Книга: Advanced PIC Microcontroller Projects in C

7.1.2 Operation of the SD Card in SPI Mode

7.1.2 Operation of the SD Card in SPI Mode

When the SD card is operated in SPI mode, only seven pins are used. Three (pins 3, 4, and 6) are used for the power supply, leaving four pins (pins 1, 2, 5, and 7) for the SPI mode of operation:

• Two power supply ground (pins 3 and 6)

• Power supply (pin 4)

• Chip select (pin 1)

• Data out (pin 7)

• Data in (pin 2)

• CLK (pin 5)

At power-up, the SD card defaults to the SD bus protocol. The card is switched to SPI mode if the Chip Select (CS) signal is asserted during reception of the reset command. When the card is in SPI mode, it only responds to SPI commands. The host may reset a card by switching the power supply off and then on again.

The mikroC compiler provides a library of commands for initializing, reading, and writing to SD cards. It is not necessary to know the internal structure of an SD card in detail before using one, since the library functions are available. However, a basic understanding of the internal structure of an SD card is helpful in making the best use of the card. In this section we will look briefly at the internal architecture and operation of SD cards.

An SD card has a set of registers that provide information about the status of the card. When the card is operated in SPI mode these are:

• Card identification register (CID)

• Card specific data register (CSD)

• SD configuration register (SCR)

• Operation control register (OCR)

The CID register consists of 16 bytes and contains the manufacturer ID, product name, product revision, card serial number, manufacturer date code, and a checksum byte. Table 7.3 shows the structure of the CID register.

Table 7.3: Structure of the CID register

The CSD register consists of 16 bytes and contains card-specific data such as the card data transfer rate, read/write block lengths, read/write currents, erase sector size, file format, write protection flags, and checksum. Table 7.4 shows the structure of the CSD register.

Table 7.4: Structure of the CSD register

The SCR register is 8 bytes long and contains information about the SD card’s special features and capabilities, such as security support and data bus widths supported.

The OCR register is only 4 bytes long and stores the VDD voltage profile of the card. The OCR shows the voltage range in which the card data can be accessed.

All SD-card SPI commands are 6 bytes long, with the MSB transmitted first. Figure 7.5 shows the command format. The first byte is known as the command byte, and the remaining five bytes are called command arguments. Bit 6 of the command byte is set to 1 and the MSB bit is always 0. With the remaining six bits we have sixty-four possible commands, named CMD0 to CMD63. Some of the important commands are:

• CMD0 GO_IDLE_STATE (resets the SD card)

• CMD1 SEND_OP_COND (initializes the card)

• CMD9 SEND_CSD (gets CSD register data)

• CMD10 SEND_CID (gets CID register data)

• CMD16 SET_BLOCKLEN (selects a block length in bytes)

• CMD17 READ_SINGLE_BLOCK (reads a block of data)

• CMD24 WRITE_BLOCK (writes a block of data)

• CMD32 ERASE_WR_BLK_START_ADDR (sets the address of the first write block to be erased)

• CMD33 ERASE_WR_BLK_END_ADDR (sets the address of the last write block to be erased)

• CMD38 ERASE (erases all previously selected blocks)

Figure 7.5: SD card SPI command format

In response to a command, the card sends a status byte known as R1. The MSB bit of this byte is always 0 and the other bits indicate the following error conditions:

• Card in idle state

• Erase reset

• Illegal command

• Communication CRC error

• Erase sequence error

• Address error

• Parameter error

The SD card in SPI mode supports single-block and multiple-block read operations. The host should set the block length. After a valid read command the card responds with a response token, followed by a data block and a CRC check. The block length can be between 1 and 512 bytes. The starting address can be any valid address in the address range of the card.

In multiple-block read operations, the card sends data blocks with each block having its own CRC check attached to the end of the block.

The SD card in SPI mode supports single-or multiple-block write operations. After receiving a valid write command from the host, the card responds with a response token and waits to receive a data block. A one-byte “start block” token is added to the beginning of every data block. After receiving the data block the card responds with a “data response” token, and the card is programmed as long as the data block is received with no errors.

In multiple-block write operations the host sends the data blocks one after the other, each preceded by a “start block” token. The card sends a response byte after receiving each data block.

Card Size Parameters SD cards are available in various sizes. At the time of writing, SanDisk Corporation (www.sandisk.com) offered the models and capacities shown in Table 7.5. The company may now be offering models with 4GB or even greater capacity.

Table 7.5: SanDisk card models and capacities

In addition to the normal storage area on the card, there is also a protected area pertaining to the secured copyright management. This area can be used by applications to save security-related data and can be accessed by the host using secured read/write commands. The card write protection mechanism does not affect this area. Table 7.6 shows the size of the protected area and the data area available to the user for reading and writing data. For example, a 1GB card has 20,480 blocks (one block is 512 bytes) of protected area and 1,983,744 blocks of user data area.

Table 7.6: Protected area and data area sizes

1 block = 512 bytes.

Data can be written to or read from any sector of the card using raw sector access methods. In general, SD card data is structured as a file system and two DOS-formatted partitions are placed on the card: the user area and the security protected area. The size of each area is shown in Table 7.7. For example, in a 1GB card, the size of the security protected area is 519 sectors (1 sector is 512 bytes), and the size of the user data area is 1,982,976 sectors.

Table 7.7: Size of the security protected area and the user area in a DOS-formatted card

1 sector = 512 bytes.

A card can be inserted and removed from the bus without any damage. This is because all data transfer operations are protected by cyclic redundancy check (CRC) codes, and any bit changes caused by inserting or removing a card can easily be detected. SD cards typically operate with a supply voltage of 2.7V. The maximum allowed power supply voltage is 3.6V. If the card is to be operated from a standard 5.0V supply, a voltage regulator should be used to drop the voltage to 2.7V.

Using an SD card requires the card to be inserted into a special card holder with external contacts (see Figure 7.6) so connections are easily made to the required card pins.

Figure 7.6: SD card holder

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