Detailed explanation of SD card driver under STM32

In the past few months, I put aside my study of S3C2440 for my graduation project, and started to develop modules on STM32. I used STM32 to make an MP3 with an interface, so I had to use an SD card (of course, nand flash can also be used, but the PCB is difficult to draw, and the most important thing is that it is easy to break). SD cards are convenient and fast.

When talking about SD card, we will think of its specific driver. There are many examples on the Internet, but none of them explain the process in such detail that it is difficult for novices to understand. Of course, this article is of no use to students who have done it. Let's analyze it in detail now.

 

The SD card we are talking about is just a type of flash memory. Its full name is Secure Digital, which means "secure digital". It was jointly launched by two major Japanese electrical groups, Panasonic and Toshiba, and SanDisk, and was first released in August 1999. Common flash memory includes: SD card, CF card, XD card, MMC card, SM card, Mini SD card, T-Flash card. SD cards are classified into:

SD, MiniSD, MicroSD, SDHC, MiniSD, MicroSD, SDXC, smart SD, SDI/O. They are different in appearance, speed, capacity and command. Speaking of this, we have to talk about the recent anti-Japanese movement. In fact, we really don’t need to cause trouble for the country. Many things around us are others’, but we don’t know it. So strengthening your own knowledge and doing more for the country is the real patriotism.

SD card supports two bus modes: SD mode and SPI mode. The SD mode uses 6-wire system, using CLK, CMD, DAT0~DAT3 for data communication. The SPI mode uses 4-wire system, using CS, CLK, DataIn, DataOut for data communication. The data transmission speed in SD mode is faster than that in SPI mode. When using a single-chip microcomputer to read and write SD cards, SPI mode is generally used. Using different initialization methods can make the SD card work in SD mode or SPI mode.

This means that there is a controller in the SD card, with corresponding registers. When we use the SD card, we are actually setting the corresponding registers. After setting, we can use commands to operate it. (That is why SD cards are easy to use, because SD card manufacturers have already done a lot of things).

 1. How to drive SD card using SPI    

The SPI communication interface of the SD card allows it to read and write data through the SPI channel. From the application point of view, the advantage of using the SPI interface is that many microcontrollers have their own SPI controller, which not only brings convenience to development, but also reduces development costs. However, it also has disadvantages, losing the fast performance advantage of the SD card. (The speed of USB download test in SPI mode is only 100K~300K, while the speed in SD mode can be as high as 3M~10M), this is a serious disadvantage of SPI, but it is the simplest for us students to use SPI. The SD mode has a very complex mode conversion, and it is difficult to switch back and forth, but it can also be used in SD mode, and many controllers have SDIO.

Second, SD card commands

The most important thing about SD card is initialization, and the important thing about initialization is the use of commands. After the SD card is successfully initialized, it can almost be used, so we should focus on the initialization. Different cards have different initialization processes (pay attention to the SPI speed not exceeding 400K during initialization)

The typical initialization process of the SD card is as follows:

1. Initialize the hardware conditions for connecting to the SD card (MCU SPI configuration, IO port configuration);

2. Power-on delay (>74 CLK) (required);

3. Reset the card (CMD0);

4. Activate the card, initialize internally and get the card type (CMD1 (for MMC card), CMD55, CMD41) (the difference is here)

5. Query OCR and obtain the power supply status (CMD58) (that is, voltage. Generally, many cards cannot be used if it is not judged);

6. Whether to use CRC (CMD59) (CRC check);

7. Set the read/write block data length (CMD16);

8. Read CSD to obtain other information of the memory card (CMD9);

9. After sending 8CLK, chip selection is disabled;

The following diagram shows: (available online)

 

 

 

 

 It can be seen that the operations of the SD card are all commands. As long as you understand the functions of these commands, you can drive the SD card yourself. I will post all the commands:

If you want to use SD mode in SPI mode, STM official has very detailed library files;

Class0: (basic command set such as card identification and initialization)

CMD0: Reset SD card.

CMD1: Read OCR register.

CMD9: Read CSD register.

CMD10: Read CID register.

CMD12: Stop data transfer when reading multiple blocks

CMD13: Read Card_Status register

Class2 (card reading command set):

CMD16: Set the length of the block

CMD17: Read single block.

CMD18: Read multiple blocks until the host sends CMD12.

Class4 (card writing command set):

CMD24: Write single block.

CMD25: Write multiple blocks.

CMD27: Write CSD register.

Class5 (Erase card command set):

CMD32: Set the starting address of the erase block.

CMD33: Set the end address of the erase block.

CMD38: Erase the selected block.

Class6 (write protection command set):

CMD28: Set the address of the write protection block.

CMD29: Erase the address of the write-protected block.

CMD30: Ask the card for the status of the write protection bits

Class7: Card lock and unlock command set

class8: Apply for a specific command set.

class10-11: reserved

Among them, class1, class3, class9: SPI mode does not support

Now that we know the command, let's take a look at the format of the command:

The SD card command consists of 6 bytes, as follows:

Byte1: 0 1 xxxxxx (command number, defined by the command flag, such as CMD39 is 100111, which is hexadecimal 0x27, then the first byte of the complete CMD39 is 01100111, which is 0x27+0x40)
Byte2-5: Command Arguments, command parameters, some commands have no parameters
Byte6: The first 7 bits are CRC (Cyclic Redundacy Check) check bits, and the last bit is stop bit 1

byte1 byte2-5 byte6

01 Command number Parameter CRC check+1

Parameters are what the command requires. If there is no parameter, use 0. For example, CMD16 would have 512 written in it.