I'm using the i.MX6SL EVK and I have the SD card in SD1. I'm seeing that it boots from the SD card even when I have the BOOT MODE switches set for Serial Download. I've checked that the fuses are not blown (especially BT_FUSE_SEL), though that shouldn't matter. The only way I can make it enter serial download mode is to remove the SD card and then reset/POR.
This does not happen with SD3.
What would make it boot from SD (internal boot) when I've set BOOT MODE for serial download?