https://www.jedec.org/sites/default/files/docs/JESD84-B451.pdf https://www.jedec.org/sites/default/files/docs/JESD84-B451.p... You can use http://bugmenot.com/

https://www.jedec.org/sites/default/files/docs/JESD84-B451.pdf https://www.jedec.org/sites/default/files/docs/JESD84-B451.p... You can use http://bugmenot.com/view/jedec.org http://bugmenot.com/view/jedec.org to get free-to-use credentials. The section in point is "A.6.1Bus initialization". I'd say that if your MMC works fine going full-speed out of the gate and the IC on the board supports this speed, then you unlikely to encounter any issues.