SPRUJ28E November 2021 – September 2024 AM68 , AM68A , TDA4AL-Q1 , TDA4VE-Q1 , TDA4VL-Q1
This section describes the sequence for switching the bus speed mode: Default Speed, High Speed mode and UHS-I mode. The switch command (CMD6) is used to change memory card bus speed mode. The EHS bit (SDIO Version 2.00) or BSS[2-0] bits (SDIO Version 3.00) in the CCCR register is used to change the bus speed mode for SDIO card. In case of Combo card, either of the switch method changes both memory and IO bus speed mode. This means the first switch is effective. Refer to the Physical Layer Specification Version 3.0x and the SDIO Specification Version 3.00 for more information about switching bus speed. Figure 1-1 shows the sequence for switching bus speed mode for Combo Card. Note that if the card is locked, bus width cannot be changed. Unlock the card is required before changing bus width.
(1) The Host Driver checks if the card supports memory. If not supported, go to (5).
(2) Issue CMD6 with mode 1 to change bus speed mode (Default, High Speed mode or UHS-I mode).
(3) Check the response of CMD6. If the card does not supports CMD6 (no response) or bus speed is not changed successfully, go to step (5). In this case, the card is in Default Speed mode.
(4) The Host Driver changes the Host Controller bus speed mode to the same mode.
(5) The Host Driver checks if the card supports SDIO. If not supported, go to the end.
(6) Issue CMD52 to write EHS bit or BSS[2-0] bits in CCCR to change bus speed mode (the same bus speed mode of (2) shall be set).
(7) If EHS or BSS[2-0] are not changed successfully, go to the end.
(8) The Host Driver changes the Host Controller bus speed to the same mode. In case of Combo card, bus speed is already changed at step (4) and this step does not affect changing bus speed.