SD/MMC简介

常见的外设接口之一。

现在比较流行的手机，Android手机中，常常就支持再插入一个外接存储卡，就是SD卡。

只不过是小型接口的MicroSD。

SD/MMC的名称解释

之所以常常看到，把SD和MMC连在一起叫。

那是因为：

SD接口，算是MMC接口的升级版，

理论上，SD接口和更早的MMC接口是兼容的。

所以，可以看做是同样的物理上的一个接口

所以一般是把SD和MMC连在一起叫：

SD/MMC接口

SD/MMC的物理外形和接口说明

MMC卡长啥样

先上图，看看常见的MMC卡，是什么样的：

正面：

注：最下面那个是MMC卡的插槽

反面：

SD卡长啥样

再来看看SD卡长啥样：

另外，还有一个小型的SD卡，叫做MicroSD，其明显小了一号

对应的可以通过普通的SD卡的卡套，变成和普通SD类似的效果：

MMC和SD卡引脚说明

直接上图：

真实的SD卡和MMC卡，背面对比：

再加一个，带引脚说明的对比图：

很明显：

MMC和SD，在接口上，都还是很类似，很兼容的。

对应的，SD和MMC的接口的名称和含义，

如下：

MMC卡的引脚名称和功能

对应的，MMC，分MMC模式和SPI模式，引脚和功能，不太一样：

MMC卡模式（MultiMedia Card mode）时的MMC引脚名称和功能

Pin	Signal	Description
1	RSV NC	Not connected or Always ?1? (data 3?)
2	CMD I/O	Command/Response
3	VSS1 S	Supply Voltage Ground
4	VDD S	Supply Voltage
5	CLK I	Clock
6	VSS2 S	Supply Voltage Ground
7	DAT0 I/O	PP Data 0

SPI模式时的MMC引脚名称和功能

Pin	Signal	Description
1	CS	Chip select (neg. true)
2	DI	DATA in
3	Vss	Ground
4	Vcc	Power supply
5	SCLK	Clock
6	Vss2	Ground
7	DO	DATA out

SD卡的引脚名称和功能

SD卡，也分对应的SD模式和SPI模式

Pin	SD Mode			SPI Mode
Pin	Name	Type	Description	Name	Type	Description
1	CD/DAT3	I/O/PP	Card detection / Connector data line 3	CS	I	Chip selection in low status
2	CMD	PP	Command/Response line	DI	I	Data input
3	Vss1	S	GND	VSS	S	GND
4	Vdd	S	Power supply	VDD	S	Power supply
5	CLK	I	Clock	SCLK	I	Clock
6	Vss2	S	GND	VSS2	S	GND
7	DAT0	I/O/PP	Connector data line 0	DO	O/PP	Data output
8	DAT1	I/O/PP	Connector data line 1	RSV
9	DAT2	I/O/PP	Connector data line 2	RSV

SD卡或MMC卡在卡Card的模式时的引脚功能说明

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.

SD卡或MMC卡在SPI模式时的引脚功能说明

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	–