Hi WayneWWW,
In the Jetson nano product design guide, the sdmmc3 can be designed as sd card.
[url]https://developer.nvidia.com/embedded/dlc/jetson-nano-product-design-guide[/url]
After doing some research, I have some conclutions below.
The SD card and eMMC actually use the same signal. The main difference is eMMC only have 4 bit data line and SD has 8. eMMC support 1-bit, 4-bit,and 8-bit mode.
[url]http://www.zeroplus.com.tw/software_download/201207-eMMC_en.pdf[/url] (see page 5)
So, there are some conversion boards in the market to convert an eMMC chip to microSD card form factor, like this:
[url]https://www.friendlyarm.com/index.php?route=product/product&product_id=208[/url]
I wonder why sdmmc3 cannot support eMMC.