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©Copyright 2000-2011 SD Card Association
SDIO Simplified Specification Version 3.00
7
3.1.2 Initialization by I/O Aware Host
An SDIO aware host sends CMD5 prior to the CMD55/ACMD41 pair, and thus would receive a valid OCR in
the R4 response to CMD5 and continue to initialize the card. Figure 3-2 shows the operation of an SDIO
aware host operating in the SD modes and Figure 3-3 shows the same operation for a host that operates in
the SPI mode.
If the I/O portion of a card has received no CMD5, the I/O Section remains inactive and shall not respond to
any command except CMD5. A combo card stays in the memory-only mode. If no memory is included on the
card, the card would not respond to any memory command. If the I/O aware host sends a CMD5 to the card,
the card responds with R4. The host then reads that R4 value and knows the number of available I/O
functions and if any SD memory exists.
A host that supports UHS-I sets S18R to 1 in the argument of CMD5 to request a change of the signal voltage
to 1.8V. If the card supports UHS-I and the current signal voltage is 3.3V, S18A is set to 1 in the R4 response.
If the signal voltage is already 1.8V, the card sets S18A to 0 so that host maintains the current signal voltage.
UHS-I is supported in SD mode and S18A is always 0 in SPI mode.
After the host has initialized the I/O portion of the card, it then reads the Common Information Area (CIA) of
the card (Refer to Section 6.8). This is done by issuing a read command, starting with the byte at address
00h, of I/O function 0. The CIA contains the Card Common Control Registers (CCCR) and the Function
Basic Registers (FBR). Also included in the CIA are pointers to the card's common Card Information
Structure (CIS) and each individual function's CIS. The CIS structure is defined in Chapter 16. The CIS
includes information on power, function, manufacturer and other things the host needs to determine if the I/O
function(s) is appropriate to power-up. If the host determines that the card should be activated, a register in
the CCCR area enables the card and each individual function. At this time, all functions of the I/O card are
fully available. In addition, the host can control the power consumption and enable/disable interrupts on a
function-by-function basis. This access to I/O does not interfere with memory access to the card if present.
Figure 3-2 shows Card Initialization Flow in SD mode and Figure 3-3 shows Card Initialization Flow in SPI
mode. UHS-I is not supported in SPI mode. Initialization Flow for an embedded device can be simplified by
removing unnecessary checks.
When receiving CMD5 with arg=0, the SDIO card returns an R4 response but does not start initialization of
the I/O function. The Version 3.00 SDIO card should start initialization sequence by receiving CMD5 without
waiting for CMD5 with WV=0.
Combo Cards can accept CMD15 with RCA=0000, as described in the Physical Layer Specification, but
there is an exception for SD memory only cards. Memory only cards require a non-zero RCA before the host
may issue CMD15. Thus, CMD15 shall be issued after CMD3 in the Standby state. In the case of ACMD41,
it shall accept RCA=0000h.
Application Notes:
As shown in Figure 3-2 and Figure 3-3, an SDIO aware host should send CMD5 arg=0 as part of the
initialization sequence after either Power On or a CMD 52 with write to I/O Reset. CMD5 arg=0 is
required for using a removal card due to the following reasons:
(1) There was 3.1V-3.5V voltage range legacy card and then whether host power supply can provide
the voltage in this range should be checked.
(2) There may be the card which requires CMD5 with WV= 0 to initialize the card.
CMD5 arg=0 may not be necessary for embedded system if the device does not require it.