Hi,
We have a custom board and are having problems reliably booting from MMC0 with a 16GB sdcard. The sysboot[4:0] strapping is configured to 10111 (MMC0, SPI0, UART0, USB0). Most of the time it boots up fine. When it does not, it prints 'CCCCCCCC' on the UART line. Sometimes after ~4 minutes, it will boot up, or print another set of 'CCCCCCCC'. ~4 minutes after that at ~8 minutes, it will usually then boot up.
So, it looks like it's skipping over MMC0 and trying to boot from UART.
We have also noticed that some card brands boot up 100% (Lexar), some about 90% (Sandisk), some only 50% (PNY). We also have some off-brand cards that work ~75% of the time.
There are 10K pull-up resistors on the clk, cmd, and dat0:3 lines and I just added a series 22ohm resistor on the clk line, but it made no difference.
Any ideas on what to check next?
Thanks.