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SDIO Simplified Specification Version 2.00
5.New I/O Read/Write Commands
Two additional data transfer instructions have been added to support I/O. IO_RW_DIRECT, a direct I/O command similar to the MMC 'Fast I/O' command, and IO_RW_EXTENDED, which allows fast access with byte or block addresses. Both commands are in class 9 (I/O Commands).
5.1IO_RW_DIRECT Command (CMD52)
The IO_RW_DIRECT is the simplest means to access a single register within the total 128K of register space in any I/O function, including the common I/O area (CIA). This command reads or writes 1 byte using only 1 command/response pair. A common use is to initialize registers or monitor status values for I/O functions. This command is the fastest means to read or write single I/O registers, as it requires only a single command/response pair.
S
D
Command
Index
110100b
R/W
flag
Function Number
RAW
flag
Stuff
Register Address
Stuff
Write Data or Stuff Bits
CRC7
E
1 | 1 | 6 | 1 | 3 | 1 | 1 | 17 | 1 | 8 | 7 | 1 |
Figure 5-1 IO_RW_DIRECT Command
The IO_RW_DIRECT Command contains the following fields:
S(tart bit): | Start bit. Always 0 |
D(irection): | Direction. Always1 indicates transfer host to card. |
Command Index: | Identifies the “IO_RW_DIRECT” command with a value of 110100b |
R/W Flag: | This bit determines the direction of the I/O operation. If this bit is 0, this command |
| shall read data from the SDIO card at the address specified by the Function Number |
| and the Register Address to the host. The data byte is returned in the response, R5. |
| If this bit is set to 1, the command shall write the bytes in the Write Data field to the |
| I/O location addressed by the Function Number and the Register Address. If the |
| RAW flag is 0, then the data in the register that was written shall be read and that |
| value returned in the response. |
RAW Flag: | The Read after Write flag. If this bit is set to 1 and the R/W flag is set to 1, then the |
| command shall read the value of the register after the write. This is useful to allow |
| writing to a control register and reading the status at the same address. If this bit is |
| cleared, the value returned in the R5 response shall be the same as the write data in |
| the command. If this bit is set, the data field of the R5 response shall contain the |
| value read from the addressed register after the write operation. |
Function Number: | The number of the function within the I/O card you wish to read or write. Note that |
| function 0 selects the common I/O area (CIA). |
Register Address: | This is the address of the byte of data inside of the selected function to read or write. |
| There are 17 bits of address available so the register is located within the first 128K |
| (131,072) addresses of that function. |
Write Data/Stuff Bits: | For a direct write command (R/W=1), this is the byte that is written to the selected |
| address. For a direct read (R/W=0), this field is not used and shall be set to 0. |
CRC7: | 7 bits of CRC data |
E(nd bit): | End bit, always 1 |
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