How to use the SD slot on a CM4 with EMMC

[tyon2006]

I see the warning about using an SD card with CM4 units with emmc - I'm curious to learn more about the problem here. The only CM4s available in my area with WiFi also had EMMC, but it was my intent to use this board with an SD card (given the limitations of the CM$ on board storage anyway, I always needed something much larger). I could use some kind of external storage device, but it vexes me to tack on a huge external drive with the SD card port just sitting right there looking at me.

That being said, is there anything that can be done to the carrier board or the CM4 itself that can get around the risk you posted?

[magic-blue-smoke]

@tyon2006
This issue is on CM4 side. Processor pins for storage are shared between soldered eMMC chip and external SD card. Also, there's no way to multiplex or address particular storage when both are present. Ie processor can't tell eMMC "please tristate your output buffers and disable receiving commands, I'm talking to SD-card" and vice versa. This results in two storage devices getting same commands and end up one device is pulling wire low and another to high at some point, effectively making direct connection between power and ground, which is not good.

Going to super extremes, you can try to check if someone have tried to desolder eMMC, but this is not something I've recommended to you :)

There are only two things I can really recommend here:

• keep watching Raspberry Pi locator, but I guess this is something you're already doing
• use WiFi dongle at the price of occupied USB connector (alternatively, if skills and aesthetics allows, solder it to the GND, DM, DP and 5V golden pads on a back side of TV Stick, keeping both USB connectors available for other devices)