Cypress Semiconductor CY8C21534 Dokumentacja Pobierz pdf (Strona 15)

The following four bytes in the command are 0x00. These are called the argument

bytes. For a read or write operation the address would be given with these bytes.

There is no argument for a command 0, so these are all given as 0x00.

The final byte is the Cyclic Redundancy Check (CRC). This is used by the SD card to

ensure that the information was read correctly. The default setting for the use of the

CRC is off, so while the SD card is in SPI mode, it ignores all CRC values, and dummy

bytes such as 0x95 can be used. The only CRC that is necessary to give is for the

command 0. After the dummy clock pulses are sent to the SD card, it will wait for an

incoming command.

Every full command issued to the SD card will be responded to with a response token

from the SD card. This shows if the command was received and carried out correctly.

Figure 6 shows the makeup of the token response.

Figure 6: Response Token. Taken from SD Specifications Part 1 Version 2.00 [4, 111].

As the figure shows there are three bits, S1 to S3, that make up the response

token. These bits show the results of the command given. For example, there are

three possible response tokens for a write operation which are 101 indicating a CRC

error, 110 indicating a data write error, and 010 indicating the data has been

accepted. [5.]

Many commands need to be issued in order to determine how to proceed with

the communications of the SD card as there are many different makes and

models of SD cards that should be able to be used in the same systems. These

commands are used for example to ensure the card is working at the correct

voltage level and that the commands given to the card are in the correct

format. This process alone could easily take up a lot of the microcontroller's

memory.

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Cypress Semiconductor CY8C21534 Dokumentacja Strona 15