https://e2e.ti.com/support/processors/f/791/t/943883

Initialization should use the same register setting as Default Speed.  Once in 1.8 V, SDR12 register settings should be set.

When they tested SD card read/write, they found that four SD cards of 8 cards didn’t work in 1.8V mode even though these worked fine in 3.3V. As a result of their investigation, they found no response from SD card immediately after changing from 3.3V to 1.8V even though AM6526 sends a command (CMD2) to SD card.

They confirmed a timing of CLK and CMD signals. (See the attached document.)

Usually, CMD signal is detected at CLK rising. There is no problem on this timing in the case of 3.3V, but CMD timing was changing immediately after changing to 1.8V such like CMD was changed at CLK rising in spite of not changing the frequency from 25MHz. From this behavior, set up time and hold time was not enough.

(This timing is the same between OK card and NG card.)

It seems that working fine or not depends on unique characteristics of each SD card.

They think that the above problem is caused by the timing between CLK and CMD signal at 1.8V.

They’re using Processor SDK RTOS driver of am65xx.

mmcsd driver is not modified.

The followings are modified in MMCSD_socSetInitCfg.

• supportedBusVoltages

• switchVoltageFxn

• supportedModes

Then, they have a question and request as follows;

1. After changing to 1.8V, immediately CMD behavior was changed like CMD signal was toggled at CLK rising. This is different from AM6526 timing specification in attached file. This is problem, right ?

2. Could you share any sample code and document with them to control changing 3.3V to 1.8V correctly ?

 [^CLK and CMD signal of MMCSD.pptx]