Power over SDIO iM8MQ

[evgenii_platono]

Hello,

I have a question regarding the evaluation board with iMX8MQ. I would like to use one external Wifi module, which should be supplied over SDIO 3.3V from the host platform. When I plug in SD adapter which connects my Wifi module with iMX to appropriate SD slot, I've figured out that some seconds EVB supply my module with 3.3V, but after a small period of time it completely switches off the voltage rail.

From iMX8MQ-EVB schematic, I could see that there is a transistor Q1602, which is responsible for SDIO power and looks like that it disconnects power from the module. Moreover, I see two control signals which directly go to the processor - SD2_NRST and POR_B. Also, there is bypass resistor R1606 which allows simple supplying scheme without any logic signals. I soldered this resistor in and with this workaround the power goes well to my module.

Could I ask you kindly explain what is the reason why the processor switches off the power from SD slot? Also, could you clarify what are the signals SD2_NRST and POR_B and how they play role in such case?

Thank you very much in advance!

[igorpadykov]

Hi evgenii_platono

> reason why the processor switches off the power from SD slot

transistor Q1602 is depicted on schematic p.19 "SD3.0 PWR", NVCC_SD2 may be 1.8V/3.3V as described
on p.19, SD2_VSELECT. This is required by voltage swithching to 1.8V signaling mode,
voltage switching sequence is described in SD 3.0 specification.
SD2_NRST and POR_B are used as "power cycle" signals and also described in SD 3.0 specification.

Best regards
igor

[evgenii_platono]

Hi Igor,

Thanks for your reply!

Yes, I see that SD2_VSELECT is related to switching between 1.8 and 3.3 Volts, which basically connected to speed mode of SDIO communication - high speed or ultra-high-speed. I'm using the module which generally works only in UHS mode and it means that the host and module should use 1.8 V for SDIO signals.

However, that is not related to power rail on SD slot - VSD_3V3 - it should always be 3.3V which intended to supply external devices. Hence I see that especially it doesn't work for my case and somehow processor doesn't want to switch on the power.

Sorry for my ignorance, but I didn't find information about SD2_NRST and POR_B, could you please explain to me what is the working algorithm with which processor recognize SD device and send logic levels at these pins?

Thank you very much!

Best regards,

Evgenii

[igorpadykov]

>VSD_3V3 - it should always be 3.3V which intended to supply external devices.

during switching to 1.8v signaling mode it may be turned off for a short time.

Suggest to check signals with logic analyzer and verify them with SD 3.0 specification.

Also reason may be that SD port can not provide sufficient current to wifi module,

one can try external power supply.

Best regards
igor