMediaCard and/or SD Card. The ability to have software power control of the cards makes the design more flexible and robust. The host will have the ability to turn power to the card on or off independent of whether the card is inserted or removed. This can help with card initialization when there is contact bounce during card insertion. The host waits a specified time after the card is inserted before powering up the card and starting the initialization process. Also, if the card goes into an unknown state, the host can cycle the power and start the initialization process again. When card access is unnecessary, allowing the host to power-down the bus can reduce overall power consumption.

Theory of operation

The Serial Peripheral Interface (SPI) bus is a very loose de facto standard for controlling almost any digital electronics that accepts a clocked serial stream of bits. SPI operates in full duplex (sending and receiving at the same time).

Fail-safe operation

Unexpected Reset
The data will be preserved.

Power failure
Power failure can be critical: If the card does not have sufficient time to complete a write operation, data may be lost. Countermeasures: make sure the power supply for the card drops slowly.

Wear leveling

MMC / SD cards are controlled by an internal controller, this controller also handles wear leveling. Therefore, the driver does not need to handle wear leveling.