SDI Technologies SDIO Card Sdio High-Speed Mode, Switching Bus Speed Mode in a Combo Card

©Copyright 2000-2007 SD Card Association

SDIO Simplified Specification Version 2.00

12. High-Speed Mode

High-Speed mode increases the bus clock rate to 50MHz and the SD bus throughput from 12.5MB/sec to 25MB/sec. For information on High-Speed mode for SD memory cards see Part 1 Physical Layer Specification Version 2.00, sections 4.3.10, 4.3.11 and 6.8. SDIO and combo cards may also support High-Speed mode.

12.1SDIO High-Speed Mode

SDIO version 1.20 cards indicate their support for High-Speed mode with the SHS (Support High-Speed) bit in the CCCR (See section 6.9). Hosts switch between default and High-Speed mode with the EHS (Enable High-Speed) bit in the CCCR by CMD52. Following a command to set or clear EHS, cards shall switch speed mode within 8 clocks after the end bit of the corresponding response.

When switching from default to High-Speed mode the host can try to set EHS without first checking SHS. The host issues CMD52 in RAW mode, setting EHS to one, and after getting the response of CMD52 the host checks SHS and EHS. If SHS=0 or EHS=0, the command will be ignored and the card is still in default mode. If SHS=1 and EHS=1, the card is in High-Speed Mode.

12.2Switching Bus Speed Mode in a Combo Card

A combo card that supports High-Speed shall support it for both memory and IO. Two bus speed switch commands are defined; SD memory command (CMD6) and SDIO command (EHS in CCCR is changed using CMD52).

A part of this section is not included in the Simplified Specification.

When one bus speed switch commands is executed successfully, the card switches the card bus speed mode. If two bus speed switch commands are executed in turn (to the same bus speed mode), only the first successful command is effective to switch bus speed mode.

The host needs to check success of executing bus speed switch command and then the host can switch the host bus speed mode to the same one.

Success of switching bus speed mode is determined by checking receipt of a good response and the result of switching bus speed mode is the same as the switch requested.

The status of current bus speed mode is read by bus speed switch commands. For example, when bus speed mode is switched by CMD6, the result can be read from EHS. If switching by RAW mode of CMD52 has failed, there are two kinds of responses. One is no response with illegal command error. The other is that CMD52 is accepted and the status of RAW mode indicates EHS is not changed.

A reset of either the memory or IO portion of a combo card will also reset both portions to default speed mode. Within 8 clocks after response of a reset by CMD52 (write to RES in CCCR) or CMD0 the card shall change the speed mode to default speed mode.

Note that when changing the bus speed the host bus driver should treat a bus speed change request from any driver as an atomic operation. The host should mask interrupts and not issue any command to the card until the bus speed change is complete.

If a combo card supports the Lock/unlock function, a locked card cannot change bus speed mode. A locked card indicates an illegal command error to a bus speed switch command. The host needs to unlock the card by CMD42 before changing bus speed. It also implies that a host should not change bus speed mode during initialization before managing a locked card.

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