IOP 16 SD Card - douggilliland/IOP16 GitHub Wiki

Table of Contents

SD card interface

  • VHDL Code
  • Reads and writes a single block of data as a data stream
-- Address Register
--    0    SDDATA        read/write data
--    1    SDSTATUS      read
--    1    SDCONTROL     write
--    2    SDLBA0        write-only
--    3    SDLBA1        write-only
--    4    SDLBA2        write-only (only bits 6:0 are valid)
  • For both SDSC and SDHC (high capacity) cards, the block size is 512bytes (9-bit value) and the SDLBA registers select the block number.
    • SDLBA2 is most significant, SDLBA0 is least significant.
  • For SDSC, the read/write address parameter is a 512-byte aligned byte address. ie, it has 9 low address bits explicitly set to 0. 23 of the 24 programmable address bits select the 512-byte block.
    • This gives an address capacity of 2^23 * 512 = 4GB .. BUT maximum SDSC capacity is 2GByte.

SDLBA registers 

-- 31 30 29 28.27 26 25 24.23 22 21 20.19 18 17 16.15 14 13 12.11 10 09 08.07 06 05 04.03 02 01 00
--+------- SDLBA2 -----+------- SDLBA1 --------+------- SDLBA0 --------+ 0  0  0  0  0  0  0  0  0
  • For SDHC cards, the read/write address parameter is the ordinal number of 512-byte block ie, the 9 low address bits are implicity 0. The 24 programmable address bits select the 512-byte block.
  • This gives an address capacity of 2^24 * 512 = 8GByte. SDHC can be upto 32GByte but this design can only access the low 8GByte (could add SDLBA3 to get the extra address lines if required).

SDLBA registers 

-- 31 30 29 28.27 26 25 24.23 22 21 20.19 18 17 16.15 14 13 12.11 10 09 08.07 06 05 04.03 02 01 00
--  0  0  0  0  0  0  0  0+---------- SDLBA2 -----+------- SDLBA1 --------+------- SDLBA0 --------+
  • The end result of all this is that the addressing looks the same for SDSC and SDHC cards.

SDSTATUS (RO) 

--    b7     Write Data Byte can be accepted
--    b6     Read Data Byte available
--    b5     Block Busy
--    b4     Init Busy
--    b3     Unused. Read 0
--    b2     Unused. Read 0
--    b1     Unused. Read 0
--    b0     Unused. Read 0

SDCONTROL (WO) 

--    b7:0   0x00 Read block
--           0x01 Write block

Programming

To read a 512-byte block from the SDCARD

  • Wait until SDSTATUS=0x80 (ensures previous cmd has completed)
  • Write SDLBA0, SDLBA1 SDLBA2 to select block index to read from
  • Write 0 to SDCONTROL to issue read command
  • Loop 512 times:
    • Wait until SDSTATUS=0xE0 (read byte ready, block busy)
    • Read byte from SDDATA

To write a 512-byte block to the SDCARD

  • Wait until SDSTATUS=0x80 (ensures previous cmd has completed)
  • Write SDLBA0, SDLBA1 SDLBA2 to select block index to write to
  • Write 1 to SDCONTROL to issue write command
  • Loop 512 times:
    • Wait until SDSTATUS=0xA0 (block busy)
    • Write byte to SDDATA

Transfer Details

  • At HW level each data transfer is 515 bytes: a start byte, 512 data bytes, 2 CRC bytes. CRC need not be valid in SPI mode, *except* for CMD0.
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