Receiving data from SD card in SPI mode

[ptjw]

hello all,

i am to configure a spartan 3A to interface with a SD card reader in SPI mode using MOSI, MISO, CS and CLK.

i have written code in VHDL to implement this, but cannot verify whether it works until the actual hardware is here (which is still a few weeks away).

i currently have a few simple questions:

lets say i have a txt file in the SD card called abc.txt with the text 'abc' inside. i know that to access a block of data (512 bytes) from a SD card in SPI mode i would have to send CMD 17 (read block) to the SD card. i know that CMD17 requires an address argument to be sent to the SD card, specifying where on the card do i want the data...so how do i find the address of the txt file?

i used winhex and opened a SDcard connected to my computer, i created the abc.txt file with the text 'abc'. from what i can see from winhex, does it mean that the address i have to specify is 0422EE00?

also, would the reply from the SDcard be (plain and simply) 'abc' in binary form? eg: 61h 62h 63h -> 01100001 01100010 01100011

thats all the questions i have for now, would appreciate any help! :grin:

 

[permute]

so how do i find the address of the txt file?
you are using a file system. you would need to use its mechanisms for finding and retrieving files.

the sd card is essentially a block device. so you'd get blocks of data back. likewise, you'd need a block address. notice that the file starts on a 512B boundary? the block is then 0x0422EE00 / 0x200 = 0x21177. a read of that block will return 512B. (for sd, more for some of the SDHC formats).

 

[ptjw]

oh ok i think i get it, SD cards are accessed in blocks of 512bytes!

but will the data sent over the MISO be the binary equivalent of 'abc'? 01100001 01100010 01100011?

thanks!

 

[permute]

It should be, and keep in mind that the protocol does have possible busy states and a block checksum.