How to make sure SDIO VDDIO works at 1.8V for SD3.0

[yuchaohsu]

Dear all

My development environment is i.MX 6ULL evk, the dts file is imx6ull-14x14-evk.dts

I tried connect SD1 to a WiFi module with VDDIO=1.8V in order to run with SDR104 mode.
From the schematic, the VDDIO of SD1 is reference to NVCC_SD,
which the voltage is control by SD1_VSELECT (GPIO1_IO05)

So I force to raise the GPIO1_IO05 to output-high and had measure the NVCC is going to 1.8V from 3.3V
However when I connect the WiFi module with VIO=1V8, the module cannot be found.
If I connect the WiFi module with VIO=3V3, the module can be found but it cannot pass sdhci_execute_tuning,
and shows error like below:

[ 4.275997] sdhci: Tuning procedure failed, falling back to fixed sampling clock
[ 4.283461] mmc0: tuning execution failed
[ 4.287524] mmc0: error -5 whilst initialising SDIO card

So looks the VDDIO on SOC side didn't work with 1.8V but 3.3V,
not sure if I had miss anything...?

[igorpadykov]

Hi YuChao

swithching to 1.8V and SDR104 mode is performed by MMC driver

automatically according to sd specifications

Simplified Specifications - SD Association 

MMC driver description can be found in Linux 4.19.35_1.1.0 Documentation

Tuning procedure fail may be caused by signal integrity problems, like

long cable, noisy power supplies.

Best regards
igor
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