CM4 Using EMMC together with SD Card on the second SD/SDIO interface on GPIO22...GPIO27

[Frauholle64]

While my application is booting and running from the internal EMMC i'd like to be able to connect a SD card (simple sd card breakout connected to GPIO22..27) and access it via circles fatfs lib. I see that people are able to do so running the linux distros with overlays.
My question is if that would be 'theoretical' possible and access it just like "SD:" , "EMMC:" or "USB:
Thanks

[rsta2]

It's theoretical possible, but currently not implemented. I will see, what I can do. I will need some time.

[rsta2]

Do you need WiFi access from your CM4 at the same time? That would be a problem, because it uses the same EMMC device, which is needed for accessing the SD card at GPIO22-27.

[Frauholle64]

No, I don't need Wifi and I am aware of that.
If it helps then I can send you a ready-to-use devboard.

[rsta2]

Good. Thanks for the offer, but I already ordered this board. It already works with the Circle SD card driver with a small modification on a RPi 1B+. I suppose the CM4 would also work, but if you want to use the eMMC memory of the CM4 together with the external SD card at the same time, I need to refactor the SD card driver, because currently it can be loaded for one interface only at a time and it is configured with defines at build time. I need to make this dynamic. I'm working on this now.

[Frauholle64]

Yes I figured that out that it defines for either one or the other.
Out of curiousity: It is capable to be used with 2 interfaces, one as a USB medium and the hardware eMMC or SD card.
Aren't that already 2 interfaces for it or does it just work different in the background (as in like 2 hardcoded fatfs diskio depending on the destination)?
Anyways, thanks for looking into it!

[rsta2]

USB is independent from the SD card interface(s) and will remain available also with FatFs. I guess there will be a new prefix like "SD2:" for FatFs for the external SD card at GPIO22-27 then. So there will be "SD:", "SD2:" and "USB:". BTW. There are already additional interfaces "USB2:", "USB3:" and "NVME:" (RPi 5 only).

[rsta2]

There is a new branch sdcard-external with the necessary modifications to addon/SDCard/ and addon/fatfs/ to support an external SD card at GPIO22..27 in an SDIO breakout board along with the eMMC memory on a Compute Module 4. The class CEMMCDevice has to be instantiated and initialized twice for this purpose:

#include <SDCard/emmc.h>

CEMMCDevice eMMC (&m_Interrupt, &m_Timer, nullptr, CEMMCDevice::EmbeddedMMC);
CEMMCDevice SDCardext (&m_Interrupt, &m_Timer, &m_ActLED, CEMMCDevice::SDCardExternal);

eMMC.Initialize ();
SDCardext.Initialize ();

After that the eMMC can be accessed as volume "SD:" in FatFs and the external SD card as "SD2:".

Can you please test this? Let me know, when you have further questions on this!

[rsta2]

OK, good to know. So maybe there is a problem with some data line D1-D3? The driver switches from 1-bit to 4-bit mode in-between. Card detection works in 1-bit mode. You can suppress 4-bit mode by out-commenting this line. Also this line could make a difference. I tested another two v3.xx SD cards with a CM4 and the Adafruit breakout board and all worked well.

[Frauholle64]

The 4 bit flag does the difference and in 'slow' 1bit mode both EMMC and SD do work together. (V4.x card)
Now is the question how to get turbo. :-)

[rsta2]

Can it be, that you mixed the data lines to the breakout board? Looking at the Adafruit board they are not in a row. That's error prone.

[Frauholle64]

The SD-board is fine. I triple checked everything.
I did connect a logic analyzer now and it turns out that the SD speed (SD_CLOCK_HIGH, 50MHz) seem to be too fast. Changing this to normal (25MHz) and it does work. It could be possible though that my cabling ruins the signal. 25MHz seem to work fine in 4 bit mode (confirmed with the Logic Analyzer).
I will do some more deeper testing and will give it another go when I receive a better board (SD reader directly on the PCB). But in general it does work! Thanks!

[rsta2]

The system option NO_SD_HIGH_SPEED can be defined, if you want to reduce SD clocking to 25 MHz without patching the driver. You are welcome and thanks for info!