Introduction to SD card (based on spec3.0)

1.SD Basic introduction to card （Secure Digital Memrory Card）

SD The card is based on MMC（MultiMedia card ） Format , It's Toshiba MMC Upgraded from the card （ therefore ,SD Card is supported MMC Card agreement ）.SD There is usually a switch on the right side of the card , That is, the write protection switch . Once on , here SD The data of can only be read but not written . There is no discussion here SPI Mode

1.1 Characteristics of the card

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1.1.1 Card capacity ：

1） Standard capacity cards （SDSC）: No more than 2GB

2)    High capacity cards （SDHC）: Greater than 2GB By no more than 32GB

3） Expansion capacity card （SDXC）: Greater than 32GB No more than 2TB The card of

Operating voltage range ：2.7V~3.6V

1.1.2 Bus rate ：（SDR-single Data Rate）  DDR（Double Data Rate）

1) Default rate mode ：3.3V The signal , the height is 25MHz, Data rate 12.5MB/S
2) High rate mode ：3.3V The signal , the height is 50MHz, Data rate 25MB/S
3) SDR12：1.8V The signal , the height is 25MHz, Data rate 12.5MB/S
4) SDR25：1.8V The signal , the height is 50MHz, Data rate 25MB/S
5) SDR50：1.8V The signal , the height is 100MHz, Data rate 50MB/S
6) SDR104：1.8V The signal , the height is 208MHz, Data rate 104MB/S
7) DDR50：1.8V The signal , the height is 50MHz, Double clock edge sampling data , Data rate 50MB/S 

1.1.3 Speed level ：

1 Class 0—— There is no specific performance
2 Class 2—— The rate is greater than or equal to 2M/S
3 Class 4—— The rate is greater than or equal to 4M/S
4 Class6—— The rate is greater than or equal to 6M/S
5 Class 10—— The rate is greater than or equal to 10M/S
SDHC and SDXC All cards should support the rate level , And in Class 2 above . Be careful ： In the use of 50MHz System clock , Single edge , The rate level cannot be directly equal to 25M. Because there will be orders ,CRC etc. , These will reduce the amount of data transmission .

1.2 bus protocol

You can see a total of

Power signal ：VSS1,VSS2,VDD

CLK：Host Clock signal to card

CMD： Two way signal command / response signal

cable ：4 Root bidirectional data signal

stay Default speed,SD Card bus can be one master and many slaves , But for high-speed and UHS_I, Only single master-slave （ That is, when the speed is slow, you can get more cards , When fast, only one card ）. The specific multi card operation mode will be mentioned later .

1.2.1 Bus protocol

1.Command: Specific orders , This is by HOST Driven , Give the card . This is through CMD Signal line serial transmission

2.Response： The response is given by the card Host drive , adopt CMD Signal line transmission .

3.data: cable

1.2.2 Command and data transfer

Generally speaking , All operations are carried out by sending commands to the card and replying to the card , For data operation .

You can see that there are two kinds of commands , One is that there is RESponse, One is that there is no RESponse, Specific orders and RESPONSE You need to see the agreement yourself

Data transmission is also transmitted through commands , Issue block transfer command （ Single block read-write and multi block read-write ）, And then received Response, Enter data transmission mode .

Data blocks ： Data blocks +crc To transmit . Be careful ： Multiple blocks are transmitted through STOP Command to end  

Data Pass during block write operation data0 Indicating busy form .